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Modulation of Tropical Cyclogenesis in the Western North Pacific by the Quasi-Biweekly Oscillation


doi: 10.1007/s00376-016-5267-z

  • The quasi-biweekly oscillation (QBWO) is the second most dominant intraseasonal mode over the western North Pacific (WNP) during boreal summer. In this study, the modulation of WNP tropical cyclogenesis (TCG) by the QBWO and its association with large-scale patterns are investigated. A strong modulation of WNP TCG events by the QBWO is found. More TCG events occur during the QBWO's convectively active phase. Based on the genesis potential index (GPI), we further evaluate the role of environmental factors in affecting WNP TCG. The positive GPI anomalies associated with the QBWO correspond well with TCG counts and locations. A large positive GPI anomaly is spatially correlated with WNP TCG events during a life cycle of the QBWO. The low-level relative vorticity and mid-level relative humidity appear to be two dominant contributors to the QBWO-composited GPI anomalies during the QBWO's active phase, followed by the nonlinear and potential intensity terms. These positive contributions to the GPI anomalies are partly offset by the negative contribution from the vertical wind shear. During the QBWO's inactive phase, the mid-level relative humidity appears to be the largest contributor, while weak contributions are also made by the nonlinear and low-level relative vorticity terms. Meanwhile, these positive contributions are partly cancelled out by the negative contribution from the potential intensity. The contributions of these environmental factors to the GPI anomalies associated with the QBWO are similar in all five flow patterns——the monsoon shear line, monsoon confluence region, monsoon gyre, easterly wave, and Rossby wave energy dispersion associated with a preexisting TC. Further analyses show that the QBWO strongly modulates the synoptic-scale wave trains (SSWs) over the WNP, with larger amplitude SSWs during the QBWO's active phase. This implies a possible enhanced (weakened) relationship between TCG and SSWs during the active (inactive) phase. This study improves our understanding of the modulation of WNP TCG by the QBWO and thus helps with efforts to improve the intraseasonal prediction of WNP TCG.
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  • Aiyyer A. R., J. Molinari, 2003: Evolution of mixed Rossby-gravity waves in idealized MJO environments. J. Atmos. Sci., 60, 2837- 2855.10.1175/1520-0469(2003)0602.0.CO;28d72e764d510909c93d75480dac34de0http%3A%2F%2Fwww.ams.org%2Fmathscinet-getitem%3Fmr%3D2020243http://www.ams.org/mathscinet-getitem?mr=2020243Abstract A linear shallow water model is used to simulate the evolution of mixed Rossby–gravity (MRG) waves in background states representative of the convective phase of the Madden–Julian oscillation (MJO). Initial MRG wave structures are obtained analytically. The MJO basic state is defined by the steady response of the tropical atmosphere to localized heating. Results from the simulations reveal that variations in the background flow play a significant role in the evolution of the MRG waves. When the basic state is symmetric about the equator, the MRG wave amplifies within the convergent region of the background flow and the ensuing development remains symmetric. When the heating is asymmetric, both the basic state and the MRG wave evolution exhibit significant asymmetries. Prominent features of this simulation are the development and growth of a series of small-scale, off-equatorial eddies that resemble tropical-depression-type disturbances. The results suggest that a persistent large-scale heating th...
    Bessafi M., M. C. Wheeler, 2006: Modulation of South Indian Ocean tropical cyclones by the Madden-Julian oscillation and convectively coupled equatorial waves. Mon. Wea. Rev., 134, 638- 656.e5858e0f1831f7253abf7bea562b1078http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2006MWRv..134..638B%26db_key%3DPHY%26link_type%3DABSTRACThttp://xueshu.baidu.com/s?wd=paperuri%3A%2845090657e479867fe962775927dea3e2%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2006MWRv..134..638B%26db_key%3DPHY%26link_type%3DABSTRACT&ie=utf-8&sc_us=16554579101997842744
    Bister M., K. A. Emanuel, 2002: Low frequency variability of tropical cyclone potential intensity 1. Interannual to interdecadal variability. J. Geophys. Res.,107 (D24), ACL 26-1-ACL26- 15.10.1029/2001JD0007761fded005d76564cca3a0b55455b7309fhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2001JD000776%2Fcitedbyhttp://onlinelibrary.wiley.com/doi/10.1029/2001JD000776/citedbyRecent research suggests that anthropogenic global warming would be associated with an increase in the intensity of tropical cyclones. A recent statistical analysis of observed tropical cyclone intensity shows that its variability with location and season is strongly tied to the variability of the thermodynamic potential intensity (PI) of tropical cyclones, as calculated using a theory described in an earlier work by the authors. Thus it is of interest to look for possible trends in global measures of PI, which are far more stable than those of actual storm intensity. We estimate global trends of PI from 1958 to 1996, averaged over the region where it exceeds 40 m s, using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis and the NCEP Empirical Orthogonal Function (EOF) sea surface temperature (SST) analysis. We adjust the Reanalysis temperatures for a large, spurious temperature increase that occurred around 1979. We do this by subtracting from the Reanalysis the atmospheric temperature difference between pairs of years with similar tropical SST before and after 1979. The value of the global mean PI is very large for the SST of the corresponding region in the mid-1990s. Supported by a recent study on the effects of ozone decrease on tropospheric temperatures, we suggest that the ozone decrease might be one of the factors contributing to increase of PI during the 1990s.
    Camargo S. J., A. H. Sobel, 2005: Western North Pacific tropical cyclone intensity and ENSO. J.Climate, 18, 2996- 3006.10.1175/JCLI3457.1f4a2f6bf47d61f1b984d7765878a520dhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2005JCli...18.2996Chttp://adsabs.harvard.edu/abs/2005JCli...18.2996CThe influence of the El Nino-Southern Oscillation (ENSO) on tropical cyclone intensity in the western North Pacific basin is examined. Accumulated cyclone energy (ACE), constructed from the best-track dataset for the region for the period 1950-2002, and other related variables are analyzed. ACE is positively correlated with ENSO indices. This and other statistics of the interannually varying tropical cyclone distribution are used to show that there is a tendency in El Nino years toward tropical cyclones that are both more intense and longer-lived than in La Nina years. ACE leads ENSO indices: during the peak season (northern summer and fall), ACE is correlated approximately as strongly with ENSO indices up to six months later (northern winter), as well as simultaneously. It appears that not all of this lead-lag relationship is easily explained by the autocorrelation of the ENSO indices, though much of it is. Interannual variations in the annual mean lifetime, intensity, and number of tropical cyclones all contribute to the ENSO signal in ACE, though the lifetime effect appears to be the most important of the three.
    Camargo S. J., M. C. Wheeler, and A. H. Sobel, 2009: Diagnosis of the MJO modulation of tropical cyclogenesis using an empirical index. J. Atmos. Sci., 66, 3061- 3074.de20a42c0b2156021bdef396f87c9e51http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2009JAtS...66.3061C%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D18603http://xueshu.baidu.com/s?wd=paperuri%3A%285e784fccba36292f3d67846b07b3405c%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2009JAtS...66.3061C%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D18603&ie=utf-8&sc_us=7206240984391299254
    Chan J. C. L., 2005: Interannual and interdecadal variations of tropical cyclone activity over the western North Pacific. Meteor. Atmos. Phys., 89, 143- 152.10.1007/s00703-005-0126-ybf5f12dced348abd0dfd4d54070ac148http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00703-005-0126-yhttp://link.springer.com/article/10.1007/s00703-005-0126-yThis paper reviews the interannual and interdecadal variations in tropical cyclone (TC) activity over the western North Pacific (WNP) and the possible physical mechanisms responsible for such variations. Interannual variations can largely be explained by changes in the planetary-scale flow patterns. Sea-surface temperatures (SSTs) in the WNP, however, do not contribute to such variations. Rather, SSTs in the central and eastern equatorial Pacific are significantly correlated with TC activity over the WNP. Causality can be established: changes in the SST in the equatorial Pacific are related to the El Nino/Southern Oscillation (ENSO) phenomenon, and modifications of the planetary-scale flow associated with ENSO alter the conditions over the WNP and hence TC activity there. Variations in annual TC activity are also associated with different phases of the stratospheric quasi-biennial oscillations due to its modification of the vertical wind shear of the environment in which TCs form. Interdecadal variations in TC activity are apparently related to the location, strength and extent of the North Pacific subtropical high. However, the mechanisms responsible for modifying these characteristics of the subtropical high have yet to be identified.
    Chan J. C. L., 2008: Decadal variations of intense typhoon occurrence in the western North Pacific. Proc. Roy. Soc.London, 464, 249- 272.10.1098/rspa.2007.018369162e322bb2d2b21168acb26e16196fhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FADS%3Fid%3D2008RSPSA.464..249Chttp://onlinelibrary.wiley.com/resolve/reference/ADS?id=2008RSPSA.464..249Cchi-Conopeptide MrIA (chi-MrIA) is a 13-residue peptide contained in the venom of the predatory marine snail Conus marmoreus that has been found to inhibit the norepinephrine transporter (NET). We investigated whether chi-MrIA targeted the other members of the monoamine transporter family and found no effect of the peptide (100 microM) on the activity of the dopamine transporter and the serotonin transporter, indicating a high specificity of action. The binding of the NET inhibitors, [3H]nisoxetine and [3H]mazindol, to the expressed rat and human NET was inhibited by chi-MrIA with the conopeptide displaying a slight preference toward the rat isoform. For both radioligands, saturation binding studies showed that the inhibition by chi-MrIA was competitive in nature. It has previously been demonstrated that chi-MrIA does not compete with norepinephrine, unlike classically described NET inhibitors such as nisoxetine and mazindol that do. This pattern of behavior implies that the binding site for chi-MrIA on the NET overlaps the antidepressant binding site and is wholly distinct from the substrate binding site. The inhibitory effect of chi-MrIA was found to be dependent on Na+ with the conopeptide becoming a less effective blocker of [3H]norepinephrine by the NET under the conditions of reduced extracellular Na+. In this respect, chi-MrIA is similar to the antidepressant inhibitors of the NET. The structure-activity relationship of chi-MrIA was investigated by alanine scanning. Four residues in the first cysteine-bracketed loop of chi-MrIA and a His in loop 2 played a dominant role in the interaction between chi-MrIA and the NET. H alpha chemical shift comparisons indicated that side-chain interactions at these key positions were structurally perturbed by the replacement of Gly-6. From these data, we present a model of the structure of chi-MrIA that shows the relative orientation of the key binding residues. This model provides a new molecular caliper for probing the structure of the NET.
    Chang C.-P., G.T.-J. Chen, 1995: Tropical circulations associated with southwest monsoon onset and westerly surges over the South China Sea. Mon. Wea. Rev., 123( 11), 3254- 3267.10.1175/1520-0493(1995)1232.0.CO;2de3e65b31e0bac6b504748eede8b0a0ahttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013127318%2Fhttp://ci.nii.ac.jp/naid/10013127318/The earliest onset of the Asian summer monsoon occurs in early to middle May over the South China Sea. This onset is signified by the development of low-level westerlies and leads to heavy convective rainfall over southern China (pre-Mei-Yu). In June, low-level westerly surges over the northern South China Sea are associated with the Mei-Yu rainfall system in the Yangtze region and southern Japan. In this work, the ECMWF data for 1981-86 are used to study the tropical circulations associated with the development of low-level westerlies during both events. Composites of horizontal wind, geopotential height, moisture, and vertical velocity during six May onsets and nine June surges, respectively, indicate that both events occur with the approach of a midlatitude trough-front system. The possible triggering of the South China Sea summer monsoon onset by the midlatitude system may explain why the South China Sea onset occurs prior to other regions of the Asian monsoon. During boreal spring, this is the only Asian monsoon region where midlatitude fronts can move into the Tropics without having to overcome significant terrain barriers. Following the two events, opposite teleconnection-like patterns develop in the Tropics in both hemispheres. During the May onsets, the arrival of the midlatitude trough/front appears to lead to a southwestward extension of a cyclogenesis zone into the equatorial Indian Ocean. Along this zone, cyclonic vortices develop over the Andaman Sea, the Bay of Bengal, and perhaps the southern equatorial Indian Ocean, and increased deep convection is indicated by the OLR composites. During the June surges, a pair of anticyclones develop straddling the equator at the longitudes of Indochina. This anticyclonic couplet is associated with decreased deep convection and propagates westward to dominate the flow changes over the Bay of Bengal and the southern Indian Ocean. The steady 4-5 m s
    Chen G. H., C. H. Sui, 2010: Characteristics and origin of quasi-biweekly oscillation over the western North Pacific during boreal summer. J. Geophys. Res., 115( D14), D14113.10.1029/2009JD01338941ea9c35ef87e69a4a8da2aaf9034044http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2009JD013389%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1029/2009JD013389/pdfABSTRACT This study investigates the structure, energetic, and origin of quasi-biweekly oscillation (QBWO) over the western North Pacific (WNP), using NCEP reanalyses for the years 2000–2007. In the context of vorticity there appears to be a significant QBWO mode over the WNP during the summer. QBWO emerges from the equatorial region and propagates northwestward. Its horizontal structure exhibits a slight southwest-northeast tilt but mainly longitudinal elongation. In the vertical the QBWO has a northwest tilt with height that gives rise to a structure of the first baroclinic mode. The centers of vorticity and vertical motion near the equator show a phase lag of about one-quarter wavelength, consistent with the characteristics of equatorial waves, whereas the cyclonic circulation is tightly coupled with anomalous convection as the wave moves away from the equator. Energetic analysis of the QBWO reveals that diabatic heating in the tropics and baroclinic processes in the subtropics play important roles in the generation of eddy available potential energy (EAPE). In turn, the conversion from EAPE to eddy kinetic energy (EKE) and the barotropic conversion are major sources for EKE to compensate the loss by EKE redistribution and dissipation. Tracing the QBWO to equatorial disturbances, our results show some features of equatorially trapped n = 1 Rossby mode, such as phase speed and group velocity. This mode is generally characterized by a zonal planetary wave number of about 6 and nearly symmetric circulation about the equator. A typical case from 2002 is chosen to illustrate that the origin of the QBWO is closely associated with the theoretical equatorial Rossby wave.
    Chen T. C., J. M. Chen, 1993: The 10-20-day mode of the 1979 Indian monsoon: Its relation with the time variation of monsoon rainfall. Mon. Wea. Rev., 121, 2465- 2482.10.1175/1520-0493(1993)121<2465:TDMOTI>2.0.CO;2ce8a9dba04b2c8d83462c41768ed7c30http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1993MWRv..121.2465Chttp://adsabs.harvard.edu/abs/1993MWRv..121.2465CThe synoptic structure of the 10–20-day monsoon mode and this intraseasonal monsoon mode's relationship with the Indian monsoon rainfall are examined with the 1979 summer First GARP Global Experiment IIIb data of the European Centre for Medium-Range Weather Forecasts and the daily 1° × 1° rainfall estimates retrieved from the satellite data by the Goddard Laboratory for Atmospheres. The major findings of this study are as follows. 1) The 10–20-day monsoon mode exhibits a double-cell (either double-high or double-low) structure; one cell is centered at about 15°–20°N and the other at the equator. 2) Both cells of the 10–20-day monsoon mode propagate coherently westward along the Indian monsoon trough and along the equator, respectively. 3) Based upon the zonal wind and local Hadley circulation, the vertical structure of the 10–20-day monsoon mode does not exhibit a phase change. 4) A significant rainfall occurs around low centers of the 10–20-day monsoon mode through the modulation of this monsoon mode on the Indian monsoon rainfall. 5) The northern-cell lows are initiated from the redevelopment of the westward-propagating residual lows over the Bay of Bengal when the 30–60-day transient monsoon trough is present there. In addition to these findings, the possible genesis and westward-propagation mechanisms of the 10–20-day monsoon mode are also discussed in this study.
    Chen T. C., M. C. Yen, and P. S. Weng, 2000: Interaction between the summer monsoons in East Asia and the South China Sea: Intraseasonal monsoon modes. J. Atmos. Sci., 57( 9), 1373- 1392.10.1175/1520-0469(2000)0572.0.CO;25ddbf662879e5360dadcc13a11288ed2http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FADS%3Fid%3D2000JAtS...57.1373Chttp://onlinelibrary.wiley.com/resolve/reference/ADS?id=2000JAtS...57.1373CThe summer monsoons in East and Southeast Asia are characterized, respectively, by the Mei-yu (in eastern China)-Baiu (in Japan) front (MBF) and by the monsoon trough stretching from northern Indochina to the Philippine Sea. These two major monsoon elements are separated by the North Pacific anticyclone. As indicated by the 850-mb zonal wind and cumulus convection over some key areas, a distinct opposite-phase intraseasonal variation exists between the two monsoon elements. Two approaches are adopted to explore the cause of this opposite-phase variation (which reflects the coupling between the two monsoon components): 1) the correlation coefficient patterns between the 850-mb zonal-wind monsoon index and the 850-mb streamfunction field and 2) the composite 850-mb streamline charts and the 120E zonal-wind cross sections. It is shown that the opposite-phase variation between the two monsoon elements is caused by the anomalous circulation associated with the northward-migrating 30-60-day monsoon trough/ridge from the equator to 20N and with the westward-propagating 12-24-day monsoon low-high along the latitude of 15-20N. Results obtained in this study are used to address two often discussed phenomena of the East Asian monsoon: 1) the rapid northward shift of the MBF across the Yangtze River basin during the Mei-yu onset is related to the north-south meridional oscillation of the MBF, and 2) the three longitudinally oriented location zones of extremely heavy rain events in eastern China are formed by the alternation of deep cumulus convection zones associated with the intraseasonal monsoon vortices (centered in the northern part of the South China Sea) between extreme monsoon conditions.
