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Role of the Oceanic Channel in the Relationships between the Basin/Dipole Mode of SST Anomalies in the Tropical Indian Ocean and ENSO Transition


doi: 10.1007/s00376-016-6048-4

  • The relationships between the tropical Indian Ocean basin (IOB)/dipole (IOD) mode of SST anomalies (SSTAs) and ENSO phase transition during the following year are examined and compared in observations for the period 1958-2008. Both partial correlation analysis and composite analysis show that both the positive (negative) phase of the IOB and IOD (independent of each other) in the tropical Indian Ocean are possible contributors to the El Niño (La Niña) decay and phase transition to La Niña (El Niño) about one year later. However, the influence on ENSO transition induced by the IOB is stronger than that by the IOD. The SSTAs in the equatorial central-eastern Pacific in the coming year originate from subsurface temperature anomalies in the equatorial eastern Indian and western Pacific Ocean, induced by the IOB and IOD through eastward and upward propagation to meet the surface. During this process, however the contribution of the oceanic channel process between the tropical Indian and Pacific oceans is totally different for the IOB and IOD. For the IOD, the influence of the Indonesian Throughflow transport anomalies could propagate to the eastern Pacific to induce the ENSO transition. For the IOB, the impact of the oceanic channel stays and disappears in the western Pacific without propagation to the eastern Pacific.
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  • Annamalai H., S. Kida, and J. Hafner, 2010: Potential impact of the tropical Indian Ocean-Indonesian Seas on El Niño characteristics. J.Climate, 23, 3933- 3952.10.1175/2010JCLI3396.1741f7f05b7753c79422a73697d0454a8http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20103282279.htmlhttp://www.cabdirect.org/abstracts/20103282279.htmlEncouraged by these results, the authors further examined the processes that cause cold SST anomalies over the Indonesian seas using an ocean model. Sensitivity experiments suggest that local wind anomalies, through stronger surface heat loss and evaporation, and subsurface upwelling are the primary causes. The present results imply that in coupled models, a proper representation of regional air–sea interactions over the equatorial Indo-Pacific warm pool may be important to understand and predict the amplitude of El Ni09o.
    Annamalai H., S. P. Xie, J. P. McCreary, and R. Murtugudde, 2005: Impact of Indian Ocean sea surface temperature on developing El Niño. J.Climate, 18, 302- 319.10.1175/JCLI-3268.15b37f54ddd8847967ae1442cc9bf8e48http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2005JCli...18..302Ahttp://adsabs.harvard.edu/abs/2005JCli...18..302APrior to the 1976-77 climate shift (1950-76), sea surface temperature (SST) anomalies in the tropical Indian Ocean consisted of a basinwide warming during boreal fall of the developing phase of most El Ninos, whereas after the shift (1977-99) they had an east-west asymmetry -- a consequence of El Nino being associated with the Indian Ocean Dipole/Zonal mode. In this study, the possible impact of these contrasting SST patterns on the ongoing El Nino is investigated, using atmospheric reanalysis products and solutions to both an atmospheric general circulation model (AGCM) and a simple atmospheric model (LBM), with the latter used to identify basic processes. Specifically, analyses of reanalysis products during the El Nino onset indicate that after the climate shift a low-level anticyclone over the South China Sea was shifted into the Bay of Bengal and that equatorial westerly anomalies in the Pacific Ocean were considerably stronger. The present study focuses on determining influence of Indian Ocean SST on these changes.A suite of AGCM experiments, each consisting of a 10-member ensemble, is carried out to assess the relative importance of remote (Pacific) versus local (Indian Ocean) SST anomalies in determining precipitation anomalies over the equatorial Indian Ocean. Solutions indicate that both local and remote SST anomalies are necessary for realistic simulations, with convection in the tropical west Pacific and the subsequent development of the South China Sea anticyclone being particularly sensitive to Indian Ocean SST anomalies. Prior to the climate shift, the basinwide Indian Ocean SST anomalies generate an atmospheric Kelvin wave associated with easterly flow over the equatorial west-central Pacific, thereby weakening the westerly anomalies associated with the developing El Nino. In contrast, after the shift, the east-west contrast in Indian Ocean SST anomalies does not generate a significant Kelvin wave response, and there is little effect on the El Nino-induced westerlies. The Linear Baroclinic Model (LBM) solutions confirm the AGCM's results.
    Balmaseda M. A., K. Mogensen, and A. Weaver, 2013: Evaluation of the ECMWF ocean reanalysis system ORAS4. Quart. J. Roy. Meteor.Soc, 139, 1131- 1161 .e13b6b23d5a8db75db04cc2154c053f2http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fqj.2063%2Ffullhttp://xueshu.baidu.com/s?wd=paperuri%3A%280c8f8f63dff5e881b0ccca8f1b8a74bd%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fqj.2063%2Ffull&ie=utf-8&sc_us=7680070861911868793
    Behera S. K., T. Yamagata, 2003: Influence of the Indian Ocean dipole on the southern oscillation. J. Meteor. Soc.Japan, 81, 169- 177.10.2151/jmsj.81.16965f69e4664873efc1464da5a2468deb9http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013696273%2Fenhttp://ci.nii.ac.jp/naid/10013696273/enThe influence of the Indian Ocean Dipole (IOD) on the interannual atmospheric pressure variability of the Indo-Pacific sector is investigated. Statistical correlation between the IOD index and the global sea level pressure anomalies demonstrates that loadings of opposite polarity occupy the western and the eastern parts of the Indian Ocean. The area of positive correlation coefficient in the eastern part even extends to the Australian region, and the IOD index has a peak correlation coefficient of about 0.4 with the Darwin pressure index, i.e. the western pole of the Southern Oscillation, when the former leads the latter by one month. The correlation analysis with seasonally stratified data further confirms the lead role of the IOD. The lOD-Darwin relation has undergone interdecadal changes ; in the last 50 years the correlation is highest during the most recent decade of 1990-99, and weakest during 1980-89.a
    Carton J. A., B. S. Giese, 2008: A reanalysis of ocean climate using Simple Ocean Data Assimilation (SODA). Mon. Wea. Rev., 136, 2999- 3017.454a78caf21a0c20ffebed73838f4b6ahttp%3A%2F%2Fwww.nrcresearchpress.com%2Fservlet%2Flinkout%3Fsuffix%3Drefg15%2Fref15%26dbid%3D16%26doi%3D10.1139%252Fcjfas-2014-0524%26key%3D10.1175%252F2007MWR1978.1http://xueshu.baidu.com/s?wd=paperuri%3A%28d49e7ec0aacf61ec20ac4c5d8331d5d0%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fwww.nrcresearchpress.com%2Fservlet%2Flinkout%3Fsuffix%3Drefg15%2Fref15%26dbid%3D16%26doi%3D10.1139%252Fcjfas-2014-0524%26key%3D10.1175%252F2007MWR1978.1&ie=utf-8&sc_us=17162769380036851920
    Cohen J., P. Cohen, 1983: Applied Multiple Regression/ Correlation Analysis for the Behavioral Sciences. Lawrence Erlbaum Associates,545 pp.9fdc6933a15a95f1939027689ca6086dhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F245453524_Applied_multiple_regressioncorrelationhttp://www.researchgate.net/publication/245453524_Applied_multiple_regressioncorrelation
    Ding R. Q., J. P. Li, 2012: Influences of ENSO teleconnection on the persistence of sea surface temperature in the tropical Indian Ocean. J. Climate,25, 8177-8195, doi: 10.1175/JCLI-D-11-00739.1.10.1175/JCLI-D-11-00739.1b4692c38cd731e2f7407b3adf0b71137http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2013EGUGA..15.1162Dhttp://adsabs.harvard.edu/abs/2013EGUGA..15.1162DThe Indian Ocean dipole (IOD) alone can cause a weak WPB in the SEIO. El Nino events co-occurring with positive IOD further strengthen the SEIO WPB. The SEIO WPB appears to be more strongly influenced by ENSO than by the IOD. In contrast, the WIO SPB and the SCS FPB are relatively independent of the IOD.