    Chia, H.-H, C.-F. Ropelewski, 2002: The interannual variability in the genesis location of tropical cyclones in the northwest Pacific. J.Climate, 15, 2934- 2944.ecf6ffaee1f227640081adf028ccfa72http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2002JCli...15.2934C%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D29636http://xueshu.baidu.com/s?wd=paperuri%3A%2893ef9fa1a483080e87f4bf4bc561f9e7%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2002JCli...15.2934C%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D29636&ie=utf-8&sc_us=9626832475982641344
    Dare R. A., J. L. McBride, 2011: The threshold sea surface temperature condition for tropical cyclogenesis. J.Climate, 24, 4570- 4576.c4c14c543f6d2902f056aa1fa42ecb24http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2011JCli...24.4570D%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D21781http://xueshu.baidu.com/s?wd=paperuri%3A%28973ef37380883e091d27280395cdb2b0%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2011JCli...24.4570D%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D21781&ie=utf-8&sc_us=3139392462552848225
    Dickinson M., J. Molinari, 2002: Mixed Rossby-gravity waves and western Pacific tropical cyclogenesis. Part I: Synoptic evolution. J. Atmos. Sci., 59( 14), 2183- 2196.10.1175/1520-0469(2002)0592.0.CO;2a146e447cccfb34b1769deb0568e6384http%3A%2F%2Fwww.ams.org%2Fmathscinet-getitem%3Fmr%3D1913036http://www.ams.org/mathscinet-getitem?mr=1913036Abstract A large-amplitude mixed Rossby–gravity wave packet is identified in the western Pacific using 6–10-day bandpass-filtered winds. Individual disturbances of 2300–3000-km wavelength propagated westward as the packet moved slowly eastward. The packet first appeared, and subsequently amplified, within a region of active convection associated with the Madden–Julian oscillation (MJO), which was isolated by low-pass-filtered outgoing longwave radiation. The packet lasted about 5 weeks, then rapidly dispersed as the active MJO moved away from it to the east. West of 150°E, individual disturbances within the packet turned northwestward away from the equator, indicating an apparent transition from mixed Rossby–gravity waves to off-equatorial tropical depression (TD)-type disturbances. Cyclones filled with cloud and anticyclones cleared during the transition. Nevertheless, convective structure consistent with mixed Rossby–gravity waves remained outside the circulation centers, and three tropical cyclones for...
    Emanuel K. A., D. S. Nolan, 2004: Tropical cyclone activity and the global climate system. Preprints, 26th Conf. on Hurricanes and Tropical Meteorology, Miami, FL, Amer. Meteor. Soc., 10A. 2.a58e0e19-b7b3-4bf9-bde3-500523e6553ac3c20a9a72adb1c17dcae21faf2ab535http%3A%2F%2Fwww.mendeley.com%2Fcatalog%2Ftropical-cyclone-activity-global-climate-system-1%2Frefpaperuri:(f35ef06aacc83753942b0d22f4051f9b)http://www.mendeley.com/catalog/tropical-cyclone-activity-global-climate-system-1/
    Frank W. M., P. E. Roundy, 2006: The role of tropical waves in tropical cyclogenesis. Mon. Wea. Rev., 134, 2397- 2417.e2e211ce5a649eeb14aa9b268acfe226http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2006MWRv..134.2397F%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D18603http://xueshu.baidu.com/s?wd=paperuri%3A%282a7b0b9cba6d43e947661f1252a98b14%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2006MWRv..134.2397F%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D18603&ie=utf-8&sc_us=16448713356602648062
    Fu B., T. Li, M. S. Peng, and F. Z. Weng, 2007: Analysis of tropical cyclogenesis in the western North Pacific for 2000 and 2001. Wea. Forecasting, 22, 763- 780.10.1175/WAF1013.1297d4249b3d89daa24c78855fd157a32http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2007WtFor..22..763Fhttp://adsabs.harvard.edu/abs/2007WtFor..22..763FAbstract High-resolution satellite data and NCEP-CAR reanalysis data are used to analyze 34 tropical cyclone (TC) genesis events in the western North Pacific during the 2000 and 2001 typhoon seasons. Three types of synoptic-scale disturbances are identified in the pregenesis stages. They are tropical cyclone energy dispersions (TCEDs), synoptic wave trains (SWTs) unrelated to preexisting TCs, and easterly waves (EWs). Among the total 34 TC genesis cases, 6 are associated with TCEDs, 11 cases are associated with SWTs, and 7 cases are associated with EWs. The analyses presented herein indicate that the occurrence of a TCED depends on the TC intensity and the background flow, with stronger cyclones and weaker background easterlies being more likely to induce a Rossby wave train. Not all Rossby wave trains would lead to the formation of a new TC. Among the 11 SWT cases, 4 cases are triggered by equatorial mixed Rossby ravity waves. Cyclogenesis events associated with EWs are identified by the westward propagation of the perturbation kinetic energy and precipitation fields. For all three types of prestorm disturbances, it seems that scale contraction of the disturbances and convergence forcing from the large-scale environmental flow are possible mechanisms leading to the genesis. Further examination of the remaining 10 genesis cases with no significant prior synoptic-scale surface signals suggests three additional possible genesis scenarios: 1) a disturbance with upper-tropospheric forcing, 2) interaction of a preexisting TC with southwesterly monsoon flows, and 3) preexisting convective activity with no significant initial low-level vorticity. Tropical intraseasonal oscillations have a significant modulation on TC formation, especially in 2000.
    Gray W. M., 1968: Global view of the origin of tropical disturbances and storms. Mon. Wea. Rev., 96, 669- 700.10.1175/1520-0493(1968)0962.0.CO;2e5b6389640eff865a75f988c3e608f80http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1968MWRv...96..669Ghttp://adsabs.harvard.edu/abs/1968MWRv...96..669GThe basic report is a progressive step in an updating of tropical storm climatology. It discusses areas which subsequently became tropical geographic areas: the northwest Pacific Ocean, the Indian Ocean and the West Indies. Mean motion fields were computed with respect to a coordinate system with its origin at the moving potential storm center.
    Gray W. M., 1979: Hurricanes: Their formation, structure and likely role in the tropical circulation. Meteorology Over the Tropical Oceans, D. B. Shaw, Ed., Roy, Meteorological Society, 155- 218.10.1038/276445b04eb39ad53a2e948f6e81f59920363b99http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1978natur.276..445shttp://adsabs.harvard.edu/abs/1978natur.276..445sNot Available
    Hartmann D. L., M. L. Michelsen, S. A. Klein, 1992: Seasonal variations of tropical intraseasonal oscillations: A 20-25-day oscillation in the western Pacific. J. Atmos. Sci., 49( 14), 1277- 1289.9d62693aaa8e51b1fd977916ece74404http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D1992JAtS...49.1277H%26db_key%3DPHY%26link_type%3DABSTRACThttp://xueshu.baidu.com/s?wd=paperuri%3A%2819924684d9ce1d6c769d7992b0b85bac%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D1992JAtS...49.1277H%26db_key%3DPHY%26link_type%3DABSTRACT&ie=utf-8&sc_us=4338870126816510019
    Ho C. H., J. J. Baik, J. H. Kim, D. Y. Gong, and C. H. Sui, 2004: Interdecadal changes in summertime typhoon tracks. J.Climate, 17( 9), 1767- 1776.fe9165cdd72c5d696b54d0aad0cdf332http%3A%2F%2Fir.lib.ncu.edu.tw%2Fhandle%2F987654321%2F36093%3Flocale%3Dzh-TWhttp://ir.lib.ncu.edu.tw/handle/987654321/36093?locale=zh-TW提供台灣中央大學的博碩士論文、考古題、期刊論文、研究計畫等下載
    Hsu H.-H., C.-H. Weng, and C. H. Wu, 2004: Contrasting characteristics between the northward and eastward propagation of the intraseasonal oscillation during the boreal summer. J.Climate, 17, 727- 743.10.1175/1520-0442(2004)0172.0.CO;2b7f5d7cf919fc3f5168c276a24eef125http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2004jcli...17..727hhttp://adsabs.harvard.edu/abs/2004jcli...17..727hAbstract This study investigates the structural and evolutionary characteristics of the eastward- and northward-propagating intraseasonal oscillation (ISO) in the Indian Ocean and western Pacific during the boreal summer. Along the equator, the near-surface moisture convergence located to the east of the deep convection region appears to result in the eastward propagation of the ISO, consistent with the frictional wave–CISK (conditional instability of the second kind) mechanism proposed in previous studies. The eastward propagation is characterized by sequentially downstream development of deep convection occuring mainly in certain regions such as 60°, 95°, 120°, and 145°E, and the date line. The northward propagation of deep convection can be attributed to the low-level moisture convergence located to the north. This convergence is a deep structure extending from the surface to the middle troposphere. Near-surface convergence appears only after the systems approach the landmass in the north. It is sugges...
    Huang P., C. Chou, and R. H. Huang, 2011: Seasonal modulation of tropical intraseasonal oscillations on tropical cyclone geneses in the western North Pacific. J.Climate, 24, 6339- 6352.10.1175/2011JCLI4200.12b4e7d2e0e43529cbd883f139092b0dahttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2011JCli...24.6339Hhttp://adsabs.harvard.edu/abs/2011JCli...24.6339HAbstract The seasonal modulation of tropical intraseasonal oscillation (TISO) on tropical cyclone (TC) geneses over the western North Pacific Ocean (WNP) is investigated in three periods of the WNP TC season: May–June (MJ), July–September (JAS), and October–December (OND). The modulation of the TISO–TC geneses over the WNP is strong in MJ, while it appears weaker in JAS and OND. In MJ, TISO propagates northward via two routes, the west route through the South China Sea and the east route through the WNP monsoon trough region, which are two clustering locations of TC geneses. TISO can synchronously influence most TC geneses over these two regions. In JAS, however, the modulation is out of phase between the monsoon trough region and the East Asian summer monsoon region, as well as the WNP subtropical high region, as a result of further northward propagation of TISO and scattered TC geneses. The TISO–TC genesis modulation in each individual region is comparable to that in MJ, although the modulation over the entire WNP in JAS appears weaker. In OND, TISO has a stronger influence on TC geneses west than east of 150°E because TISO decays and its convection center located at the equator is out of the TC genesis region when propagating eastward into east of 150°E. Midlevel relative humidity is the primary contribution to the modulations of TISO on the genesis environment, while vorticity could contribute to the modulation over the subtropics in JAS.
    Huffman, G. J., Coauthors, 2007: The TRMM multi-satellite precipitation analysis: Quasi-global, multi-year, combined-sensor precipitation estimates at fine scales. Journal of Hydrometeorology, 8, 38- 55.10.1007/978-90-481-2915-7_1aa0a8e32bf5a118992029370c1c95703http%3A%2F%2Fciteseer.ist.psu.edu%2Fshowciting%3Fcid%3D7512548http://citeseer.ist.psu.edu/showciting?cid=7512548The Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) is intended to provide a “best” estimate of quasi-global precipitation from the wide variety of modern satellite-borne precipitation-related sensors. Estimates are provided at relatively fine scales (0.25° × 0.25°, 3-h) in both real and post-real time to accommodate a wide range of researchers. However, the errors inherent in the finest scale estimates are large. The most successful use of the TMPA data is when the analysis takes advantage of the fine-scale data to create time/space averages appropriate to the user’s application. We review the conceptual basis for the TMPA, summarize the processing sequence, and focus on two new activities. First, a recent upgrade for the real-time version incorporates several additional satellite data sources and employs monthly climatological adjustments to approximate the bias characteristics of the research quality post-real-time product. Second, an upgrade for the research quality post-real-time TMPA from Versions 6 to 7 (in beta test at press time) is designed to provide a variety of improvements that increase the list of input data sets and correct several issues. Future enhancements for the TMPA will include improved error estimation, extension to higher latitudes, and a shift to a Lagrangian time interpolation scheme.
    Jiang X. A., T. Li, and B. Wang, 2004: Structures and mechanisms of the northward propagating boreal summer intraseasonal oscillation. J.Climate, 17, 1022- 1039.10.1175/1520-0442(2004)017<1022:SAMOTN>2.0.CO;28e4bda294ab6437900146e047d43aae6http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2004JCli...17.1022Jhttp://adsabs.harvard.edu/abs/2004JCli...17.1022JThe spatial and temporal structures of the northward-propagating boreal summer intraseasonal oscillation (BSISO) are revealed based on the analysis of both the ECHAM4 model simulation and the NCEP-NCAR reanalysis. The BSISO structure and evolution characteristics simulated by the model bear many similarities to those derived from the NCEP-NCAR reanalysis. The most notable features are the remarkable meridional asymmetries, relative to the BSISO convection, in the vorticity and specific humidity fields. A positive vorticity perturbation with an equivalent barotropic structure appears a few latitude degrees north of the convection center. The maximum specific humidity also shows a clear northward shift in the lower troposphere. Two internal atmospheric dynamics mechanisms are proposed to understand the cause of the northward propagation of the BSISO. The first is the vertical shear mechanism. The key process associated with this mechanism is the generation of barotropic vorticity due to the coupling between the free-atmosphere baroclinic and barotropic modes in the presence of the vertical shear of the mean flow. The induced barotropic vorticity in the free atmosphere further causes a moisture convergence in the planetary boundary layer (PBL), leading to the northward shift of the convective heating. The second mechanism is the moisture-convection feedback mechanism. Two processes contribute to the northward shift of the low-level moisture. One is the moisture advection by the mean southerly in the PBL. Another is the moisture advection by the BSISO wind due to the mean meridional specific humidity gradient. The asymmetric specific humidity contributes to the northward shift of the convective heating. A theoretical framework is constructed to investigate the instability of the northward-propagating BSISO mode and the relative roles of various mechanisms including air-sea interactions. An eigenvalue analysis indicates that the northward propagation of the BSISO is an unstable mode of the summer mean flow in the monsoon region. It has a typical wavelength of 2500 km. While the easterly shear contributes to the northward propagation primarily north of 5N, the moisture feedback and the air-sea interaction also contribute significantly, particularly in the region near and south of the equator. The internal atmospheric dynamics are essential in causing the northward propagation of the BSISO over the tropical Indian Ocean.
    Jiang X. A., M. Zhao, and D. E. Waliser, 2012: Modulation of tropical cyclones over the eastern Pacific by the intraseasonal variability simulated in an AGCM. J.Climate, 25, 6524- 6538.10.1175/JCLI-D-11-00531.14d28331ca79875eb6fc7f236f643c09bhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2012JCli...25.6524Jhttp://adsabs.harvard.edu/abs/2012JCli...25.6524JNot Available
    Kim J. H., C. H. Ho, H. S. Kim, C. H. Sui, and S. K. Park, 2008: Systematic variation of summertime tropical cyclone activity in the western North Pacific in relation to the Madden-Julian oscillation. J.Climate, 21, 1171- 1191.10.1175/2007JCLI1493.164e1c0243fe1ad0e83201f312759f45fhttp%3A%2F%2Fwww.dbpia.co.kr%2FArticle%2F780513http://www.dbpia.co.kr/Article/780513A statistical analysis of TC landfalls by MJO category is applied in seven selected subareas: the Philippines, Vietnam, South China, Taiwan, East China, Korea, and Japan. While a robust and significant modulation in the number of TC landfalls is observed in south China, Korea, and Japan, the modulation is marginal in the remaining four subareas.
    Klotzbach P. J., 2014: The Madden-Julian Oscillation's impacts on worldwide tropical cyclone activity. J.Climate, 27( 6), 2317- 2330.10.1175/JCLI-D-13-00483.1f577fd614e574362f9da1ccd7fb728e0http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2014JCli...27.2317Khttp://adsabs.harvard.edu/abs/2014JCli...27.2317KNot Available
    Klotzbach, P. J. and W. M. Gray, 2008: Multidecadal variability in North Atlantic tropical cyclone activity. J.Climate, 21, 3929- 3935.10.1175/2008JCLI2162.132f7e0bd52966b86f206879793cd89f6http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2008JCli...21.3929Khttp://adsabs.harvard.edu/abs/2008JCli...21.3929KRecent increases in Atlantic basin tropical cyclone activity since 1995 and the associated destructive U.S. landfall events in 2004 and 2005 have generated considerable interest into why there has been such a sharp upturn. Natural variability, human-induced global warming, or a combination of both factors, have been suggested. Several previous studies have discussed observed multidecadal variability in the North Atlantic over 25-40-yr time scales. This study, using data from 1878 to the present, creates a metric based on far North Atlantic sea surface temperature anomalies and basinwide North Atlantic sea level pressure anomalies that shows remarkable agreement with observed multidecadal variability in both Atlantic basin tropical cyclone activity and in U.S. landfall frequency.
    Klotzbach P. J., E. C. J. Oliver, 2015: Variations in global tropical cyclone activity and the Madden-Julian oscillation since the midtwentieth Century. Geophys. Res. Lett.,42, 4199-4207, doi: 10.1002/2015GL06396.10.1002/2015GL06396637606a4bfeaedbefb0ad98d7717f44bahttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2015GL063966%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1002/2015GL063966/fullThe Madden-Julian oscillation (MJO) has been documented in previous studies to significantly impact tropical cyclone activity in all ocean basins. Most of these studies have utilized the Wheeler-Hendon index. This index is only available since 1974, the period over which remotely sensed outgoing longwave radiation data has been available. Our study utilizes a long reconstructed MJO index, based on surface pressures, which extends back to 1905. We document consistent modulation of tropical cyclone activity by the MJO in all basins over this time period. These modulations are shown to be remarkably stable over the entire analysis period. We also examine the combined impacts of El Nino-Southern Oscillation and the MJO on tropical cyclone activity in each basin over multidecadal time scales.
    Land er, M. A., 1994: An exploratory analysis of the relationship between tropical storm formation in the western North Pacific and ENSO. Mon. Wea. Rev., 122, 636- 651.10.1175/1520-0493(1994)1222.0.CO;251cf8aeb7b40aed7f6fa94a8f36df927http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1994MWRv..122..636Lhttp://adsabs.harvard.edu/abs/1994MWRv..122..636LNot Available
    Lau K.-H., N.-C. Lau, 1990: Observed structure and propagation characteristics of tropical summertime synoptic scale disturbances. Mon. Wea. Rev., 118, 1888- 1913.10.1175/1520-0493(1990)1182.0.CO;27e1bac0a83d85475ef35af3d09978797http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1990MWRv..118.1888Lhttp://adsabs.harvard.edu/abs/1990MWRv..118.1888LAbstract The three-dimensional structure and propagation characteristics of tropical synoptic scale transients during the northern summer we studied with twice daily ECMWF global gridded analyses for the 1980-1987 period. Regions of enhanced variability in relative vorticity at 850 mb are identified in the western Pacific, eastern Pacific, Bay of Bengal/northern India and eastern Atlantic/western Africa sectors. Dominant spectral peaks with time scales ranging from 3 to 8 days are noted in the power spectra for these locations. The lag-correlation and regression statistics of tropical fluctuations with synoptic time scales are examined. Strong teleconnectivity and temporal coherence are found over all of the active sites with enhanced vorticity variance, as well as over the western Atlantic/Caribbean and the Indochinese Peninsula. These results indicate that a substantial amount of synoptic scale variability in the tropics is associated with propagating wavelike disturbances that remain coherent over seve...
    Lee J.-Y., B. Wang, M. C. Wheeler, X. H. Fu, D. E. Waliser, and I.-S. Kang, 2013: Real-time multivariate indices for the boreal summer intraseasonal oscillation over the Asian summer monsoon region. Climate Dyn., 40, 493- 509.10.1007/s00382-012-1544-413e33dc2946aadad564f7cdf0db52fd8http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-012-1544-4http://link.springer.com/10.1007/s00382-012-1544-4The boreal summer intraseasonal oscillation (BSISO) of the Asian summer monsoon (ASM) is one of the most prominent sources of short-term climate variability in the global monsoon system. Compared with the related Madden-Julian Oscillation (MJO) it is more complex in nature, with prominent northward propagation and variability extending much further from the equator. In order to facilitate detection, monitoring and prediction of the BSISO we suggest two real-time indices: BSISO1 and BSISO2, based on multivariate empirical orthogonal function (MV-EOF) analysis of daily anomalies of outgoing long-wave radiation (OLR) and zonal wind at 850 hPa (U850) in the region 10°S―40°N, 40°―160°E, for the extended boreal summer (May-October) season over the 30-year period 1981-2010. BSISO1 is defined by the first two principal components (PCs) of the MV-EOF analysis, which together represent the canonical northward propagating variability that often occurs in conjunction with the eastward MJO with quasi-oscillating periods of 30-60 days. BSISO2 is defined by the third and fourth PCs, which together mainly capture the northward/northwestward propagating variability with periods of 10-30 days during primarily the pre-monsoon and monsoon-onset season. The BSISO1 circulation cells are more Rossby wave like with a northwest to southeast slope, whereas the circulation associated with BSISO2 is more elongated and front-like with a southwest to northeast slope. BSISO2 is shown to modulate the timing of the onset of Indian and South China Sea monsoons. Together, the two BSISO indices are capable of describing a large fraction of the total intraseasonal variability in the ASM region, and better represent the northward and northwestward propagation than the real-time multivariate MJO (RMM) index of Wheeler and Hendon.
    Li C. Y., Y. P. Zhou, 1995: On quasi-two-week (10-20-day) oscillation in the tropical atmosphere. Scientia Atmospherica Sinica, 19, 435- 444. (in Chinese)ccc7b5e11f360c8e540d218d1e657dd9http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-DQXK504.005.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-DQXK504.005.htmBased on the 10-20 day band-pass filtered data which are computed from ECMWF global dais (1981-1987), the 10-20 day (quasi-two-week) atmospheric oscillation in the tropics is studied systematically in this paper. The oscillation includes that in the distribution and evolution of kinetic energy,the structure and propagation feature of this kind of low-frequency and so on. It is shown that the 10-20 day osCillation is another important low-frequency system in the tropical atthosphere. Its kinetic energy is even stronger than 30-60 day oscillation. Its structllre and activity is different from the 30-60day oscillation. For instance, the 10-20 day oscillation in the tropical atmosphere is dominated by the perturbances of the zonal wavenumber 2-4. Its vertical structure mainly displays barotropic feature. It mainly propagates westwards. The meridional wind component and zonal wind component are same important. Therefore, the study focused on the 10-20 day oscillation in the tropical atmoSPhere is very necessary.
    Li R. C. Y., W. Zhou, 2013a: Modulation of Western North Pacific tropical cyclone activity by the ISO. Part I: Genesis and intensity. J.Climate, 26, 2904- 2918.10.1175/JCLI-D-12-00211.14eb5bfe78e1906c6b54dfa27a66033b2http%3A%2F%2Fwww.cell.com%2Fdevelopmental-cell%2Fabstract%2FS1534-5807%2813%2900473-5%3Fscript%3Dtruehttp://www.cell.com/developmental-cell/abstract/S1534-5807(13)00473-5?script=trueAbstract This study investigates the intraseasonal variability of tropical cyclones (TCs) by systematically examining the two major components of the intraseasonal oscillation (ISO), the 30–60-day Madden–Julian oscillation (MJO) and the 10–20-day quasi-biweekly oscillation (QBWO). Results suggest that these two ISO modes exhibit different origins, spatial scales, and propagation characteristics, which result in distinctive TC modulation in the western North Pacific Ocean (WNP). The northeastward-propagating MJO predominantly controls the basinwide TC frequency. The significant increase (reduction) in cyclogenesis in the convective (nonconvective) phase is found to be associated with the concomitant strengthening (weakening) of the monsoon trough. In addition, the large contrast in TC frequency also results in a significant difference in daily accumulated cyclone energy (ACE) between the convective and nonconvective MJO phases. The northwestward-propagating QBWO, in contrast, is characterized by alternating signals of positive and negative convection. It leads to the opposite TC modulation in the WNP1 (0°–30°N, 120°–150°E) and WNP2 (0°–30°N, 150°E–180°) regions and results in a northwestward shift in TC genesis locations, which in turn causes substantial differences in intensity distribution and daily ACE for different QBWO phases. Finally, a brief examination of the dual mode situation suggests that the QBWO generally exerts modulation upon the background MJO, and the modulation seems to vary under different MJO conditions.
    Li R. C. Y., W. Zhou, 2013b: Modulation of western North Pacific tropical cyclone activity by the ISO. Part II: Tracks and landfalls. J.Climate, 26, 2919- 2930.
    Li T., 2006: Origin of the summertime synoptic-scale wave train in the western North Pacific. J. Atmos. Sci.,63, 1093-1102, doi: 10.1175/JAS3676.1.10.1175/JAS3676.1e33aac2b21666e4eef25433ff389960ehttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2006JAtS...63.1093Lhttp://adsabs.harvard.edu/abs/2006JAtS...63.1093LThe origin of the summertime synoptic wave train in the western North Pacific is investigated with a multilevel, nonlinear baroclinic model. A realistic three-dimensional summer mean state is specified and eigenvectors are calculated by introducing small perturbation initially to the model. Numerical experiments indicate that the origin of the synoptic wave train may arise from instability of the summer mean flow in the presence of a convection frictional convergence (CFC) feedback. In the lack of the CFC feedback, the summer mean flow supports only a least damped mode, characterized by a northwest southeast-oriented wave train pattern with a zonal wavelength of 2500 km. In the presence of both the realistic summer mean flow and the CFC feedback, the model reproduces a fast growing mode, whose structure and propagation characters are similar to the observed.Sensitivity experiments with different initial perturbation patterns indicate that the model solution is not sensitive to initial conditions. Further sensitivity experiments reveal that the basic-state vertical shear may affect the growth rate and propagation character of the wave train. An easterly shear may lead to a faster growth and northwestward phase propagation, whereas a westerly shear may favor a slower growth and southeastward phase propagation.
    Li T., B. Fu, 2006: Tropical cyclogenesis associated with Rossby wave energy dispersion of a preexisting typhoon. Part I: Satellite data analyses.J. Atmos. Sci., 63, 1377- 1389.10.1175/JAS3692.11dab8ce1ddbc6cd1f6f9890be0cee0e8http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2006JAtS...63.1377Lhttp://adsabs.harvard.edu/abs/2006JAtS...63.1377LAbstract The structure and evolution characteristics of Rossby wave trains induced by tropical cyclone (TC) energy dispersion are revealed based on the Quick Scatterometer (QuikSCAT) and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) data. Among 34 cyclogenesis cases analyzed in the western North Pacific during 2000-01 typhoon seasons, six cases are associated with the Rossby wave energy dispersion of a preexisting TC. The wave trains are oriented in a northwest-outheast direction, with alternating cyclonic and anticyclonic vorticity circulation. A typical wavelength of the wave train is about 2500 km. The TC genesis is observed in the cyclonic circulation region of the wave train, possibly through a scale contraction process. The satellite data analyses reveal that not all TCs have a Rossby wave train in their wakes. The occurrence of the Rossby wave train depends to a certain extent on the TC intensity and the background flow. Whether or not a Rossby wave train can finally lead to cyclogenesis depends on large-scale dynamic and thermodynamic conditions related to both the change of the seasonal mean state and the phase of the tropical intraseasonal oscillation. Stronger low-level convergence and cyclonic vorticity, weaker vertical shear, and greater midtropospheric moisture are among the favorable large-scale conditions. The rebuilding process of a conditional unstable stratification is important in regulating the frequency of TC genesis.
    Li T., X. Y. Ge, B. Wang, and Y. T. Zhu, 2006: Tropical cyclogenesis associated with Rossby wave energy dispersion of a preexisting typhoon. Part II: Numerical simulations. J. Atmos. Sci., 63, 1390- 1409.10.1175/JAS3692.1664e9d6c8b8f887f5e91c97ff3cd435fhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2006JAtS...63.1390Lhttp://adsabs.harvard.edu/abs/2006JAtS...63.1390LSensitivity experiments with both a single mesh (with a 15-km resolution) and a nested mesh (with a 5-km resolution in the inner mesh) indicate that TC energy dispersion alone in a resting environment does not lead to cyclogenesis, suggesting the important role of the wave train–mean flow interaction. A proper initial condition for background wind and moisture fields is crucial for maintaining a continuous vorticity growth through the multioscillatory phases.
    Liebmann B., H. H. Hendon, and J. D. Glick, 1994: The relationship between tropical cyclones of the western Pacific and Indian Oceans and the Madden-Julian Oscillation. J. Meteor. Soc.Japan, 72, 401- 412.10.1063/1.532936febacbd4320d54c7dbd68efa01cf34a3http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F110001807345http://ci.nii.ac.jp/naid/110001807345The special quasiperiodic solution of the (2+1) -dimensional Kadometsev–Petviashvili equation is separated into three systems of ordinary differential equations, which are the second, third, and fourth members in the well-known confocal involutive hierarchy associated with the nonlinearized Zakharov–Shabat eigenvalue problem. The explicit theta function solution of the Kadometsev–Petviashvili equation is obtained with the help of this separation technique. A generating function approach is introduced to prove the involutivity and the functional independence of the conserved integrals which are essential for the Liouville integrability.
    Liu K. S., J. C. L. Chan, 2008: Interdecadal variability of western North Pacific tropical cyclone tracks. J.Climate, 21, 4464- 4476.fc1df8275b85bd7cfec41adfaa55cbc7http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2008JCli...21.4464L%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D545ac9cfd425044http://xueshu.baidu.com/s?wd=paperuri%3A%2848653f90585ce0a0a89c019e012ad3bc%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2008JCli...21.4464L%26db_key%3DPHY%26link_type%3DABSTRACT%26high%3D545ac9cfd425044&ie=utf-8&sc_us=9140394844520107100
    Madden R. A., P. R. Julian, 1971: Detection of a 40-50 day oscillation in the zonal wind in the tropical Pacific. J. Atmos. Sci., 28, 702- 708.a33db74c6b65a47f5bd8c4d77c78e938http%3A%2F%2Fwww.scielosp.org%2Fscielo.php%3Fpid%3DS0102-311X2008000800009%26script%3Dsci_arttexthttp://www.scielosp.org/scielo.php?pid=S0102-311X2008000800009&amp;script=sci_arttext
    McBride J. L., 1981: Observational analysis of tropical cyclone formation. Part III: Budget analysis. J. Atmos. Sci., 38, 1152- 1166.10.1175/1520-0469(1981)0382.0.CO;2865debdb-44c5-45fd-84cf-2f74a54b25bd8f560b2819997f4835b2abf4e965215ehttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1979PhDT.......165Mrefpaperuri:(dc8ac8723ded80174fe939756e62f98d)http://adsabs.harvard.edu/abs/1979PhDT.......165MAbstract Twelve composite data sets are constructed from rawinsonde data in the tropical northwest Pacific and tropical northwest Atlantic Oceans. Each data set is a composite average of approximately 80 individual disturbances. Four different types of non-developing oceanic tropical disturbance are composited. For comparison, disturbances in each ocean are composited at four different stages of intensification from pre-hurricane to hurricane and pre-typhoon to typhoon. In total, 912 different tropical weather systems go into the composites and approximately 40 000 rawinsonde observations are used. Details are presented on data density, number of individual weather systems averaged and mean position for each composite system. The basic thermodynamic and dynamic properties of the systems are discussed as well as regional differences between the Pacific and the Atlantic. The analyses presented here form a framework for Part II and subsequent papers which use these composite data sets to investigate the gene...
    McBride J. L., R. Zehr, 1981: Observational analysis of tropical cyclone formation. Part II: Comparison of non-developing versus developing systems. J. Atmos. Sci., 38( 6), 1132- 1151.10.1175/1520-0469(1981)0382.0.CO;2a6327c54d1bba8d5854aaadb86d2b856http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1981JAtS...38.1132Mhttp://adsabs.harvard.edu/abs/1981JAtS...38.1132MAbstract The thermodynamic and dynamic fields surrounding the composite tropical weather systems described in Part I (McBride, 1981a) are examined for differences between non-developing and developing systems. The main findings are as follows: (i) Both non-developing and developing systems are warm core in the upper levels. The temperature (and height) gradients are more pronounced in the developing system, but the magnitudes are so small that the differences would be difficult to measure for individual systems. (ii) The developing or pre-typhoon cloud cluster exists in a warmer atmosphere over a large horizontal scale, for example, out to 8 latitude radius in all directions. (iii) There is no obvious difference in vertical stability for moist convection between the systems. (iv) There is no obvious difference in moisture content or moisture gradient. (v) Pre-typhoon and pre-hurricane systems are located in large areas of high values of low-level relative vorticity. The low-level vorticity in the vicinit...
    Nakazawa T., 1986: Intraseasonal variations of OLR in the tropics during the FGGE year. J. Meteor. Soc.Japan, 64, 17- 34.29672e00fe351f11aa10579abbb8842dhttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F40000634345http://ci.nii.ac.jp/naid/40000634345
    Nakazawa T., 1988: Tropical super clusters within intraseasonal variations over the Western Pacific. J. Meteor. Soc.Japan, 66, 823- 839.
    Onogi, K., Coauthors, 2007: The JRA-25 reanalysis. J. Meteor. Soc.Japan, 85, 369- 432.10.2151/jmsj.85.369dbb54365-2f33-4085-8b92-f196a4906bf509e7329c86a48449c81f63085ae7280ehttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F110006318054refpaperuri:(d414385782241d61675aa2139a5f686e)http://ci.nii.ac.jp/naid/110006318054A long-term global atmospheric reanalysis, named "Japanese 25-year Reanalysis (JRA-25)" was completed using the Japan Meteorological Agency (JMA) numerical assimilation and forecast system. The analysis covers the period from 1979 to 2004. This is the first long-term reanalysis undertaken in Asia. JMA's latest numerical assimilation system, and specially collected observational data, were used to generate a consistent and high-quality reanalysis dataset designed for climate research and operational monitoring and forecasts. One of the many purposes of JRA-25 is to enhance the analysis to a high quality in the Asian region. Six-hourly data assimilation cycles were performed, producing 6-hourly atmospheric analysis and forecast fields of various physical variables. The global model used in JRA-25 has a spectral resolution of T106 (equivalent to a horizontal grid size of around 120 km) and 40 vertical layers with the top level at 0.4 hPa. In addition to conventional surface and upper air observations, atmospheric motion vector (AMV) wind retrieved from geostationary satellites, brightness temperature from TIROS Operational Vertical Sounder (TOYS), precipitable water retrieved from orbital satellite microwave radiometer radiance and other satellite data are assimilated with three-dimensional variational method (3D-Var). JMA produced daily sea surface temperature (SST), sea ice and three-dimensional ozone profiles for JRA-25. A new quality control method for TOYS data was developed and applied in advance. Many advantages have been found in the JRA-25 reanalysis. Predicted 6-hour global total precipitation distribution and amount are well reproduced both in space and time. The performance of the long time series of the global precipitation is the best among the other reanalyses, with few unrealistic variations from degraded satellite data contaminated by volcanic eruptions. Secondly, JRA-25 is the first re-analysis to assimilate wind profiles around tropical cyclones reconstructed from historical best track in-formation; tropical cyclones were analyzed properly in all the global regions. Additionally, low-level cloud along the subtropical western coast of continents is well simulated and snow depth analysis is also of a good quality. The article also covers material which requires attention when using JRA-25.