    Dommenget D., T. Bayr, and C. Frauen, 2013: Analysis of the non-linearity in the pattern and time evolution of El Niño Southern Oscillation. Climate Dyn., 40, 2825- 2847.10.1007/s00382-012-1475-0e1c30d7d8a05e20580685d6308f2b519http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-012-1475-0http://link.springer.com/10.1007/s00382-012-1475-0In the study present here is is shown that the non-linearity (skewness) of El Ni09o Southern Oscillation (ENSO) that is usually described as differences in the amplitudes of El Nino and La Nina events, also extents to the spatial SST pattern of events and to the time evolution of events. The analysis presented is based on observations, the CMIP3 model data base and on idealized simplified ENSO models. Is is shown that significant differences in the spatial pattern between positive (El Ni09o) vs. negative (La Ni09a) and strong vs. weak events exist, which is mostly describing the difference between central and east Pacific events. The Bjerknes feedbacks and time evolution of strong ENSO events also show strong asymmetries, with strong El Ni09os being forced more strongly by zonal wind stress than by thermocline depth anomalies and are followed by La Ni09a events. The key non-linearity in the Bjerknes feedbacks is the zonal wind stress response to sea surface temperature anomalies, which during strong El Ni09o events is stronger and shifted to the east relative to strong La Ni09a events, supporting the eastward shifted El Ni09o pattern and the asymmetric time evolution. The observational results are supported by analysis of state of the art coupled general circulation models and by a simplified hybrid coupled ENSO model. Out of the 24 CMIP3 coupled climate models only 4 models are capable of simulating the spatial pattern and time evolution non-linearity realistically. All these four models strongly support the asymmetric forcings of ENSO events by zonal wind stress and thermocline depth anomalies. Based on the simplified hybrid coupled RECHOZ model of ENSO it can be shown that the non-linear zonal wind stress response to SST anomalies causes the asymmetric forcings of ENSO events. On the basis of 100 perfect model ensemble forecasts with the RECHOZ model it can further be illustrated, that strong La Ni09a events are more predictable than strong El Ni09o events due to the non-linear zonal wind stress response to SST anomalies.
    Drushka K., J. Sprintall, S. T. Gille, and I. Brodjonegoro, 2010: Vertical structure of Kelvin waves in the Indonesian Throughflow exit passages.. J. Phys. Oceanogr, 40, 1965- 1987.10.1016/B978-012374473-9.00602-0865e0b31-c318-4cec-8633-33680da5c19da2eb81dfd2215c48b054ff434df861b8http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FB9780123744739006020refpaperuri:(5c1b64a6b406abce0a0553d3c2ea7787)http://www.sciencedirect.com/science/article/pii/B9780123744739006020The Indonesian seas play a unique role in providing the only open pathway that connects two major ocean basins at tropical latitudes. On average, the sea level is higher on the western Pacific side of the Indonesian archipelago compared to the eastern Indian side. This pressure gradient generates a transport of water and their properties from the Pacific toward the Indian Ocean. This flow from the Pacific Ocean into the Indian Ocean is known as the Indonesian Throughflow. The warmer water that enters from the Pacific can be traced throughout the Indonesian seas, and then followed within the surface to intermediate depths as a distinct low-salinity tongue in the Indian Ocean. As such, the Throughflow forms the ‘warm’ water route for the global thermohaline circulation and therefore impacts the regional and global climate system. Because of its proximity to Asia and Australia, the circulation and transport in the Indonesian seas has a large seasonal variation due to the influence of the reversing annual wind patterns associated with the Asian–Australian monsoon system. During the different monsoon seasons, waters of different sources from both the Indian and Pacific Oceans flow into the Indonesian archipelago and cause variability in temperature, salinity, and other properties. Local processes within the Indonesian seas related to the regional monsoon winds such as upwelling and downwelling, along with the tides, air–sea heat fluxes, and voluminous precipitation and associated river runoff, also act to change the Pacific temperature and salinity stratification into the distinctly fresh Indonesian seas profile. These changes in the physical properties of the water are linked to the behavior, migration pattern, and the seasonal distribution of the phytoplankton and pelagic fish species that live within the Indonesian seas. Thus a knowledge and understanding of the pathways and variability of the Indonesian Throughflow and its properties is important for the region's people, who depend on the sea for their very food and livelihood, and also to help develop management plans to sustain these valuable and limited maritime resources.
    Du Y., S.-P. Xie, Y.-L. Yang, X.-T. Zheng, L. Liu, and G. Huang, 2013: Indian Ocean variability in the CMIP5 multimodel ensemble: The basin mode. J. Climate,26, 7240-7266, doi: 10.1175/JCLI-D-12-00678.1.10.1175/JCLI-D-12-00678.1085ac17ba27ef1ea750fe5deab445728http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F268689332_Indian_Ocean_Variability_in_the_CMIP5_Multimodel_Ensemble_The_Basin_Modehttp://www.researchgate.net/publication/268689332_Indian_Ocean_Variability_in_the_CMIP5_Multimodel_Ensemble_The_Basin_ModeThis study evaluates the simulation of the Indian Ocean Basin (IOB) mode and relevant physical processes in models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Historical runs from 20 CMIP5 models are available for the analysis. They reproduce the IOB mode and its close relationship to El Nino-Southern Oscillation (ENSO). Half of the models capture key IOB processes: a downwelling oceanic Rossby wave in the southern tropical Indian Ocean (TIO) precedes the IOB development in boreal fall and triggers an antisymmetric wind anomaly pattern across the equator in the following spring. The anomalous wind pattern induces a second warming in the north Indian Ocean (NIO) through summer and sustains anticyclonic wind anomalies in the northwest Pacific by radiating a warm tropospheric Kelvin wave. The second warming in the NIO is indicative of ocean-atmosphere interaction in the interior TIO. More than half of the models display a double peak in NIO warming, as observed following El Nino, while the rest show only one winter peak. The intermodel diversity in the characteristics of the IOB mode seems related to the thermocline adjustment in the south TIO to ENSO-induced wind variations. Almost all the models show multidecadal variations in IOB variance, possibly modulated by ENSO.
    England, M. H., F. Huang, 2005: On the interannual variability of the Indonesian Throughflow and its linkage with ENSO. J.Climate, 18, 1435- 1444.5e5a0a1b0238a49692cfd471f80f4f2fhttp%3A%2F%2Fwww.bioone.org%2Fservlet%2Flinkout%3Fsuffix%3Di0883-1351-30-1-66-England1%26dbid%3D16%26doi%3D10.2110%252Fpalo.2013.061%26key%3D10.1175%252FJCLI3322.1http://xueshu.baidu.com/s?wd=paperuri%3A%281b04b79f391bd7ea701ce2355faa9528%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fwww.bioone.org%2Fservlet%2Flinkout%3Fsuffix%3Di0883-1351-30-1-66-England1%26dbid%3D16%26doi%3D10.2110%252Fpalo.2013.061%26key%3D10.1175%252FJCLI3322.1&ie=utf-8&sc_us=16512953132509483130
    Izumo, T., Coauthors, 2010: Influence of the state of the Indian Ocean Dipole on following year's El Niño. Nature Geoscience, 3, 168- 172
    Izumo T., M. Lengaigne, J. Vialard, J.-J. Luo, T. Yamagata, G. Madec, 2014: Influence of the Indian Ocean Dipole and Pacific recharge on the following year's El Niño: Interdecadal robustness. Climate Dyn.,42, 291-310, doi: 10.1007/s00382-012-1628-1.10.1007/s00382-012-1628-1fdb8ccb65089f95862ba4629cf18022bhttp%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00382-012-1628-1http://link.springer.com/article/10.1007/s00382-012-1628-1The Indian Ocean Dipole (IOD) can affect the El Ni09o–Southern Oscillation (ENSO) state of the following year, in addition to the well-known preconditioning by equatorial Pacific Warm Water Volume (WWV), as suggested by a study based on observations over the recent satellite era (1981–2009). The present paper explores the interdecadal robustness of this result over the 1872–2008 period. To this end, we develop a robust IOD index, which well exploits sparse historical observations in the tropical Indian Ocean, and an efficient proxy of WWV interannual variations based on the temporal integral of Pacific zonal wind stress (of a historical atmospheric reanalysis). A linear regression hindcast model based on these two indices in boreal fall explains 5002% of ENSO peak variance 1402months later, with significant contributions from both the IOD and WWV over most of the historical period and a similar skill for El Ni09o and La Ni09a events. Our results further reveal that, when combined with WWV, the IOD index provides a larger ENSO hindcast skill improvement than the Indian Ocean basin-wide mode, the Indian Monsoon or ENSO itself. Based on these results, we propose a revised scheme of Indo-Pacific interactions. In this scheme, the IOD–ENSO interactions favour a biennial timescale and interact with the slower recharge-discharge cycle intrinsic to the Pacific Ocean.
    Jin F.-F., 1997a: An equatorial ocean recharge paradigm for ENSO. Part I: Conceptual model. J. Atmos. Sci., 54, 811- 829.10.1175/1520-0469(1997)0542.0.CO;209c47e5b99df8464751dff9cd4c7d43ahttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1997JAtS...54..811Jhttp://adsabs.harvard.edu/abs/1997JAtS...54..811JA new conceptual model for ENSO has been constructed based upon the positive feedback of tropical ocean–atmosphere interaction proposed by Bjerknes as the growth mechanism and the recharge–discharge of the equatorial heat content as the phase-transition mechanism suggested by Cane and Zebiak and by Wyrtki. This model combines SST dynamics and ocean adjustment dynamics into a coupled basinwide recharge oscillator that relies on the nonequilibrium between the zonal mean equatorial thermocline depth and wind stress. Over a wide range of the relative coupling coefficient, this recharge oscillator can be either self-excited or stochastically sustained. Its period is robust in the range of 3–5 years. This recharge oscillator model clearly depicts the slow physics of ENSO and also embodies the delayed oscillator (Schopf and Suarez; Battisti and Hirst) without requiring an explicit wave delay. It can also be viewed as a mixed SST–ocean dynamics oscillator due to the fact that it arises from the merging of two uncoupled modes, a decaying SST mode and a basinwide ocean adjustment mode, through the tropical ocean–atmosphere coupling. The basic characteristics of this recharge oscillator, including the relationship between the equatorial western Pacific thermocline depth and the eastern Pacific SST anomalies, are in agreement with those of ENSO variability in the observations and simulations with the Zebiak–Cane model.
    Jin F.-F., 1997b: An equatorial ocean recharge paradigm for ENSO. Part II: A stripped-down coupled model. J. Atmos. Sci., 54, 830- 847.10.1175/1520-0469(1997)0542.0.CO;26278c3976d36c385455807a9deb73330http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1997jats...54..830jhttp://adsabs.harvard.edu/abs/1997jats...54..830jThe conceptual recharge oscillator model intuitively established in Part I is derived from a dynamical framework of a Cane-Zebiak type model for tropical ocean-atmosphere interaction. A two-strip approximation to the equatorial ocean dynamics and one-strip approximation to the SST dynamics are employed to obtain a stripped-down coupled model that captures the main physics of the Cane-Zebiak type model. It is shown that the conceptual recharge oscillator model can be obtained from the stripped-down coupled model with a two-box approximation in the zonal direction or a low-frequency approximation to filter out high-frequency modes. Linear solutions of the stripped-down model are analytically solved and the dependence of coupled modes on various model parameters is delineated. In different parameter regimes, the stripped-down coupled model describes a coupled-wave mode and a mixed SST-ocean-dynamics mode that results from the merger of a nonoscillatory ocean adjustment mode with an SST mode. These two coupled oscillatory modes undergo a further merger. In the neighborhood of this merger, the leading mode of the system becomes a generalized mixed mode. It is suggested that the slow ENSO regime can be best characterized by this generalized mixed mode whose essential physics are described by the conceptual recharge oscillator model proposed in Part I.
    Kand aga, P., A. L. Gordon, J. Sprintall, R. D. Susanto, 2009: Intraseasonal variability in the Makassar Strait thermocline. J. Mar. Res., 67, 757- 777.10.1357/0022240097920061153f7ef00644a63370af2acbeb8c308e0bhttp%3A%2F%2Fwww.ingentaconnect.com%2Fcontent%2Fjmr%2Fjmr%2F2009%2F00000067%2F00000006%2Fart00003http://www.ingentaconnect.com/content/jmr/jmr/2009/00000067/00000006/art00003Intraseasonal variability [ISV] in the Makassar Strait thermocline is examined through the analysis of along-channel flow, regional sea level anomaly and wind fields from January 2004 through November 2006. The dominant variability of 45–90 day in the Makassar Strait along-channel flow is horizontally and vertically coherent and exhibits vertical energy propagation. The majority of the Makassar ISV is uncoupled to the energy exerted by the local atmospheric ISV: instead the Makassar ISV is due to the combination of a remotely forced baroclinic wave radiating from Lombok Strait and deep reaching ISV originating in the Sulawesi Sea. Thermocline depth changes associated with ENSO influence the ISV characteristics in the Makassar Strait lower thermocline, with intensified ISV during El Ni09o when the thermocline shallows and weakened ISV during La Ni09a.
    Klein S. A., B. J. Soden, and N.- C. Lau, 1999: Remote sea surface temperature variations during ENSO: Evidence for a tropical atmospheric bridge. J.Climate, 12, 917- 932.10.1175/1520-0442(1999)0122.0.CO;283ab4967234d8e4d075060389c8e7b8ehttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1999jcli...12..917khttp://adsabs.harvard.edu/abs/1999jcli...12..917kIn an El Ni09o event, positive SST anomalies usually appear in remote ocean basins such as the South China Sea, the Indian Ocean, and the tropical North Atlantic approximately 3 to 6 months after SST anomalies peak in the tropical Pacific. Ship data from 1952 to 1992 and satellite data from the 1980s both demonstrate that changes in atmospheric circulation accompanying El Ni09o induce changes in cloud cover and evaporation which, in turn, increase the net heat flux entering these remote oceans. It is postulated that this increased heat flux is responsible for the surface warming of these oceans. Specifically, over the eastern Indian Ocean and South China Sea, enhanced subsidence during El Ni09o reduces cloud cover and increases the solar radiation absorbed by the ocean, thereby leading to enhanced SSTs. In the tropical North Atlantic, a weakening of the trade winds during El Ni09o reduces surface evaporation and increases SSTs. These relationships fit the concept of an “atmospheric bridge” that connects SST anomalies in the central equatorial Pacific to those in remote tropical oceans.