    Reed R. J., E. E. Recker, 1971: Structure and properties of synoptic-scale wave disturbances in the equatorial western Pacific. J. Atmos. Sci., 28, 1117- 1133.10.1175/1520-0469(1971)0282.0.CO;21d0c1f0a06f32f1055a2c075b515d085http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1971jats...28.1117rhttp://adsabs.harvard.edu/abs/1971jats...28.1117rAbstract A compositing technique is used to obtain the average structure of 18 disturbances which traversed an area in the equatorial western Pacific during the wet season (July–September) of 1967. Principal emphasis is placed on the wave properties in the triangular area described by Ponape, Kwajalein and Eniwetok within which it was possible to measure divergence and vertical motion and to compute moisture and heat budgets. Meridional wind maxima of nearly opposite phase occurred in the lower and upper troposphere. Negative temperature deviations were found in the vicinity of the wave trough at low and high levels; positive deviations were observed at intermediate levels. Highest relative humidities occurred in the trough region. This was also the region of strongest upward motion and greatest rainfall and cloud amount. The maximum upward velocity of 2.5 cm sec611 was found at 300 mb. Convergence was strongest in the sub-cloud layer; divergence was concentrated near 175 mb. The maximum anticyclonic vorti...
    Reed R. J., D. C. Norquist, and E. E. Recker, 1977: The structure and properties of African wave disturbances as observed during phase III of GATE. Mon. Wea. Rev., 105, 317- 333.10.1175/1520-0493(1977)1052.0.CO;2da7ee0105f91063b500f2215e273c0b3http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1977MWRv..105..317Rhttp://adsabs.harvard.edu/abs/1977MWRv..105..317RAbstract A compositing method is used to determine the average structure and properties of eight wave disturbances observed over west Africa and the eastern Atlantic during the period 23 August-19 September, 1974, a period marked by well-developed and regular wave activity. The disturbance centers propagated westward in the zone of cyclonic shear to the south of the 700 mb easterly jet, located at 16–17°N. The mean wave- length was about 25M km and the mean period 3.5 days. The mean zonal current satisfied the necessary condition for barotropic instability. The composite disturbance was most intense at 650 mb, being cold core below and warm core above. Two circulation centers were evident at the surface, one located below the upper center and the other displaced 10° to the north at about the latitude of the monsoon trough. When separate composites were constructed for land and ocean stations, the dual centers were found to be primarily a land phenomenon. Distinctive features of the high-level (200 inb) ci...
    Ritchie E. A., G. J. Holland, 1999: Large-scale patterns associated with tropical cyclogenesis in the western Pacific. Mon. Wea. Rev., 127, 2027- 2043.10.1175/1520-0493(1999)1272.0.CO;27265faeae48e51584694ce58556a4a97http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1999MWRv..127.2027Rhttp://adsabs.harvard.edu/abs/1999MWRv..127.2027RAbstract Five characteristic, low-level, large-scale dynamical patterns associated with tropical cyclogenesis in the western North Pacific basin are examined along with their capacity to generate the type of mesoscale convective systems that precede genesis. An 8-yr analysis set for the region is used to identify, and create composites for, the five characteristic patterns of monsoon shear line, monsoon confluence region, monsoon gyre, easterly waves, and Rossby energy dispersion. This brings out the common processes that contribute to tropical cyclogenesis within that pattern, which are described in detail. A 3-yr set of satellite data is then used to analyze the mesoscale convective system activity for all cases of genesis in that period and to stratify based on the above large-scale patterns. It is found that mesoscale convective systems develop in all cases of genesis except one. Seventy percent of cases developed mesoscale convective systems at more than one time during the genesis period and 44% of ...
    Sobel A. H., C. S. Bretherton, 1999: Development of synoptic-scale disturbances over the summertime tropical northwest Pacific. J. Atmos. Sci., 56, 3106- 3127.10.1175/1520-0469(1999)0562.0.CO;2bc3175d80b4f4597e001d19b129dad04http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1999JAtS...56.3106Shttp://adsabs.harvard.edu/abs/1999JAtS...56.3106SThis study addresses the origin of the synoptic-scale disturbances that occur in the tropical western North Pacific ocean (WP) region in Northern Hemisphere summer. These have been called “easterly waves” and“tropical depression–type” (TD) disturbances. This analysis uses the National Center for Environmental Prediction–National Center for Atmospheric Research reanalysis dataset. By performing a regression analysis on several terms in the vorticity equation at 850 hPa, it is shown that the TD disturbances propagate approximately as barotropic Rossby waves at 850 hPa. Given this, ray-tracing calculations and the wave activity diagnostic introduced by Plumb are used to show that wave accumulation is a promising candidate for the initial development mechanism of the TD disturbances. The expected local “growth rate” from this mechanism is simply the convergence of the group velocity, which reaches values corresponding to a growth timescale of 3 days. This convergence is dominated by, but somewhat larger than, the convergence in the time-mean flow. The wave accumulation mechanism can operate either on waves coming from outside the WP region or on those generated in situ; in particular, mature tropical cyclones are probably a climatologically important source of waves. While the results presented here provide no direct information on the nature of the feedbacks between diabatic processes and large-scale wave dynamics, they do indicate that no instability mechanism involving any diabatic process need be invoked to explain the initial development of TD disturbances. It is possible, rather, that diabatic processes do not provide a positive feedback until the disturbances reach finite amplitude, whether at the stage of true tropical cyclogenesis or some prior intermediate stage.
    Takayabu N. Y., T. Nitta, 1993: 3-5 day-period disturbances coupled with convection over the tropical Pacific Ocean, J. Meteor. Soc.Japan, 71, 221- 246.41e1e8c6423b224aba9285a25421aa54http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10003552502http://ci.nii.ac.jp/naid/100035525023-5 day-period disturbances coupled with convection over the tropical Pacific ocean TAKAYABU Y. N. J. Met. Soc. Japan 71, 221-246, 1993
    Wang B., X. H. Xu, 1997: Northern hemisphere summer monsoon singularities and climatological intraseasonal oscillation. J.Climate, 10( 5), 1071- 1085.10.1175/1520-0442(1997)0102.0.CO;29cfd137d08ddf5caf4703a3d9a42118ehttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1997JCli...10.1071Whttp://adsabs.harvard.edu/abs/1997JCli...10.1071WUsing climatological pentad mean outgoing longwave radiation (OLR) and European Centre for Medium-Range Weather Forecasts analysis winds, the authors show that the Northern Hemisphere summer monsoon displays statistically significant climatological intraseasonal oscillations (CISOs). The extreme phases of CISO characterize monsoon singularities-monsoon events that occur on a fixed pentad with usual regularity, whereas the transitional phases of CISO represent the largest year-to-year monsoon variations. The CISO results from a phase-locking of transient intraseasonal oscillation to annual cycle. It exhibits a dynamically coherent structure between enhanced convection and low-level convergent (upper-level divergent) cyclonic (anticyclonic) circulation. Its phase propagates primarily northward from the equator to the northern Philippines during early summer (May-July), and westward along 15N from 170E to the Bay of Bengal during August and September. The propagation of CISO links monsoon singularities occurring in different regions. Four CISO cycles are identified from May to October. The first cycle has a peak wet phase in mid-May that starts the monsoon over the South China Sea and Philippines. Its dry phase in late May and early June brings the premonsoon dry weather over the regions of western North Pacific summer monsoon (WNPSM), Meiyu/Baiu, and Indian summer monsoon (ISM). The wet phase of Cycle II peaking in mid-June marks the onsets of WNPSM, continental ISM, and Meiyu, whereas the dry phase in early to mid-July corresponds to the first major breaks in WNPSM and ISM, and the end of Meiyu. The wet phase of Cycle III peaking in mid-August benchmarks the height of WNPSM, which was followed by a conspicuous dry phase propagating westward and causing the second breaks of WNPSM (in early September) and ISM (in mid-September). The wet phase of Cycle IV represents the last active WNPSM and withdrawal of ISM in mid-October. The relationships among ISM, WNPSM, and East Asian Subtropical Monsoon (EASM) are season dependent. During Cycle II, convective activities in the three monsoon regions are nearly in phase. During Cycle III, however, the convective activities are out of phase between ISM and WNPSM; meanwhile, little linkage exists between WNPSM and EASM. The causes of unstable relationships and the phase propagation of CISO are discussed.
    Wang B., J. C. L. Chan, 2002: How strong ENSO events affect tropical storm activity over the western North Pacific. J.Climate, 15, 1643- 1658.10.1175/1520-0442(2002)0152.0.CO;2248bb77da7a55c482abfa57c1313880ehttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2002JCli...15.1643Whttp://adsabs.harvard.edu/abs/2002JCli...15.1643WAn analysis of 35-yr (1965-99) data reveals vital impacts of strong (but not moderate) El Ni09o and La Ni09a events on tropical storm (TS) activity over the western North Pacific (WNP). Although the total number of TSs formed in the entire WNP does not vary significantly from year to year, during El Ni09o summer and fall, the frequency of TS formation increases remarkably in the southeast quadrant (0°-17°N, 140°E-180°) and decreases in the northwest quadrant (17°-30°N, 120°-140°E). The July-September mean location of TS formation is 6° latitude lower, while that in October-December is 18° longitude eastward in the strong warm versus strong cold years. After the El Ni09o (La Ni09a), the early season (January-July) TS formation in the entire WNP is suppressed (enhanced). In strong warm (cold) years, the mean TS life span is about 7 (4) days, and the mean number of days of TS occurrence is 159 (84) days. During the fall of strong warm years, the number of TSs, which recurve northward across 35°N, is 2.5 times more than during strong cold years. This implies that El Ni09o substantially enhances poleward transport of heat-moisture and impacts high latitudes through changing TS formation and tracks.The enhanced TS formation in the SE quadrant is attributed to the increase of the low-level shear vorticity generated by El Ni09o-induced equatorial westerlies, while the suppressed TS generation over the NW quadrant is ascribed to upper-level convergence induced by the deepening of the east Asian trough and strengthening of the WNP subtropical high, both resulting from El Ni09o forcing. The WNP TS activities in July-December are noticeably predictable using preceding winter-spring Ni09o-3.4 SST anomalies, while the TS formation in March-July is exceedingly predictable using preceding October-December Ni09o-3.4 SST anomalies. The physical basis for the former is the phase lock of ENSO evolution to the annual cycle, while for the latter it is the persistence of Philippine Sea wind anomalies that are excited by ENSO forcing but maintained by local atmosphere-ocean interaction.
    Wang B., X. Zhou, 2008: Climate variation and prediction of rapid intensification in tropical cyclones in the western North Pacific, Meteor. Atmos. Phys., 99, 1-16, doi: 10.1007/s00703-006-0238-z.10.1007/s00703-006-0238-z3a8bdfab2c0a6d8f5754d422e983233dhttp%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00703-006-0238-zhttp://link.springer.com/article/10.1007/s00703-006-0238-zThe RI is an essential characteristic of category 4 and 5 hurricanes and super typhoons because all category 4 and 5 hurricanes in the Atlantic basin and 90% of the super typhoons in the western North Pacific experience at least one RI process in their life cycles. Over the past 40 years, the annual total of RI in the western North Pacific shows pronounced interdecadal variation but no significant trend. This result suggests that the number of supper typhoons has no upward trend in the past 40 years. Our results also suggest that when the mean latitude, where the tropical storms form, shifts southward (either seasonally or from year to year) the proportion of super typhoon or major hurricane will likely increase. This shift is determined by large scale circulation change rather than local SST effects. This idea differs from the current notion that increasing SST can lead to more frequent occurrence of category 4 or 5 hurricanes through local thermodynamics.
    Wang L., G. H. Chen, and R. H. Huang, 2009: The modulation of quasi-biweekly oscillation on tropical cyclone activity over the western North Pacific. Chinese Journal of Atmospheric Sciences, 33, 416- 424. (in Chinese)10.1016/S1003-6326(09)60084-46f123a5c901e34bdc56d6e9bed142a8chttp%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-DQXK200903001.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-DQXK200903001.htmThis paper explores the modulation of quasi-biweekly oscillation(QBWO) on tropical cyclogenesis over the western North Pacific(WNP),using the Japanese reanalysis(JRA) daily data,NOAA/NCEP data and tropical cyclones(TCs) dataset obtained from the Joint Typhoon Warming Center.Through the classification of four phases,the composites indicate that the zonal wind and convective activity on 10-20-day scale propagate northwestward.During phases 1 and 4,the number and the occurrence probability of the TCs are lower.In contrast,the number of the TCs is relatively large and the occurrence probability of the TCs is higher during phases 2 and 3.Moreover,during phase 3,the number of the TCs landing in China is largest.From phases 1 to 4,the location of monsoon trough shifts westward over the WNP.The westward-propagating synoptic-scale disturbances due to the convergence of zonal wind on the eastern flank of the monsoon trough over the WNP are prone to translating to TD-type(tropical depression-type) disturbances,which is favorable for developing into TC by means of kinetic energy conversion from basic state in the monsoon trough.
    Wheeler M. C., H. H. Hendon, 2004: An all-season real-time multivariate MJO index: Development of an index for monitoring and prediction. Mon. Wea. Rev., 132, 1917- 1932.10.1175/1520-0493(2004)1322.0.CO;2ff3949520e2db3649691459062f6df4dhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2004mwrv..132.1917whttp://adsabs.harvard.edu/abs/2004mwrv..132.1917wAbstract A seasonally independent index for monitoring the Madden-ulian oscillation (MJO) is described. It is based on a pair of empirical orthogonal functions (EOFs) of the combined fields of near-equatorially averaged 850-hPa zonal wind, 200-hPa zonal wind, and satellite-observed outgoing longwave radiation (OLR) data. Projection of the daily observed data onto the multiple-variable EOFs, with the annual cycle and components of interannual variability removed, yields principal component (PC) time series that vary mostly on the intraseasonal time scale of the MJO only. This projection thus serves as an effective filter for the MJO without the need for conventional time filtering, making the PC time series an effective index for real-time use. The pair of PC time series that form the index are called the Real-time Multivariate MJO series 1 (RMM1) and 2 (RMM2). The properties of the RMM series and the spatial patterns of atmospheric variability they capture are explored. Despite the fact that RMM1 and RMM...
    Wu L., J. Liang, and C.-C. Wu, 2011: Monsoonal Influence on Typhoon Morakot (2009). Part I: Observational Analysis. J. Atmos. Sci., 68, 2208- 2221.10.1175/2011JAS3730.10db2c50e2ed7226b3123cb576d113467http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2011JAtS...68.2208Whttp://adsabs.harvard.edu/abs/2011JAtS...68.2208WTyphoon Morakot made landfall on Taiwan with a record rainfall of 3031.5 mm during 6-13 August 2009. While previous studies have emphasized the influence of southwesterly winds associated with intraseasonal oscillations and monsoon surges on moisture supply, the interaction between Morakot and low-frequency monsoon flows and the resulting influence on the slow movement and asymmetric precipitation structure of the typhoon were examined observationally. Embedded in multi-time-scale monsoonal flows. Morakot generally moved westward prior to its landfall on Taiwan and underwent a coalescence process first with a cyclonic gyre on the quasi-biweekly oscillation time scale and then with a cyclonic gyre on the Madden-Julian oscillation time scale. The coalescence enhanced the synoptic-scale southwesterly winds of Morakot and thus decreased its westward movement and turned the track northward, leading to an unusually long residence time in the vicinity of Taiwan. The resulting slow movement and collocation with the low-frequency gyres also maintained the major rainfall in southern Taiwan because the low-frequency flows played an important role in enhancing the winds on the southern side, especially during 6-9 August 2009. In addition to the lifting effect of the Taiwan terrain and the moisture supply associated with monsoon flows, the study suggests that the monsoonal influence maintained the major rainfall area in southern Taiwan through reducing the translation speed, shifting Morakot northward, and enhancing the low-frequency flows on the southern side of the typhoon. Since the enhanced low-frequency flows did not shift northward with the movement of Morakot, its primary rainfall expanded outward with time as the typhoon center moved northwestward after its landfall on Taiwan.
    Wu L., J. Duan, 2015: Extended simulation of tropical cyclone formation in the western North Pacific monsoon trough. J. Atmos. Sci., 72, 4469- 4485.10.1175/JAS-D-14-0375.1fde9587710ea1d84c4464360573ebaf7http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015JAtS...72.4469Whttp://adsabs.harvard.edu/abs/2015JAtS...72.4469WAbstractPrevious studies suggest that the low-frequency background makes an important contribution to the predictability of tropical cyclone (TC) activity on the intraseasonal time scale by providing large-scale conditions favorable for TC formation. Extended numerical experiments were conducted to demonstrate additional low-frequency influence on TC activity, which results from the development of a synoptic-scale wave train. The cyclonic circulation of the wave train provides low-level synoptic-scale disturbances for TC formation.The observed TC formation events over the western North Pacific during 14 August to 10 September 2004 were first successfully simulated with the initial and lateral conditions derived from the National Centers for Environmental Prediction (NCEP) Final (FNL) Operational Global Analysis. Then the 27-day extended experiment was repeated only with the initial and lateral boundary conditions derived from the FNL low-frequency (longer than 20 days) background. It is found that the dev...
    Yasunari T., 1979: Cloudiness fluctuations associated with the northern hemisphere summer monsoon. J. Meteor. Soc.Japan, 57, 227- 242.1ed18e95e9491edb8fc3e6e9a31cde65http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F266375479_Cloudiness_fluctuations_associated_with_the_northern_hemisphere_summer_monsoonhttp://www.researchgate.net/publication/266375479_Cloudiness_fluctuations_associated_with_the_northern_hemisphere_summer_monsoonABSTRACT The broad-scale fluctuations of cloudiness over the Eastern Hemisphere during the northern summer monsoon were investigated by using daily satellite mosaic pictures taken from June 1 to September 30, 1973. Spectral analysis revealed two dominant periodicities, of around 40 days and around 15 days. Cross-spectral, time-sectional, time-lag correlation and phase-lag vector analysis were applied to reveal the characteristics of these two modes in the time-space field. The fluctuation of 40-day period shows marked northward movement of cloudiness from the equatorial zone to the mid-latitudes (around 30 * E) over the whole Asian monsoon area, and southward movement over Africa and the central Pacific. The northward move-ment is most apparent over the India-Indian Ocean sector. The fluctuation of this mode is associated with the major "active"-"break" cycle of the monsoon over the whole Asian monsoon area. The fluctuation of 15-day period shows similar features to that of 40-day period, but includes two clockwise rotations, one over India and Southeast Asia and the other over the western Pacific. A southward movement from the equatorial zone to the Southern Hemisphere middle latitudes is also prominent to the east and west of Australia. The fluctuation of this mode seems to correspond with the movements of equatorial, monsoon (or tropical), and westerly disturbances. It is also suggested that the fluctuation of 40-day period may be closely connected with the global-scale zonal oscillation in the equatorial zone and that of 15-day period may exist as a result of meridional wave interactions.
    Yoshida R., H. Ishikawa, 2013: Environmental factors contributing to tropical cyclone genesis over the western North Pacific. Mon. Wea. Rev., 141, 451- 467.10.1175/MWR-D-11-00309.1ace7580b4017190c6a06eaad6667bf15http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2013MWRv..141..451Yhttp://adsabs.harvard.edu/abs/2013MWRv..141..451YNot Available
    Yoshida R., Y. Kajikawa, and H. Ishikawa, 2014: Impact of boreal summer intraseasonal oscillation on environment of tropical cyclone genesis over the Western North Pacific. SOLA, 10, 15- 18.10.2151/sola.2014-004f31fcf92b84e87013e1452d55f666779http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F130004448381http://ci.nii.ac.jp/naid/130004448381ABSTRACT The impact of intraseasonal oscillation on tropical cyclone genesis (TCG) over the western North Pacific during boreal summer was examined in association with categorized synoptic-scale low-level flow patterns. Five synoptic-scale flow patterns were considered as synoptic-scale environment connecting the large-scale intraseasonal oscillation and meso-scale TCG: a shear line, a confluence region, an easterly wave, a monsoon gyre, and a pre-existing tropical cyclone. The phase and amplitude of the boreal summer intraseasonal oscillation (BSISO) were used. It is found that the phase 7 was favorable for TCG associated with the shear line, and the phases 7 and 8 were also favorable for TCG associated with the confluence region. During the favorable phases for each flow pattern associated with TCG, the BSISO affected the large-scale zonal wind distribution, and the large-scale zonal wind forms synoptic-scale flow patterns over the suitable environment for TCG. The link between the intraseasonal oscillation and synoptic-scale flow patterns is an important factor in TCG.
    Zhang Q., Q. Liu, and L. Q. Wu, 2009: Tropical cyclone damages in China 1983-2006. Bull. Amer. Meteor. Soc., 90, 489- 495.10.1175/2008BAMS2631.12a4db01a52380d08715ffaa06781a7e4http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20093158281.htmlhttp://www.cabdirect.org/abstracts/20093158281.htmlBased on damage records released by the Department of Civil Affairs of China, direct economic losses and casualties associated with tropical cyclones that made landfall over China during 1983-2006 are examined. In an average year, landfalling tropical cyclones cause 472 deaths and 28.7 billion yuans (2006 RMB) in direct economic losses, accounting for 0.38% of the annual total gross domestic pr...
    Zhao H. K., L. G. Wu, 2014: Inter-decadal shift of the prevailing tropical cyclone tracks over the western North Pacific and its mechanism study. Meteor. Atmos. Phys., 125, 89- 101.10.1007/s00703-014-0322-8ec5b2727e7cdbc11d8dc8a1e3b2f7593http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00703-014-0322-8http://link.springer.com/article/10.1007/s00703-014-0322-8On the basis of observations and results from the combination of a statistical formation model and a trajectory model, the inter-decadal shift of prevailing TC tracks in the western North Pacific (WNP) are examined. The contributions of the changes in large-scale steering flows and tropical cyclone (TC) formation locations to the observed inter-decadal shift are investigated and their relative importance is determined. This study focuses on two periods, 1965-1986 (ID1) and 1987-2010 (ID2), which are determined based on the abrupt change of the annual category 4 and 5 TC frequency derived from the Bayesian change-point detection analysis. It is found that the models can well simulate the primary features of prevailing TC tracks on the inter-decadal timescale. From ID1 to ID2, a significant decrease in the frequency of TC occurrences is observed over the central South China Sea and well simulated by the models. Areas with a remarkable increase in the TC frequency, which extends from the Philippine Sea to the eastern coast of China and in the west of the WNP basin, are also reasonably simulated. Above changes in the prevailing TC tracks are attributed to (1) intensified cyclonic circulation centered over the western part of China and (2) more westward-southward expansion and intensification of the subtropical high over the WNP. Further analysis reveals that the inter-decadal shift in prevailing TC tracks is mainly resulted from the combined effects of changes in large-scale steering flows and TC formation locations. Although both contribute to the inter-decadal shift in the prevailing TC tracks, changes in large-scale steering flows play a more important role compared to changes in TC formation locations.
    Zhao H. K., C. Z. Wang, 2015: Interdecadal modulation on the relationship between ENSO and typhoon activity during the late season in the Western North Pacific. Climate Dyn., doi: 10.1007/s00382-015-2837-1.10.1007/s00382-015-2837-145b88c18f5ba07b5d232ec3c80fef36fhttp%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-015-2837-1http://link.springer.com/10.1007/s00382-015-2837-1The present study identifies an interdecadal modulation of the Pacific decadal oscillation (PDO) on the relationship between El Nino-Southern oscillation (ENSO) and typhoon activity during the late season (October-December) in the western North Pacific. The PDO is uncorrelated with ENSO during the warm phase of 1979-1997, while the PDO is positively correlated with ENSO during the cold phase of 1998-2012. Further analyses show that the warm phase is associated with the reduced ENSO-typhoon activity relationship and more typhoons, whereas the cold phase is corresponded to the enhanced ENSO-typhoon activity relationship and fewer typhoons. These variations are mainly manifested by a significant difference of typhoon activity in the southeastern part of the western North Pacific. Moreover, the change of ENSO-typhoon relationship is largely due to changes in large-scale environmental conditions especially from low-level vorticity and vertical wind shear between the two phases, which are related to the changes in tropical Indo-Pacific sea surface temperature. The study implies that the phase of the PDO should be taken into account when ENSO is used as a predictor for predicting typhoon activity in the western North Pacific.
    Zhao H. K., L. G. Wu, and W. C. Zhou, 2010: Assessing the influence of the ENSO on tropical cyclone prevailing tracks in the western North Pacific. Adv. Atmos. Sci.,27(6), 1361-1371, doi: 10.1007/s00376-010-9161-9.10.1007/s00376-010-9161-94de0053304f5dc937f96a235425bc874http%3A%2F%2Fwww.ams.org%2Fmathscinet-getitem%3Fmr%3D2243122http://d.wanfangdata.com.cn/Periodical_dqkxjz-e201006012.aspxUsing a statistical model for simulating tropical cyclone (TC) formation and a trajectory model for simulating TC tracks, the influence of the El Nino-Southern Oscillation (ENSO) on the peak-season (July-September) TC prevailing tracks in the western North Pacific basin is assessed based on 14 selected El Nino and 14 selected La Nina years during the period 1950-2007. It is found that the combination of statistical formation model and a trajectory model can simulate well the primary features of TC prevailing tracks on the interannual timescale. In the El Nino years, the significant enhancement of TC activity primarily occurs south of 20°N, especially east of 130°E. TCs that take the northwestward prevailing track and affect East Asia, including Taiwan Island, the Chinese mainland, Korea, and Japan, tend to move more westward in the El Nino years, while taking a more northward track in the La Nina years. Numerical simulations confirm that the ENSO-related changes in large-scale steering flows and TC formation locations can have a considerable influence on TC prevailing tracks.
    Zhao H. K., L. G. Wu, and W. C. Zhou, 2011: Interannual changes of tropical cyclone intensity in the western north Pacific. J. Meteor. Soc. Japan,89(3), 243-253, doi: 10.2151/jmsj.2011-305.10.2151/jmsj.2011-305c925b66825ff413aac105baebab1e96fhttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F40018855001http://ci.nii.ac.jp/naid/40018855001The individual contributions of changes in sea surface temperature(SST),vertical wind shear and tropical cyclone(TC) tracks to the interannual TC intensity change in the western North Pacific(WNP) basin are examined based on the selected 7 warm years and 7 cold years during the period 1970-2007.The selected warm and cold years are defined by the Nino-3.4 SST anomalies index,corresponding to the El Ni o and La Ni a events,respectively.The intensity model used in this study can simulate the spatial distribution and differences of TC intensity when the model is integrated along the observed TC tracks in the warm and cold years.It is found that the change of TC tracks plays a dominant role in the observed TC intensity difference between the warm and cold years.During the warm years,TC formation is enhanced in the southeast quadrant,and more TCs take a northwestward track during the warm years than during the cold years because of the interannual change in the large-scalesteering flows.As a result,more TCs have longer time for intensification and develop into intense TCs during the warm years than during the cold years.
    Zhao H. K., L. G. Wu, and R. F. Wang, 2014: Decadal variations of intense tropical cyclones over the western North Pacific during 1948-2010. Adv. Atmos. Sci.,31(2), 57-65, doi: 10.1007/s00376-013-3011-5.10.1007/s00376-013-3011-59e8dfaea731895442aacbf74c9e6b7e0http%3A%2F%2Fwww.cnki.com.cn%2FArticle%2FCJFDTotal-DQJZ201401007.htmhttp://d.wanfangdata.com.cn/Periodical_dqkxjz-e201401007.aspxUsing Joint Warning Typhoon Center(JTWC) best track data during the period 1948–2010, decadal and interdecadal changes of annual category 4 and 5 tropical cyclone(TC) frequency in the western North Pacific basin were examined. By allowing all of the observed TCs in the JTWC dataset to move along the observed TC tracks in a TC intensity model, the annual category 4 and 5 TC frequency was simulated. The results agreed well with observations when the TC intensity prior to 1973 was adjusted based on time-dependent biases due to changes in measurement and reporting practices. The simulated and adjusted time series showed significant decadal(12–18 years) variability, while the interdecadal(18–32 years) variability was found to be statistically insignificant. Numerical simulations indicated that changes in TC tracks are the most important factor for the decadal variability in the category 4 and 5 TC frequency in the western North Pacific basin, while a combined effect of changes in SST and vertical wind shear also contributes to the decadal variability. Further analysis suggested that the active phase of category 4 and 5 TCs is closely associated with an eastward shift in the TC formation locations, which allows more TCs to follow a longer journey, favoring the development of category 4 and 5 TCs. The active phase corresponds with the SST warming over the tropical central and eastern Pacific and the eastward extension of the monsoon trough, thus leading to the eastward shift in TC formation locations.
    Zhao H. K., X. A. Jiang, and L. G. Wu, 2015a: Modulation of Northwest Pacific tropical cyclone genesis by the intraseasonal variability. J. Meteor. Soc. Japan,93(2), 81-97, doi: 10.2151/jmsj.2015-006.10.2151/jmsj.2015-006c97a9e2ebc7615c4c27e5d3c56b3f162http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F130004788806http://ci.nii.ac.jp/naid/130004788806The modulation of tropical cyclone (TC) genesis over the western North Pacific (WNP) by the intraseasonal variability (ISV) is investigated in this study. The two leading ISV modes, i.e., the 40-day Madden-Julian oscillation (MJO) and the 16-day quasi-biweekly oscillation, are found to exert significant impacts on TC genesis over the WNP. A majority of TC geneses over the WNP is found to occur during the period when both the modes are active, suggesting a joint influence of the two modes on TC genesis over the WNP. -he modulation of TC genesis over the WNP by the two leading ISV modes can be well depicted by the genesis potential index (GPI). Contributions of the four terms to the total GPI anomalies are further analyzed to determine the key factors involved in modulations of TC genesis by both of the ISV modes. Results indicate that while, in general, the low-level absolute vorticity and the mid-level relative humidity are the two most important factors affecting WNP TC genesis, relative roles of the four GPI factors also tend to be dependent on the ISV phases. This study provides further understanding of the ISV modulation of WNP TC genesis, which could benefit the intraseasonal prediction of the TC activity.
    Zhao H. K., R. Yoshida, and G. B. Raga, 2015b: Impact of the madden-Julian oscillation on Western North Pacific tropical cyclogenesis associated with large-scale patterns. Journal of Applied Meteorology and Climatology,54, 1423-1429, doi: 10.1175/JAMC-D-14-0254.1.10.1175/JAMC-D-14-0254.134488b09b4f0520ccd551d10eb32c277http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015JApMC..54.1413Zhttp://adsabs.harvard.edu/abs/2015JApMC..54.1413ZNot Available
    Zhao H. K., X. A. Jiang, and L. G. Wu, 2016: Boreal summer synoptic-scale waves over the western North Pacific in multimodel simulations. J.Climate, doi: 10.1175/JCLI-D-15-0696.10.1175/JCLI-D-15-0696.153cb537b1e6c031cca7c0a9e49ad1890http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F299500831_Boreal_Summer_Synoptic-Scale_Waves_over_the_Western_North_Pacific_in_Multi-model_Simulationshttp://www.researchgate.net/publication/299500831_Boreal_Summer_Synoptic-Scale_Waves_over_the_Western_North_Pacific_in_Multi-model_Simulations
    Zhou C. H., T. Li, 2010: Upscale feedback of tropical synoptic variability to intraseasonal oscillations through the nonlinear rectification of the surface latent heat flux. J.Climate, 23, 5738- 5754.10.1175/2010JCLI3468.190ae5e329de5e94695151d317f091b26http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20103379136.htmlhttp://www.cabdirect.org/abstracts/20103379136.htmlABSTRACT Analysis of observational data suggests two-way interactions between the tropical intraseasonal oscillation (ISO) and synoptic-scale variability (SSV). On one hand, SSV is strongly modulated by the ISO; that is, a strengthened (weakened) SSV appears during the enhanced (suppressed) ISO phase. The northwest- southeast-oriented synoptic wave train is strengthened and well organized in the northwestern Pacific during the enhanced ISO phase but weakened during the suppressed ISO phase. On the other hand, SSV may exert an upscale feedback to ISO through the nonlinearly rectified surface latent heat flux (LHF). The maximum synoptic contribution exceeds 20%-30% of the total intraseasonal LHF over the tropical Indian Ocean, western Pacific, and northeastern Pacific. The nonlinearly rectified LHF leads the ISO convection and boundary layer specific humidity, and thus it may contribute to the propagation of the ISO in boreal summer through the preconditioning of the surface moisture and moist static energy ahead of the convection.
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Manuscript received: 19 December 2015
Manuscript revised: 27 May 2016
Manuscript accepted: 13 June 2016
通讯作者: 陈斌, bchen63@163.com
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Modulation of Tropical Cyclogenesis in the Western North Pacific by the Quasi-Biweekly Oscillation