    Kug J.-S., I.-S. Kang, 2006: Interactive feedback between ENSO and the Indian Ocean. J.Climate, 19, 1784- 1801.d9bfd7a7-8e95-4330-850e-bef98d8096107df42a06f2d5a4d80619fedd8b033c7chttp%3A%2F%2Fwww.mendeley.com%2Fresearch%2Finteractive-feedback-between-indian-ocean-enso%2Frefpaperuri:(29d454f12b688a451a6f70791244be15)http://www.mendeley.com/research/interactive-feedback-between-indian-ocean-enso/
    Kug J.-S., T. Li, S.-I. An, I.-S. Kang, J.-J. Luo, S. Masson, and T. Yamagata, 2006: Role of the ENSO-Indian Ocean coupling on ENSO variability in a coupled GCM. Geophys. Res. Lett., 33,L09710, doi: 10.1029/2005GL024916.10.1029/2005GL02491632fac0464a64e68af8a6a3bd635cd1a7http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2005GL024916%2Ffullhttp://xueshu.baidu.com/s?wd=paperuri%3A%2877477221b24d999ab807dfce0bc9943b%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2005GL024916%2Ffull&ie=utf-8&sc_us=11024636832908850019The effect of the Indian Ocean on El Nino/La Nina life cycles has been studied using 200-yrs simulation data of a coupled GCM. The results show that the interactive feedback between the ENSO and the Indian Ocean holds the key to the rapid transition to an opposite phase. This remote impact of the Indian Ocean SST anomaly is linked to the change of zonal wind stress in the western Pacific, which leads to a rapid demise of El Nino/La Nina. Without the involvement of the Indian Ocean, the phase transition is much slower. This role of the ENSO-Indian Ocean coupling on ENSO transition is consistent with that derived from the observational analysis.
    Liang X. S., 2014: Unraveling the cause-effect relation between time series. Physical Review E, 90, 052150.10.1103/PhysRevE.90.05215025493782276c5348e5b7a66897379fb097a097f3http%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpubmed%2F25493782http://www.ncbi.nlm.nih.gov/pubmed/25493782Given two time series, can one faithfully tell, in a rigorous and quantitative way, the cause and effect between them ? Based on a recently rigorized physical notion, namely, information flow, we solve an inverse problem and give this important and challenging question, which is of interest in a wide variety of disciplines, a positive answer. Here causality is measured by the time rate of information flowing from one series to the other. The resulting formula is tight in form, involving only commonly used statistics, namely, sample covariances; an immediate corollary is that causation implies correlation, but correlation does not imply causation. It has been validated with touchstone linear and nonlinear series, purportedly generated with one-way causality that evades the traditional approaches. It has also been applied successfully to the investigation of real-world problems; an example presented here is the cause-and-effect relation between the two climate modes, El Nino and the Indian Ocean Dipole (IOD), which have been linked to hazards in far-flung regions of the globe. In general, the two modes are mutually causal, but the causality is asymmetric: El Nino tends to stabilize IOD, while IOD functions to make El Nino more uncertain. To El Nino, the information flowing from IOD manifests itself as a propagation of uncertainty from the Indian Ocean.
    Luo J.-J., R. C. Zhang, S. K. Behera, Y. Masumoto, F.-F. Jin, R. Lukas, and T. Yamagata, 2010: Interaction between El Niño and extreme Indian Ocean Dipole. J.Climate, 23, 726- 742.10.1175/2009JCLI3104.126dad4ffbfcfcaf40cb5514664d9e531http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20103092247.htmlhttp://www.cabdirect.org/abstracts/20103092247.htmlClimate variability in the tropical Indo-Pacific sector has undergone dramatic changes under global ocean warming. Extreme Indian Ocean dipole (IOD) events occurred repeatedly in recent decades with an unprecedented series of three consecutive episodes during 200609“08, causing vast climate and socioeconomic effects worldwide and weakening the historic El Ni01±o09“Indian monsoon relationship. Major attention has been paid to the El Ni01±o influence on the Indian Ocean, but how the IOD influences El Ni01±o and its predictability remained an important issue to be understood. On the basis of various forecast experiments activating and suppressing air09“sea coupling in the individual tropical ocean basins using a state-of-the-art coupled ocean09“atmosphere model with demonstrated predictive capability, the present study shows that the extreme IOD plays a key role in driving the 1994 pseudo09“El Ni01±o, in contrast with traditional El Ni01±o theory. The pseudo09“El Ni01±o is more frequently observed in recent decades, coincident with a weakened atmospheric Walker circulation in response to anthropogenic forcing. The study0964s results suggest that extreme IOD may significantly enhance El Ni01±o and its onset forecast, which has being a long-standing challenge, and El Ni01±o in turn enhances IOD and its long-range predictability. The intrinsic El Ni01±o09“IOD interaction found here provides hope for enhanced prediction skill of both of these climate modes, and it sheds new light on the tropical climate variations and their changes under the influence of global warming.
    McPhaden M. J., 2008: Evolution of the 2006-2007 El Niño: The role of intraseasonal to interannual time scale dynamics. Advances in Geosciences, 14, 219- 230.10.5194/adgeo-14-219-20089db316c6a9888be2c77fbdc2e1bc2f77http%3A%2F%2Fwww.oalib.com%2Fpaper%2F1369778http://www.oalib.com/paper/1369778The relationship between monthly mean sea surface temperature (SST) anomalies in the commonly used El Ni o regions and precipitation for 44 stations in Perú is documented for 1950–2002. Linear lag correlation analysis is employed to establish the potential for statistical precipitation forecasts from SSTs. Useful monthly mean precipitation anomaly forecasts are possible for several locations and calendar months if SST anomalies in El Ni o 1+2, Ni o 3.4, and Ni o 4 regions are available. Prediction of SST anomalies in El Ni o regions is routinely available from Climate Prediction Center, NOAA, with reasonable skill in the El Ni o 3.4 region, but the prediction in El Ni o 1+2 region is less reliable. The feasibility of using predicted SST anomalies in the El Ni o 3.4 region to predict SST anomalies in El Ni o 1+2 region is discussed.
    Mogensen K., M. Alonso Balmaseda, and A. Weaver, 2012: The NEMOVAR ocean data assimilation system as implemented in the ECMWF ocean analysis for System 4. ECMWF Technical Memorandum 668,59 pp.e48b9b1f77509bc9ed54499d6dc76661http%3A%2F%2Fwww.mendeley.com%2Fresearch%2Fnemovar-ocean-data-assimilation-system-implemented-ecmwf-ocean-analysis-system-4%2Fhttp://www.mendeley.com/research/nemovar-ocean-data-assimilation-system-implemented-ecmwf-ocean-analysis-system-4/
    Ohba M., 2013: Important factors for long-term change in ENSO transitivity. International Journal of Climatology, 33, 1495- 1509.10.1002/joc.352933a81e0460aa21f0e74f5abb025747aehttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fjoc.3529%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1002/joc.3529/fullNot Available
    Ohba M., H. Ueda, 2005: Basin-wide warming in the equatorial Indian Ocean associated with El Niño. SOLA,1, 89-92, doi: 10.2151/sola.2005-024.10.2151/sola.2005-02491ac1ebf54c66c0814bdb3a978bc1353http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F130004940829http://ci.nii.ac.jp/naid/130004940829Abstract A physical process of basin-wide warming in the equatorial Indian Ocean (IO) is examined in view of the seasonally different coupling processes between the monsoon circulation and the modulated Walker circulation associated with major El Ni09o. In the
    Ohba M., H. Ueda, 2007: An impact of SST anomalies in the Indian Ocean in acceleration of the El Niño to La Niña transition. J. Meteor. Soc.Japan, 85, 335- 348.10.2151/jmsj.85.335cf8b144b0d2ce2bb255b75c883c06657http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F130004788592%2Fhttp://ci.nii.ac.jp/naid/130004788592/One possible impact of sea surface temperature (SST) anomalies in the Indian Ocean (IO) on the ongoing El Ni09o is investigated using an air-sea coupled general circulation model (CGCM). A suite of CGCM experiments by imposing the IO basin-wide warming (BW) and IO dipole/zonal mode (IODZM) are conducted to assess the feedback effects of the El Ni09o-related SST anomalies on the Pacific. While the IODZM during boreal fall does not have a significant impact on the Pacific sector, the BW during boreal winter enhances the surface easterlies over the equatorial Western Pacific (WP) during the mature-decay phase of El Ni09o. The strengthened easterlies act to enhance the SST cooling over the WP, thereby the zonal gradient of SST between the IO and WP is greater than the climatology. These enhanced WP easterlies induce an advanced transition to the La Ni09a phase, which is caused by the upwelling ocean Kelvin waves. These results imply that the BW in the IO, to some extent, can be hastening the El Ni09o to La Ni09a transition.
    Ohba M., H. Ueda, 2009: Role of nonlinear atmospheric response to SST on the asymmetric transition process of ENSO. J.Climate, 22, 177- 192.10.1175/2008JCLI2334.10445825725a7e703b1368f5f014e3a6dhttp%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20093063980.htmlhttp://www.cabdirect.org/abstracts/20093063980.htmlA suite of idealized atmospheric general circulation model (AGCM) experiments are performed by imposing two different ENSO-related SST anomalies, which have equal amplitudes but opposite signs. The nonlinear climate response in the AGCM is found at the mature-to-decaying phase of ENSO that closely resembles the observations, including a zonal and meridional shift in the equatorial positions of the atmospheric wind. By using a simple ocean model, it is determined that the asymmetric responses of the equatorial zonal wind result in different recovery times of the thermocline in the eastern Pacific. Thus, the differences in transition processes between the warm and cold ENSO event are fundamentally due to the nonlinear atmospheric response to SST, which originates from the distribution of climatological SST and its seasonal changes. By including the asymmetric wind responses the intermediate airea coupled model herein demonstrates that the essential elements of the redevelopment of La Nina arise from the nonlinear atmospheric response to SST anomalies.
    Ohba M., M. Watanabe, 2012: Role of the Indo-Pacific interbasin coupling in predicting asymmetric ENSO transition and duration. J.Climate, 25, 3321- 3335.10.1175/JCLI-D-11-00409.164bbe636f30af39e9efe613cb2f34b56http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2012EGUGA..14.1201Ohttp://adsabs.harvard.edu/abs/2012EGUGA..14.1201OWarm and cold phases of El Ni09o-Southern Oscillation (ENSO) exhibit a significant asymmetry in their transition/duration such that El Ni09o tends to shift rapidly to La Ni09a after the mature phase, while La Ni09a tends to persist for up to two years. The possible role of sea surface temperature (SST) anomalies in the Indian Ocean (IO) in this ENSO asymmetry is investigated using a coupled general circulation model (CGCM). Decoupled-IO experiments are conducted to assess asymmetric IO feedbacks to the ongoing ENSO evolution in the Pacific. Identical-twin forecast experiments show that a coupling of the IO extends the skillful prediction of the ENSO warm phase by about one year, which was about eight months in the absence of the IO coupling, in which a significant drop of the prediction skill around the boreal spring (known as spring barriers) is found. The effect of IO coupling on the predictability of the Pacific SST is significantly weaker in the decay phase of La Ni09a. Warm IO SST anomalies associated with El Ni09o enhance surface easterlies over the equatorial western Pacific and hence counteract the El Ni09o decay. However, this mechanism cannot be applied to cold IO SST anomalies during La Ni09a. The result of our CGCM experiments estimates that approximately one-half of the ENSO asymmetry arises from the phase-dependent nature of the Indo-Pacific interbasin coupling.
    Ohba M., D. Nohara, and H. Ueda, 2010: Simulation of asymmetric ENSO transition in WCRP CMIP3 multimodel experiments. J. Climate,23, 6051-6067, doi: 10.1175/2010 JCLI3608.1.10.1175/2010JCLI3608.101abf28753f8abad589c79b900b233d8http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20113021678.htmlhttp://www.cabdirect.org/abstracts/20113021678.htmlIn the CMIP3 models, four climate models simulate well the rapid transition from El Ni09o to La Ni09a. The intensity of a rapid transition of El Ni09o is mainly related to the intensity of the simulated climatological precipitation over the western–central Pacific (WCP). The models that have strong WCP precipitation can simulate the rapid termination of the equatorial zonal wind in the WCP, which tends to result in the termination of El Ni09o phase. This relationship is not applicable for the La Ni09a transition phase. The simulation of La Ni09a persistency is related to the reflection of off-equatorial Rossby waves at the western boundary of the Pacific and the seasonal evolution of the climatological precipitation in the WCP. Differences in the transition processes between El Ni09o and La Ni09a events are fundamentally due to the nonlinear atmospheric (convective) response to SST, which originates from the distribution of climatological SST and its seasonal changes. The results of the present study indicate that a realistic simulation of the climatological state and its seasonality in the WCP are important to be able to simulate the observed transition process of the ENSO.
    Okumura Y. M., M. Ohba, C. Deser, and H. Ueda, 2011: A proposed mechanism for the asymmetric duration of El Niño and La Niña. J.Climate, 24, 3822- 3829.10.1175/2011JCLI3999.173a61fa1ed08037a4cc61dc27673ce85http%3A%2F%2Fagris.fao.org%2Fagris-search%2Fsearch.do%3FrecordID%3DAV2012090472http://agris.fao.org/agris-search/search.do?recordID=AV2012090472Not Available
    Potemra J. T., 2005: Indonesian Throughflow transport variability estimated from satellite altimetry. Oceanography, 18, 98- 107.10.5670/oceanog.2005.10c0226cbb54341ce6764af8243768542ahttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F276189197_Indonesian_Throughflow_Transport_Variability_Estimated_from_Satellite_Altimetryhttp://www.researchgate.net/publication/276189197_Indonesian_Throughflow_Transport_Variability_Estimated_from_Satellite_Altimetry
    Rayner N. A., D. E. Parker, E. B. Horton, C. K. Folland , L. V. Alexand er, D. P. Rowell, E. C. Kent, and A. Kaplan, 2003: Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res., 108 ,4407, doi: 10.1029/2002JD002670.10.1029/2002JD0026700831f099871c89699f00bb6e2586346bhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2002JD002670%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2002JD002670/full[1] We present the Met Office Hadley Centre's sea ice and sea surface temperature (SST) data set, HadISST1, and the nighttime marine air temperature (NMAT) data set, HadMAT1. HadISST1 replaces the global sea ice and sea surface temperature (GISST) data sets and is a unique combination of monthly globally complete fields of SST and sea ice concentration on a 1 latitude-longitude grid from 1871. The companion HadMAT1 runs monthly from 1856 on a 5 latitude-longitude grid and incorporates new corrections for the effect on NMAT of increasing deck (and hence measurement) heights. HadISST1 and HadMAT1 temperatures are reconstructed using a two-stage reduced-space optimal interpolation procedure, followed by superposition of quality-improved gridded observations onto the reconstructions to restore local detail. The
    Saji N. H., T. Yamagata, 2003: Structure of SST and surface wind variability during Indian Ocean dipole mode events: COADS observations. J.Climate, 16, 2735- 2751.10.1175/1520-0442(2003)016<2735:SOSASW>2.0.CO;298b491d65e37b09544743261573bc3e8http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2003JCli...16.2735Shttp://adsabs.harvard.edu/abs/2003JCli...16.2735SABSTRACT A study of the detailed spatiotemporal characteristics of the Indian Ocean dipole (IOD) mode in SST and surface winds using available observations from 1958 till 1997 is reported. The analysis is used to address several of the controversial issues regarding the IOD.One key finding of this study is that interdecadal fluctuations contribute strongly to tropical Indian Ocean (TIO) SST variability; in SST anomalies (SSTA) interdecadal variance is as strong as interannual variance. Over both the western and eastern TIO, an accelerated warming of SST after the mid-1970s is apparent. The lack of anticorrelation between western and eastern TIO SSTA occurs only in this latter half of the analysis period.In order to examine the hypothesis that the IOD is a part of ENSO evolution in the TIO, the temporal characteristics of IOD indices have been compared with Ni&ntilde;o-3. On the basis of several quantitative comparisons that include wavelet and cross-wavelet analysis, several important differences between the two phenomena are reported. These differences are highlighted to argue that the IOD is not a part of ENSO evolution in the TIO. On the other hand, a striking similarity is found in the temporal structure of atmospheric and oceanic variability within the TIO that is suggestive of IOD arising from inherent coupled air-sea interactions in the TIO.ENSO events that do not co-occur with IOD have been isolated and their impacts on TIO SSTA and winds described. Similarly, the characteristics of IOD events that occur independently of ENSO are described. Based on the characteristics of these two groups a hypothesis is suggested through which both phenomena may interact. It is noted that ENSO events co-occurring with IOD events are much stronger compared to non-co-occurring events. On the other hand, IOD events that are independent of ENSO as well as those that co-occur with it appear to have the same strength.