  • 1. Key Laboratory of Meteorological Disaster, Ministry of Education/Joint International Research Laboratory of Climate and Environment Change/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disaster/Pacific Typhoon Research Center/Earth System Modeling Center, Nanjing University of Information Science and Technology, Nanjing 210044, China
  • 2. NOAA/Atlantic Oceanographic and Meteorological Laboratory, Miami, FL, 33149, USA
  • 3. RIKEN Advanced Institute for Computational Science, Kobe, 650-0047, Japan

Abstract: The quasi-biweekly oscillation (QBWO) is the second most dominant intraseasonal mode over the western North Pacific (WNP) during boreal summer. In this study, the modulation of WNP tropical cyclogenesis (TCG) by the QBWO and its association with large-scale patterns are investigated. A strong modulation of WNP TCG events by the QBWO is found. More TCG events occur during the QBWO's convectively active phase. Based on the genesis potential index (GPI), we further evaluate the role of environmental factors in affecting WNP TCG. The positive GPI anomalies associated with the QBWO correspond well with TCG counts and locations. A large positive GPI anomaly is spatially correlated with WNP TCG events during a life cycle of the QBWO. The low-level relative vorticity and mid-level relative humidity appear to be two dominant contributors to the QBWO-composited GPI anomalies during the QBWO's active phase, followed by the nonlinear and potential intensity terms. These positive contributions to the GPI anomalies are partly offset by the negative contribution from the vertical wind shear. During the QBWO's inactive phase, the mid-level relative humidity appears to be the largest contributor, while weak contributions are also made by the nonlinear and low-level relative vorticity terms. Meanwhile, these positive contributions are partly cancelled out by the negative contribution from the potential intensity. The contributions of these environmental factors to the GPI anomalies associated with the QBWO are similar in all five flow patterns——the monsoon shear line, monsoon confluence region, monsoon gyre, easterly wave, and Rossby wave energy dispersion associated with a preexisting TC. Further analyses show that the QBWO strongly modulates the synoptic-scale wave trains (SSWs) over the WNP, with larger amplitude SSWs during the QBWO's active phase. This implies a possible enhanced (weakened) relationship between TCG and SSWs during the active (inactive) phase. This study improves our understanding of the modulation of WNP TCG by the QBWO and thus helps with efforts to improve the intraseasonal prediction of WNP TCG.