    Saji N. H., B. N. Goswami, P. N. Vinayachand ran, and T. Yamagata, 1999: A dipole mode in the tropical Indian Ocean. Nature, 401, 360- 363.10.1038/438541686210831d5bd363a994efe19550bc4fc1839b4http%3A%2F%2Feuropepmc.org%2Fabstract%2Fmed%2F16862108http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_PM16862108For the tropical Pacific and Atlantic oceans, internal modes of variability that lead to climatic oscillations have been recognized, but in the Indian Ocean region a similar ocean-atmosphere interaction causing interannual climate variability has not yet been found. Here we report an analysis of observational data over the past 40 years, showing a dipole mode in the Indian Ocean: a pattern of internal variability with anomalously low sea surface temperatures off Sumatra and high sea surface temperatures in the western Indian Ocean, with accompanying wind and precipitation anomalies. The spatio-temporal links between sea surface temperatures and winds reveal a strong coupling through the precipitation field and ocean dynamics. This air-sea interaction process is unique and inherent in the Indian Ocean, and is shown to be independent of the El Nino/Southern Oscillation. The discovery of this dipole mode that accounts for about 12% of the sea surface temperature variability in the Indian Ocean--and, in its active years, also causes severe rainfall in eastern Africa and droughts in Indonesia--brightens the prospects for a long-term forecast of rainfall anomalies in the affected countries.
    Smith T. M., R. W. Reynolds, T. C. Peterson, and J. Lawrimore, 2008: Improvements to NOAA's historical merged land-ocean surface temperature analysis (1880-2006). J.Climate, 21, 2283- 2296.10.1175/BAMS-D-11-00241.1c414e21c-59c5-4c50-9c7e-e9f6fee91eeaa871f494927b97ada299e482d296dab1http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F273920081_NOAA%27s_Merged_LandOcean_Surface_Temperature_Analysisrefpaperuri:(dbc44c65ee1c9ace06b0727517ae2bee)http://www.researchgate.net/publication/273920081_NOAA's_Merged_LandOcean_Surface_Temperature_AnalysisThis paper describes the new release of the Merged Land–Ocean Surface Temperature analysis (MLOST version 3.5), which is used in operational monitoring and climate assessment activities by the NOAA National Climatic Data Center. The primary motivation for the latest version is the inclusion of a new land dataset that has several major improvements, including a more elaborate approach for addressing changes in station location, instrumentation, and siting conditions. The new version is broadly consistent with previous global analyses, exhibiting a trend of 0.076°C decade 611 since 1901, 0.162°C decade 611 since 1979, and widespread warming in both time periods. In general, the new release exhibits only modest differences with its predecessor, the most obvious being very slightly more warming at the global scale (0.004°C decade 611 since 1901) and slightly different trend patterns over the terrestrial surface.
    Sprintall J., A. L. Gordon, R. Murtugudde, and R. D. Susanto, 2000: A semiannual Indian Ocean forced Kelvin wave observed in the Indonesian seas in May 1997. J. Geophys. Res., 105, 17 217- 17 230.10.1029/2000JC9000657d19ce5e43d588664f7f3e32a8cdfbd8http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2000JC900065%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2000JC900065/fullRecent observations within the Indonesian exit passages and internal seas highly resolve the arrival and passage of a semiannual Kelvin wave. In mid-May 1997, surface and subsurface currents were to the southeast at a mooring located south of Java in the South Java Current, while local wind forcing was northwestward. Subsequent northward fluctuations in the geostrophic current through Lombok Strait and in observed currents from two moorings located in Makassar Strait are commensurate with the speed and passage of a Kelvin wave through the region. The Kelvin wave was due to westerly wind forcing in the remote equatorial Indian Ocean during the semiannual April/May monsoon transition period. This was confirmed through a simple remote wind-forced analytical Kelvin wave model of velocity at the South Java Current mooring location and sea level in Lombok Strait and also in the numerical general circulation model of Murtugudde et al. [1998]. Warm temperature anomalies measured at the south Java mooring and within Makassar Strait are associated with the passage of the Kelvin wave. Salinity anomalies measured at the south Java mooring are consistent with an Indian Ocean source. The observed passage of the Kelvin wave during May 1997 unambiguously demonstrates for the first time that equatorial Indian Ocean remote wind forcing may on occasions influence the internal Indonesian seas
    Suarez M. J., P. S. Schopf, 1988: A delayed action oscillator for ENSO. J. Atmos. Sci., 45, 3283- 3287.3f8116688108017cc92986e8e7a4fe6dhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F248381891_A_delayed_oscillator_for_ENSOhttp://www.researchgate.net/publication/248381891_A_delayed_oscillator_for_ENSOPublication &raquo; A delayed oscillator for ENSO.
    Tillinger D., A. L. Gordon, 2009: Fifty years of the Indonesian Throughflow. J.Climate, 22, 6342- 6355.10.1175/2009JCLI2981.1a6911992c7fe1ded96898a44fac5c462http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20103024888.htmlhttp://www.cabdirect.org/abstracts/20103024888.htmlSimple Ocean Data Assimilation (SODA) reanalysis data are used to produce a 50-yr record of flow through the Makassar Strait, the primary conduit for the Indonesian Throughflow (ITF). Two time series are constructed for comparison to the flow through the Makassar Strait as observed during 1997-98 and 2004-06: SODA along-channel speed within the Makassar Strait and Pacific to Indian Ocean intero...
    Wajsowicz R. C., E. K. Schneider, 2001: The Indonesian throughflow's effect on global climate determined from the COLA coupled climate system. J.Climate, 14, 3029- 3042.10.1175/1520-0442(2001)0142.0.CO;2e4a79d7f62ac092fadc81ccc4c1cf76chttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2001jcli...14.3029whttp://adsabs.harvard.edu/abs/2001jcli...14.3029wBlocking the Indonesian archipelago by a land bridge, and so preventing flow from the western equatorial Pacific Ocean into the eastern Indian Ocean, causes a global readjustment in the coupled ocean tmosphere's climate. Notable features found in 10-yr-averaged fields when compared with the control climate are an ENSO-like signal over the equatorial Pacific, with warmer SSTs in the eastern basin, westerly anomalies in wind stress, and increased precipitation along the equator. The band of increased precipitation is flanked by bands of decreased rainfall. Over the Indian Ocean, blocking of the throughflow results in signatures similar to those associated with the Indian Dipole Mode. In the eastern basin, there are cooler SSTs and heat content anomalies. The southeast trades are increased with an associated increase in latent heating. The precipitation belt is shifted northwestward to give decreased rainfall over southern Indonesia and increased rainfall to the north; there is a slight increase in rainfall over eastern Africa, and SSTs are warmer in the western half of the basin. The Atlantic, midlatitudes, and polar regions are affected via atmospheric teleconnections. Net surface heat flux differences in regions of significant SST difference in the equatorial Pacific and Indian Oceans are about 2 times as large as found previously in ocean-only simulations, indicating a positive feedback. Experiments with the atmospheric GCM component forced by the Indian Ocean SST anomalies generated by blocking the throughflow reproduce the changes in surface heat flux and winds found in the coupled model simulations over the southern Indian Ocean. In the equatorial Pacific, positive feedback is provided by the Bjerknes mechanism. In the Indian Ocean, the positive feedback loop comprises a change in oceanic heat flux divergence, which changes SST, and in turn net surface heat flux and surface winds, which further change the oceanic circulation and heat flux diver...
    Webster P. J., A. M. Moore, J. P. Loschnigg, and R. R. Leben, 1999: Coupled ocean-atmosphere dynamics in the Indian Ocean during 1997-98. Nature, 401, 356- 360.10.1038/4384816862107dcca6bbc88e576dc28c749ca84a1fc7ahttp%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpubmed%2F16862107http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_PM16862107Climate variability in the Indian Ocean region seems to be, in some aspects, independent of forcing by external phenomena such as the El Nino/Southern Oscillation. But the extent to which, and how, internal coupled ocean-atmosphere dynamics determine the state of the Indian Ocean system have not been resolved. Here we present a detailed analysis of the strong seasonal anomalies in sea surface temperatures, sea surface heights, precipitation and winds that occurred in the Indian Ocean region in 1997-98, and compare the results with the record of Indian Ocean climate variability over the past 40 years. We conclude that the 1997-98 anomalies--in spite of the coincidence with the strong El Nino/Southern Oscillation event--may primarily be an expression of internal dynamics, rather than a direct response to external influences. We propose a mechanism of ocean-atmosphere interaction governing the 1997-98 event that may represent a characteristic internal mode of the Indian Ocean climate system. In the Pacific Ocean, the identification of such a mode has led to successful predictions of El Nino; if the proposed Indian Ocean internal mode proves to be robust, there may be a similar potential for predictability of climate in the Indian Ocean region.
    Weisberg R. H., C. Z. Wang, 1997: A western Pacific oscillator paradigm for the El Niño-Southern Oscillation. Geophys. Res. Lett., 24, 779- 782.10.1029/97GL00689fd55ec8f4d00114bbfad45e7160e4a86http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F97GL00689%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/97GL00689/fullA data-based hypothesis is presented on the mechanism of the El Nino-Southern Oscillation (ENSO), a major determinant of interannual global climate variability. The hypothesis emphasizes the importance of off-equator sea surface temperature and sea level pressure variations west of the dateline for initiating equatorial easterly winds over the far western Pacific. These winds compete with westerly winds over the equatorial central Pacific enabling the coupled ocean-atmosphere system to oscillate. Consistent with this hypothesis, an analogical oscillator model is constructed that produces ENSO-like oscillations. The proposed mechanism differs from the delayed oscillator paradigm in that wave reflection at the western boundary is not a necessary condition for the coupled ocean-atmosphere system to oscillate.
    Wijffels S. E., G. Meyers, 2004: An intersection of oceanic waveguides: Variability in the Indonesian Throughflow region. J. Phys. Oceanogr., 34, 1232- 1253.10.1175/1520-0485(2004)0342.0.CO;28e945cfdd618b6a7b03599a2a9fab8fahttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2004JPO....34.1232Whttp://adsabs.harvard.edu/abs/2004JPO....34.1232WNot Available
    Wu R. G., B. P. Kirtman, 2004: Understanding the impacts of the Indian Ocean on ENSO variability in a coupled GCM. J.Climate, 17, 4019- 4031.10.1175/1520-0442(2004)0172.0.CO;240e0729f767a0c948c1b8fd34dae4c2chttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2004JCli...17.4019Whttp://adsabs.harvard.edu/abs/2004JCli...17.4019WAbstract This study investigates the impacts of the Indian Ocean on El Ni09o–Southern Oscillation (ENSO) variability through numerical simulations with a coupled atmosphere–ocean general circulation model, composite analyses with the coupled model output, and simple atmosphere model experiments with specified sea surface temperature (SST) forcing. It is found that, when the Indian Ocean is decoupled from the atmosphere, the ENSO variability in the coupled model is significantly reduced. Conditional SST distributions indicate that the warm (cold) ENSO state is stronger and occurs more frequently when the Indian Ocean SST in summer is relatively cold (warm), whereas it is weaker and occurs less frequently when the Indian Ocean is relatively warm (cold). The impacts of the Indian Ocean are suggested by a comparison of SST composites under warm, normal, and cold Indian Ocean SST conditions in the developing stage of ENSO. It is demonstrated that the Indian Ocean affects the ENSO variability through modulating ...
    Wyrtki K., 1987: Indonesian through flow and the associated pressure gradient. J. Geophys. Res., 92, 12 941- 12 946.10.1029/JC092iC12p1294160e0d48fca95de8694643c066c21c7fehttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2FJC092iC12p12941%2Fabstracthttp://xueshu.baidu.com/s?wd=paperuri%3A%28d375702e0c462b93a0478db101a17cfa%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2FJC092iC12p12941%2Fabstract&ie=utf-8&sc_us=11346542004292738428The flow of water from the western Pacific to the eastern Indian Ocean through the Indonesian archipelago is governed by a strong pressure gradient. Dynamic height computations determine the average sea level difference as 16 cm and show that most of the pressure gradient is contained in the upper 200 m. Sea level data from Davao in the Philippines and from Darwin in Australia are used to determine the annual signal and the interannual variations of the pressure gradient for the years 1966 to 1985. The annual signal has a maximum during the southeast monsoon in July and August and a minimum in January and February. Interannual variations are not related to the Southern Oscillation because sea level is low at both stations during El Nino events, and thus there is little influence on the sea level difference. The mechanism of the through flow is discussed, but a determination of its numerical value will have to await direct measurements. A comparison of the sea level difference with results from a numerical model by Kindle shows satisfactory agreement. It is concluded that the variability of the through flow can be monitored by sea level measurements.
    Xie S.-P., H. Annamalai, F. A. Schott, and J. P. McCreary Jr, 2002: Structure and mechanisms of South Indian Ocean climate variability. J.Climate, 15, 864- 878.10.1175/1520-0442(2002)015<0864:SAMOSI>2.0.CO;297708127d9485413b97b2a055b35a12bhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2002jcli...15..864xhttp://adsabs.harvard.edu/abs/2002jcli...15..864xCiteSeerX - Document Details (Isaac Councill, Lee Giles): A unique open-ocean upwelling exists in the tropical South Indian Ocean (SIO), a result of the negative wind curl between the southeasterly trades and equatorial westerlies, raising the thermocline in the west. Analysis of in-situ measurements and a model-assimilated dataset reveals a strong influence of subsurface thermocline variability on sea surface temperature (SST) in this upwelling zone. El Nino/Southern Oscillation (ENSO) is found to be the dominant forcing for the SIO thermocline variability, with SST variability off Sumatra also making a significant contribution. When either an El Nino or Sumatra cooling event takes place, anomalous easterlies appear in the equatorial Indian Ocean, forcing a westward-propagating downwelling Rossby wave in the SIO. In phase with this dynamic Rossby wave, there is a pronounced co-propagation of SST. Moreover, a positive precipitation anomaly is found over, or just to the south of, the Rossby wave-induced positive SST anomaly, resu...