1. Introduction
2. Datasets and methodology
  • The TC data are from the JTWC best-track dataset. The peak TC season (i.e., May-October) during 1998-2012 is selected for analysis. In this study, we focus on the early stage of TCG events. Thereby, only those TCs with maximum sustained wind speed greater than or equal to 13 m s-1 (25 knots) are considered.

    To extract the leading QBWO mode and SSWs over the WNP, we obtain rainfall observations for the period 1998-2012 from TRMM Product 3B42 (Huffman et al., 2007), which extends from 50°S to 50°N with a 0.25° horizontal resolution and a 3-hourly temporal resolution.

    To objectively identify basic large-scale patterns favorable for TCG and to explore key environmental factors associated with the QBWO that affect TCG over the WNP, 6-hourly variables including winds, relative humidity, temperature and SST are derived from JRA-25/the JMA Climate Data Assimilation System (Onogi et al., 2007).

  • 2.2.1. Identification of the five large-scale patterns

    Following (Yoshida and Ishikawa, 2013), the large-scale patterns associated with all the 331 TCG cases during May-October from 1998 to 2012 are classified. The large-scale pattern with the largest normalized contribution score is referred to as the determined flow pattern for each TCG. Details on the computation of the normalized contribution score of the five patterns for each individual TCG event can be found in (Yoshida and Ishikawa, 2013) and (Zhao et al., 2015b).

    Typical examples of TCG corresponding to the five major flow patterns are obtained using the objective algorithm developed by (Yoshida and Ishikawa, 2013). As shown in Fig. 1a, the contribution score is highest from the SL pattern; the second largest contribution score is from the GY pattern, followed by the relatively weak contribution score from the PTC pattern. Therefore, the SL pattern is identified to be the major contribution pattern. The scores for the three patterns are shown in Fig. 1b. It can clearly be seen that the contribution score of the CF pattern is the largest, followed by that of the SL and GY patterns. Similarly, the major EW pattern can be obtained based upon the calculated contribution score shown in Fig. 2a. In Fig. 2b, only the contribution score of the GY pattern can be detected by the algorithm, while no score is computed for the other flow patterns. The major PTC pattern is presented in Fig. 3a, in which a preexisting TC and relatively weak cyclonic vorticity are observed, although the relatively weak contribution from the SL pattern can be found. Specifically, an unclassified flow pattern is displayed in Fig. 3b, with no scores for the five flow patterns of interest.

    Figure 1.  Examples of (a) SL and (b) CF patterns. The red stars represent the locations of TCG. The purple stars indicate a preexisting TC about 1500 km to the west of the TCG location denoted by red stars at the genesis time. The yellow dotted line shows the detected leading edge of the westerly wind by the objective algorithm, and represents a part of the CF region. The blue dotted line indicates the shear line, which is detected near the genesis location by the objective algorithm. Vectors are the horizontal wind field at 850 hPa and the color-shaded area is the zonal wind at 850 hPa. Green contours represent SLP.

    Figure 2.  As in Fig. 1 but for the (a) EW and (b) GY patterns. The green dotted line represents part of the EW trough, as detected by the objective algorithm.

    Figure 3.  As in Fig. 1 but for the (a) PTC and (b) unclassified flow patterns.

    Figure 4.  Composite rainfall anomalies (color-shaded; units: mm d-1) during different QBWO phases superimposed on TCG events. TCG cases associated with the SL (in white), CF (in purple), EW (in green), PTC (in blue) and GY (in yellow) large-scale patterns are given for the QBWO phases.

    The SL appears to be the most frequent flow pattern contributing to WNP TCG. It is associated with about 42% of the 331 TCG events, followed by the CF (18%) and EW (16%) patterns. The PTC (7%) and GY (4%) patterns can be detected, but they only make small contributions to WNP TCG. The same results were found by (Zhao et al., 2015b), and these results agree well with the results of previous studies (Ritchie and Holland, 1999; Yoshida and Ishikawa, 2013). Note that there are some slight differences in the percentages of TCG frequency associated with each large-scale pattern between the results of this study and that of previous studies. There are three possible reasons for these small differences. The first is the different detecting algorithms used. The second reason is different study periods covered. And the last reason is that the impact of climate change differs among different decades.

    Further analyses illustrate no significant difference in TC locations over the peak season under the five large-scale patterns, except a more eastward shift in the average location of TCG associated with the PTC pattern, at the 5% confidence level.

    2.2.2. Identification of the QBWO

    Similar to previous studies (Wang et al., 2009; Li and Zhou, 2013a; Zhao et al., 2015a, 2015b), an extended EOF (EEOF) analysis is used for extracting the QBWO mode over the WNP basin. Before performing the EEOF analysis, the TRMM daily rainfall anomalies for the period 1998-2012 are filtered in the 10-30-day band. The domain (0°-30°N, 100°-160°E) is selected for the EEOF analyses. Similar to (Zhao et al., 2015b), the QBWO over the WNP can be identified by the EEOF1 and EEOF2. A prevailing period of about 20 days for the two EEOF modes can be found in the time series of the principal components (PCs) of EEOF1and EEOF2 (figure not shown), in accordance with the peak period of the QBWO in previous studies (Wang et al., 2009; Li and Zhou, 2013a; Zhao et al., 2015a). The lagged regression patterns of band-pass filtered TRMM rainfall for the EEOF1 and EEOF2 modes further show a pronounced northwestward propagation for the QBWO (figure not shown), which is consistent with results in previous studies (Yasunari, 1979; Hsu et al., 2004; Wang et al., 2009; Lee et al., 2013; Li and Zhou, 2013a; Zhao et al., 2015a). In summary, the first two leading EEOF modes of the observed daily rainfall anomalies can successfully capture the dominant QBWO mode over the WNP.

    Similar to previous studies (Kim et al., 2008; Li and Zhou, 2013a, Zhao et al., 2015a, 2015b), the daily QBWO amplitude and phase (ranging from 1 to 8) can be determined by the two leading EEOF PCs (i.e., PC1 and PC2) following the same method adopted in (Wheeler and Hendon, 2004). In this study, QBWO events are determined for days with amplitude greater than or equal to 1.0 (i.e., \(\sqrt\rm PC1^2+\rm PC2^2\ge 1.0\)).

    2.2.3. Diagnostics for exploring the role of environmental factors

    The genesis potential index (GPI) (Emanuel and Nolan, 2004) is used for exploring the contribution of each individual environmental factor to TCG under all the five large-scale patterns in this study. The GPI is expressed as \begin{equation} \label{eq1} {\rm GPI}=|10^5\xi|^{\frac{3}{2}}\left(\dfrac{H}{50}\right)^3\left(\dfrac{V_{\rm pot}}{70}\right)^3(1+0.1V_{\rm shear})^{-2} , (1)\end{equation} where \(\xi,H,V_\rm pot\), and V shear are the 850 hPa absolute vorticity (s-1), 600 hPa relative humidity (%), potential intensity (PI) (m s-1), and vertical wind shear, respectively. The vertical wind shear is computed as the vector difference between the winds at 200 hPa and 850 hPa (m s-1). The terms \(\eta=|10^5\xi|^\frac32,\gamma=(H/50)^3, \varphi=(V_\rm pot/70)^3\), and s=(1+0.1V shear)-2 represent the four GPI components, which are associated with 850 hPa absolute vorticity, 600 hPa relative humidity, PI, and vertical wind shear, respectively. The algorithm of (Bister and Emanuel, 2002) is used for computing PI. In this algorithm, SST and vertical profiles of temperature and specific humidity in the troposphere are considered.

    As the QBWO propagates northwestward, the anomalous GPI patterns match well with TCG counts and shifts in genesis area, which will be shown later in section 3.1. To further understand the modulation of WNP TCG by the QBWO, we assess the respective roles of the four large-scale factors involved in the GPI [i.e., η,γ,φ and s in Eq. (2)] in contributing to the GPI anomalies, following the methodology detailed in (Jiang et al., 2012).

3. TCG associated with the QBWO
  • The composite rainfall anomalies along with TCG locations during a life cycle of the QBWO are displayed in Fig. 4. The northwestward propagating QBWO can significantly modulate WNP TCG. For example, the TCG zone exhibits a clear migration coupled with the northwestward movement of the enhanced convection from phases 1 to 4 of the QBWO. In addition to the shift in TCG locations, a strong impact of the QBWO on TCG counts can be readily seen. As shown in Fig. 5a, more TCs occur during QBWO phases 1-4 and fewer cases occur during phases 5-8, corresponding to enhanced convection over the main development region of WNP TCG in phases 1-4 and suppressed convection in phases 5-8 of the QBWO. Such a distinct modulation of WNP TCG by the QBWO mode is further indicated by the TCG rate (Fig. 5b), which is computed by the ratio of TCG counts to the number of total days. Similarly, it is readily found that the main development regions of TCG events associated with each large-scale pattern have no significant difference, except that the TCG location associated with the PTC pattern shifts more eastward in all the QBWO phases. Additionally, the CF flow pattern associated with the easternmost TCG location during phase 6 (Fig. 4) can be detected. Furthermore, the ratios of occurrence of each individual large-scale pattern during the QBWO phases are also examined. The results are almost identical to those without separating them into large-scale patterns. In all the flow patterns, except the PTC pattern, more TCG cases occur during the active phases (1-4) than during the inactive phases (5-8). It is noteworthy that slightly more TCG cases occur during the inactive phases under the PTC pattern (Fig. 6).

    To have enough TCG samples and obtain more robust results, we combine phases 1-4 into one category (hereafter, the active QBWO phase) and phases 5-8 into another (hereafter, the inactive QBWO phase). Consequently, the difference in the TCG cases between the active and inactive QBWO phases becomes much clearer. 43.5% of the total number (i.e., 144) of TCG events is found in the active phase and 29.9% of the total number (i.e., 99) of TCG events is observed during the inactive phase. In summary, these results suggest a significant impact of the QBWO on TCG events over the WNP. Compared with the modulation of TCG events by the MJO, discussed in (Zhao et al., 2015a), the impact of the QBWO mode on WNP TCG is weaker. This is mainly because the QBWO has a strong impact on local TC activity, while the MJO has a strong impact on basin-wide TC activity, as suggested in previous studies (Li and Zhou, 2013a; Zhao et al., 2015a).

    Figure 6 further shows the TCG counts associated with the five large-scale patterns during the active and inactive QBWO phases. A more consistent modulation of WNP TCG by the QBWO than that by the MJO is found for all five of the flow patterns (Zhao et al., 2015b). Generally, more events occur during the active QBWO phase under all five flow patterns, except the PTC pattern. Under the PTC pattern, there are slightly more TCG events during the inactive QBWO phase than that during the active QBWO phase, which may be associated with the northwestward propagation of the QBWO. These results are in agreement with the determination of the five major flow patterns. Enhanced westerly wind, and thus intensified monsoonal circulation, can be found during the active phase, resulting in more WNP TCG events associated with the three monsoon-related flow patterns (i.e., SL, CF and GY). In contrast, enhanced easterly wind anomalies, and thus a weakened monsoon trough, over the WNP, can be detected during the inactive QBWO phase, leading to fewer TCG cases under the aforementioned three monsoon-related large-scale patterns. Additionally, there are slightly more TCG cases associated with the EW flow pattern during the active QBWO phase. This may be because the easterly wind intensifies (weakens) over the northeastern side of the enhanced (weakened) monsoon trough over the WNP basin during the active (inactive) QBWO phase, and thus provides more (less) chance for TCG. Meanwhile, TCG locations associated with the EW flow pattern shift more northeastward than that associated with the other three monsoon-related large-scale patterns.

  • Figure 7 shows the composite GPI anomalies and WNP TCG locations during the convectively active and inactive QBWO periods. Corresponding to the composite rainfall anomalies (figure not shown), the positive (negative) GPI anomalies are generally consistent with the active (inactive) phase of convection. In contrast to that during the inactive QBWO phase, enhanced TCG events can be detected in regions with positive GPI anomalies during the active phase. Furthermore, most TCG events tend to form over the monsoon region aligned in the northwest-southeast direction during the active QBWO phase, while more TCG cases occur outside the monsoon trough during the inactive phase. All of these results suggest that the QBWO plays an important role in modulating TCG by changing environmental factors. It is worth noting that a certain number of TCG events can be detected in areas of negative GPI anomalies, as shown in Fig. 7. Two possible reasons may explain this phenomenon. One is that while the GPI reduces with respect to the base state, it is still high enough to support TC genesis. The other is that other factors not included in Eq. (2) or not associated with the QBWO, such as the MJO and convectively coupled equatorial waves, may affect TCG. Further study is needed to better explore the associated physical mechanism.

    Figure 5.  The TCG (a) count and (b) rate (100× TCG count/QBWO days) during the different QBWO phases.

    Figure 6.  TCG count associated with each of the five large-scale patterns (SL, CF, EW, PTC, GY) during the active and inactive QBWO phases.

    Figure 7.  Composite GPI anomalies (color-shaded) during the (a) active and (b) inactive QBWO phase along with TCG events. The green icons represent typhoon.

    Figure 8.  Spatial distributions of the contributions of (a, e) vorticity (b, f) relative humidity, (c, g) and (d, h) potential intensity to the total GPI anomalies over the WNP during the (a-d) active and (e-h) inactive QBWO phases. The green icons represent typhoon.

    The relative importance of the four factors involved in the GPI index is further investigated in this section. The spatial distributions of the four linear terms associated with the total GPI anomalies during the active and inactive phases, along with TCG events, over the WNP, are shown in Fig. 8. During the active phase, positive GPI anomalies with varying 850 hPa relative vorticity (referred to as GPI-VOR) and 600 hPa relative humidity (referred to as GPI-RH) appear to be more consistent with WNP TCG events, and the PI variation also contributes to TCG events. However, most of TCG events over the WNP are associated with negative GPI anomalies with varying vertical wind shear (referred to as GPI-Shear). In contrast to that during the active phase, the largest contribution to the total GPI anomalies during the inactive phase is from the mid-level relative humidity. Distinct from that during the active phase, the PI variation in the eastern part of the WNP (referred to as GPI-PI) makes a positive contribution to the GPI anomalies. Generally, the contributions of environmental factors to the GPI anomalies depend on the QBWO phase. During the active phase, the two leading contributions to positive GPI anomalies are from the enhanced 850 hPa relative vorticity and increased 600 hPa relative humidity, whose contributions are slightly offset by the negative contributions from the vertical wind shear. A relatively similar and small contribution from the nonlinear term (referred to as GPI-nonlinear) can be found during both active and inactive phases (Fig. 9). This is different from the case associated with the MJO mode, as indicated in (Zhao et al., 2015b). The nonlinear term appears not to be a significant contributor,which is further enhanced by displaying the small composite GPI anomalies averaged within the 10°× 10° box centered in TCG locations for all TCG cases, without separating them into large-scale patterns, during the active and inactive QBWO phases (Fig. 10).

    Figure 9.  Spatial distributions of the contributions of nonlinear terms to the total GPI anomalies over the WNP during the (a) active and (b) inactive phases. The green icons represent typhoon.

    Figure 10.  Contribution of the different terms [(i.e., Vorticity, Vertical wind shear, RH, PI, and nonlinear terms] to the total GPI anomalies over the WNP during the (a) active and (b) inactive QBWO phases, for the five large-scale patterns, individually (SL, CF, EW, PTC, GY) and combined (Total).

  • In this section, similar analyses as above are conducted to assess the relative importance of environmental factors in modulating WNP TCG cases under all five flow patterns. Figure 10 shows the mean composite GPI anomalies with the 10°× 10° domain centered on the TCG locations for all TCG cases associated with each large-scale pattern on days with and without TCGs during the active and inactive QBWO phases, respectively. The results show that a positive contribution to the GPI under all of the five large-scale flow patterns during the active phase comes mainly from two factors, i.e., 850 hPa relative vorticity and 600 hPa relative humidity. A weak positive contribution from the nonlinear and PI terms can also be found. Meanwhile, it is readily seen that the negative contribution from the vertical wind shear can partly reduce the positive contributions discussed above. These results are similar to the results from analyzing the total 331 WNP TCG cases without considering them under the five large-scale patterns, as shown in section 3.1. Specifically, the GPI anomalies associated with the GY flow pattern appear to be larger than those associated with other flow patterns, and a weak positive contribution can be found from the vertical wind shear. During the inactive phase, the mid-level relative humidity makes the most important contribution to the positive anomalies. Positive contributions can also be found from the nonlinear term and vertical wind shear, and a weak positive contribution from the 850 hPa relative vorticity. The positive contribution is largely offset by the negative contribution from the PI term. These results are also similar to the results obtained without distinguishing the large-scale patterns, except the role of vertical wind shear. During the active phase, the positive contribution to the GPI anomalies can be found from the vertical wind shear for the GY large-scale pattern, which is distinct from the result obtained without distinguishing the large-scale patterns.

    Considering the possible contamination of the influence of TCs themselves on the magnitude of the GPI anomalies, the magnitude of the averaged composite GPI anomalies with the 10°× 10° domain centered on the TCG location for all TCG cases associated with each large-scale pattern on the 1348 non-TC days is computed for the active and inactive QBWO phases, respectively. The roles of these factors in contributing to GPI anomalies is basically similar to those without excluding TC days during the active and inactive phases, although a slight difference in magnitude can be found. Therefore, the results discussed above on the importance of environmental factors in contributing to the composite GPI anomalies are robust.

  • The SSW train is one of the most frequent occurring weather patterns during boreal summer over the WNP. Many studies have suggested that SSWs actually serve as a major source for WNP TCG events (Sobel and Bretherton, 1999; Li et al., 2006; Fu et al., 2007). In this section, the modulation of SSWs by the QBWO is investigated, which sheds light on the impact of the QBWO on WNP TCG cases.

    To extract the SSWs over the WNP from the data, an EEOF analysis of 3-8-day bandpass filtered TRMM rainfall is performed. Similar to the QBWO mode, the SSWs can be captured by the first two leading EOF modes, as shown in Fig. 11. The amplitude of the SSW activity can be computed as the magnitude of the two leading EEOF PCs, i.e., \(\rm SSW_\rm amplitude=\sqrt\rm PC1^2+\rm PC2^2\). Looking at the lead-lag SSW patterns, it is found that the SSWs are characterized by a northwestward propagation, with a typical wavelength of about 2500 km. To further explore the impact of the QBWO on the SSWs, the amplitude of the SSWs during the active and inactive QBWO phases is computed. The results are shown in Fig. 12. During the active QBWO phase, the SSW amplitude is about 1.50, which is stronger than the SSW amplitude 1.15 during the inactive phase. A similar and consistent result under all five large-scale patterns can be obtained, i.e., the amplitude is larger in the active than the inactive QBWO phase (figure not shown), which corresponds to more (fewer) TCGs over the WNP during the active (inactive) QBWO phase. SSW trains may be an important factor determining the productivity of TCG events over the WNP. Further study is necessary to reveal the detail of the possible link between them, as well as the possible underlying physical mechanism.

    Figure 11.  Evolution of SSW patterns from lag(-3) to lag(+3) days (units: mm d-1).

    Figure 12.  Spatial patterns of the WNP SSW mode during the (a) active and (b) inactive QBWO phases.

4. Summary and conclusions
  • This paper is an extension of the work of (Zhao et al., 2015b); it focuses on investigating the modulation of WNP TCG events by the QBWO, and the association with five large-scale patterns, during the peak TC season for the period 1998-2012. The five flow patterns favorable for WNP TCG events have been illustrated in previous studies (Ritchie and Holland, 1999; Yoshida and Ishikawa, 2013). Following (Yoshida and Ishikawa, 2013), the five flow patterns, i.e., SL, CF, EW, PTC and GY, are also identified in this study. The results of the present study in this regard are consistent with those of previous studies (Ritchie and Holland, 1999), which suggest that the majority of TCG cases are associated with the SL flow pattern, a moderate number of TCG cases are associated with the CF and EW patterns, and very few TCG events are linked to the PTC and GY patterns. Based on the identification of each large-scale pattern, three of the patterns (i.e. SL, CF and GY) are linked uniquely to monsoonal circulation. Meanwhile, the EW flow pattern is closely associated with enhanced easterly wind from the Central Pacific, and TC cases under the PTC pattern are closely associated with the Rossby wave energy dispersion of previous TCG events.

    The modulation of WNP TCG events by the QBWO under different large-scale patterns is investigated by assessing the relative importance of environmental factors. A strong impact of the QBWO on local WNP TCG events is found, with more TCG events forming over the WNP during the active phase compared to the inactive phase. Apart from the PTC pattern, similar results are found for all of the basic patterns. Different from (Zhao et al., 2015b), slightly more TCG cases are detected during the inactive phase than during the active phase.

    The respective contribution of each factor involved in the GPI is further assessed following previous studies (Jiang et al., 2012; Zhao et al., 2015a, 2015b). During a life cycle of the QBWO, large positive GPI anomalies are spatially correlated with WNP TCG events. Further analyses suggest that the roles of these environmental factors depend largely on the different QBWO phases. During the active QBWO phase, the mid-level relative humidity and low-level relative vorticity are the two most important factors contributing to positive GPI anomalies. Weak contributions are made by the nonlinear and PI terms. These positive contributions are partly offset by the negative contribution from the vertical wind shear term. Contrary to that during the active QBWO phase, the mid-level relative humidity appears to be the only large contributor to positive GPI anomalies during the inactive phase. The positive contribution from the nonlinear term during the inactive phase is slightly higher than the contribution from the low-level relative vorticity, which is distinctly different from the situation during the active phase when the contribution made by the low-level relative vorticity is the second most important. The vertical wind shear term makes a positive contribution to GPI anomalies during the inactive QBWO phase, also different from that during the active phase. These positive contributions are also partly cancelled out by the negative contribution made by the PI term. Almost identical results can be obtained for all of the five large-scale patterns, except GY. For the GY pattern, the total GPI anomalies and the contribution of each linear factor to the GPI anomalies appear to be larger than that for the other four flow patterns. Additionally, the role of vertical wind shear in the composite GPI anomalies for the GY flow pattern is distinct from that obtained without distinguishing the large-scale patterns. These results may partly stem from very few TCGs being associated with the monsoon-related GY pattern over the WNP, but this needs further investigation.

    Compared with the results of (Zhao et al., 2015b), it is found that the contributions of the four linear terms to the positive GPI anomalies associated with the QBWO are significantly different to those associated with the MJO. First, the contribution of the PI term to GPI anomalies associated with the QBWO (MJO) mode is positive (negative) during the active (inactive) phase of the QBWO (MJO). The contribution from the PI term appears to be out of phase between the MJO and QBWO phases. Second, the contribution of relative vorticity to GPI anomalies is second most important during the MJO inactive phase, whereas its contribution is weak during the QBWO inactive phase. Additionally, the non-linear term (contributing positively towards the positive GPI anomalies) has a relatively similar contribution during the active and inactive phases of the QBWO, different from its impact during the MJO. During the convectively active MJO period, the positive contribution of the nonlinear term tends to be larger than that during the convectively suppressed period. This may be linked to different physical mechanisms being responsible for the modulation of WNP TCG events by the two intraseasonal modes (Zhou and Li, 2010). Note that interactions between SSWs and the low-frequency large-scale oscillations of the QBWO and MJO may lead to changes in GPI anomalies and TCG cases. The relationship between GPI anomalies and TCG cases might be different under the influences of the QBWO and MJO, which is deserving of further investigation.

    The possible link between TCG and SSWs over the WNP associated with the QBWO is also investigated. The SSWs are regarded as a major seeding for WNP TCG (Li, 2006; Fu et al., 2007). During the active QBWO phase, the SSW amplitude is larger than that during the inactive phase. The result is valid for all of the large-scale patterns, except GY, and more TCGs occur during the active QBWO phase. In this study, only about 7% of the total 311 TCG events occur under the PTC pattern. Based upon the definition of TC cases associated with the PTC pattern, it is explicitly understood that these TCs correspond to SSWs over the WNP. Similarly, the considerable number of TC cases occurring under other flow patterns might also be closely associated with SSWs. For example, as suggested in previous studies, a mixed Rossby-gravity wave can transit to SSWs in an environment of active convection and zonal wind confluence (Dickinson and Molinari, 2002; Aiyyer and Molinari, 2003), suggesting TCG events under the EW pattern and CF pattern may be associated with SSWs. Thus, the variability of SSWs may bear a close relationship with WNP TCG events. Additionally, a number of studies have suggested that TCG over the WNP basin might also be related with other convectively coupled equatorial waves (Bessafi and Wheeler, 2006; Frank and Roundy, 2006). To date, studies on the equatorial waves and TCs associated with the five major large-scale patterns are relatively limited.

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