    Xie S.-P., K. M. Hu, J. Hafner, H. Tokinaga, Y. Du, G. Huang, and T. Sampe, 2009: Indian Ocean capacitor effect on Indo-western Pacific climate during the summer following El Niño. J.Climate, 22, 730- 747.10.1175/2008JCLI2544.15ca7332e21ffff9143909ee76fe9bab3http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20093117314.htmlhttp://www.cabdirect.org/abstracts/20093117314.htmlSignificant climate anomalies persist through the summer (June-August) after El Nino dissipates in spring over the equatorial Pacific. They include the tropical Indian Ocean (TIO) sea surface temperature (SST) warming, increased tropical tropospheric temperature, an anomalous anticyclone over the subtropical northwest Pacific, and increased mei-yu-baiu rainfall over East Asia. The cause of these lingering El Nino effects during summer is investigated using observations and an atmospheric general circulation model (GCM). The results herein indicate that the TIO warming acts like a capacitor anchoring atmospheric anomalies over the Indo-western Pacific Oceans. It causes tropospheric temperature to increase by a moist-adiabatic adjustment in deep convection, emanating a baroclinic Kelvin wave into the Pacific. In the northwest Pacific, this equatorial Kelvin wave induces northeasterly surface wind anomalies, and the resultant divergence in the subtropics triggers suppressed convection and the anomalous anticyclone. The GCM results support this Kelvin wave-induced Ekman divergence mechanism. In response to a prescribed SST increase over the TIO, the model simulates the Kelvin wave with low pressure on the equator as well as suppressed convection and the anomalous anticyclone over the subtropical northwest Pacific. An additional experiment further indicates that the north Indian Ocean warming is most important for the Kelvin wave and northwest Pacific anticyclone, a result corroborated by observations. These results have important implications for the predictability of Indo-western Pacific summer climate: the spatial distribution and magnitude of the TIO warming, rather than simply whether there is an El Nino in the preceding winter, affect summer climate anomalies over the Indo-western Pacific and East Asia.
    Xue Y., T. M. Smith, and R. W. Reynolds, 2003: Interdecadal changes of 30- yr SST normals during 1871-2000. J.Climate, 16, 1601- 1612.10.1175/1520-0442-16.10.1601c20a4f063700df8ee19ef55b90dc7641http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2003JCli...16.1601Xhttp://adsabs.harvard.edu/abs/2003JCli...16.1601XSST predictions are usually issued in terms of anomalies and standardized anomalies relative to a 30-yr normal: climatological mean (CM) and standard deviation (SD). The World Meteorological Organization (WMO) suggests updating the 30-yr normal every 10 yr. In complying with the WMO's suggestion, a new 30-yr normal for the 1971-2000 base period is constructed. To put the new 30-yr normal in historical perspective, all the 30-yr normals since 1871 are investigated, starting from the beginning of each decade (1871-1900, 1881-1910, . . . , 1971-2000). Using the extended reconstructed sea surface temperature (ERSST) on a 200° grid for 1854-2000 and the Hadley Centre Sea Ice and SST dataset (HadISST) on a 100° grid for 1870-1999, eleven 30-yr normals are calculated, and the interdecadal changes of seasonal CM, seasonal SD, and seasonal persistence (P) are discussed. The interdecadal changes of seasonal CM are prominent (0.300°-0.600°) in the tropical Indian Ocean, the midlatitude North Pacific, the midlatitude North Atlantic, most of the South Atlantic, and the sub-Antarctic front. Four SST indices are used to represent the key regions of the interdecadal changes: the Indian Ocean (''INDIAN''; 1000°S-2500°N, 4500°-10000°E), the Pacific decadal oscillation (PDO; 3500°-4500°N, 16000°E-16000°W), the North Atlantic Oscillation (NAO; 4000°-6000°N, 2000°-6000°W), and the South Atlantic (SATL; 2200°S-200°N, 3500°W-1000°E). Both INDIAN and SATL show a warming trend that is consistent between ERSST and HadISST. Both PDO and NAO show a multidecadal oscillation that is consistent between ERSST and HadISST except that HadISST is biased toward warm in summer and cold in winter relative to ERSST. The interdecadal changes in Nin01±o-3 (500°S-500°N, 9000°-15000°W) are small (0.200°) and are inconsistent between ERSST and HadISST. The seasonal SD is prominent in the eastern equatorial Pacific, the North Pacific, and North...
    Yamagata T., S. K. Behera, S. A. Rao, Z. Y. Guan, K. Ashok, and H. N. Saji, 2003: Comments on "Dipoles, temperature gradients, and tropical climate anomalies". Bull. Amer. Meteor. Soc., 84, 1418- 1422.10.1175/BAMS-84-10-14181945f2c54219c2d9ef838023296d20fahttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2003BAMS...84.1422Mhttp://adsabs.harvard.edu/abs/2003BAMS...84.1422MNo Abstract Available.
    Yang J. L., Q. Y. Liu, S.-P. Xie, Z. Y. Liu, and L. X. Wu, 2007: Impact of the Indian Ocean SST basin mode on the Asian summer monsoon. Geophys. Res. Lett., 34,L02708, doi: 10.1029/2006GL028571.10.1029/2006GL028571353dab2ba8ff639e582352e283adfb3ehttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2006GL028571%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2006GL028571/full[1] Following an El Nino event, a basin-wide warming takes place over the tropical Indian Ocean, peaks in late boreal winter and early spring, and persists through boreal summer. Our observational analysis suggests that this Indian Ocean warming induces robust climatic anomalies in the summer Indo-West Pacific region, prolonging the El Nino's influence after tropical East Pacific sea surface temperature has returned to normal. In response to the Indian Ocean warming, precipitation increases over most of the basin, forcing a Matsuno-Gill pattern in the upper troposphere with a strengthened South Asian high. Near the ground, the southwest monsoon intensifies over the Arabian Sea and weakens over the South China and Philippine Seas. An anomalous anticyclonic circulation forms over the subtropical Northwest Pacific, collocated with negative precipitation anomalies. All these anomaly patterns are reproduced in a coupled model simulation initialized with a warming in the tropical Indian Ocean mixed layer, indicating that the Indian Ocean warming is not just a passive response to El Nino but important for summer climate variability in the Indo-West Pacific region. The implications for seasonal prediction are discussed.
    Yasunari T., 1985: Zonally propagating modes of the global east-west circulation associated with the Southern Oscillation. J. Meteor. Soc.Japan, 63, 1013- 1029.10.1175/1520-0469(1985)042<2695:IOGWOA>2.0.CO;2e8e955d2852f7dfd127e58e6563595a4http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F248496020_zonally_propagating_modes_of_the_global_east-west_circulation_associated_with_the_southern_oscillathttp://www.researchgate.net/publication/248496020_zonally_propagating_modes_of_the_global_east-west_circulation_associated_with_the_southern_oscillatSeveral schemes for correcting temperature and velocity measurements from moored current meters are tested on two moorings, one of which experienced large vertical excursions as the Gulf Stream meandered over it. The addition of two extra temperature recorders 100 m above and below the current meter permits the calculation of a reference corrected series through time dependent interpolation. By assuming that the current meter profiles the vertical temperature structure as it is pulled up and down it is possible to calculate the mean vertical temperature gradient and its dependence on temperature. A quadratic dependence is suggested by hydrographic measurements and the direct in situ measurements on the moorings. It is found possible, through weighted polynomial regression, to calculate this dependence directly from measurements of temperature and pressure on a single instrument and, thereby, to remove more than 95 percent of the mooring motion induced temperature variance. The correction of temperature for mooring motion is found necessary for accurate estimation of heat flux from moorings in energetic areas. Correction of velocity is more difficult but it is not found to have much effect on flux calculations.
    Yasunari T., 1987: Global structure of the El Niño/Southern oscillation. Part II. Time evolution. J. Meteor. Soc.Japan, 65, 81- 102.2f1a4910d66c7f114a5075818c50f25ehttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F40000634425http://ci.nii.ac.jp/naid/40000634425
    Yu J.-Y., C. R. Mechoso, J. C. McWilliams, and A. Arakawa, 2002: Impacts of the Indian Ocean on the ENSO cycle. Geophys. Res. Lett.,29, 46-1-6-4, doi: 10.1029/2001GL014098.10.1029/2001GL014098aa43b7b57395f9da3be917632afd85f3http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2001GL014098%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2001GL014098/fullThis study examines the impacts of the Indian Ocean on the ENSO (El Nino-Southern Oscillation) cycle by performing experiments with a coupled atmosphere-ocean general circulation model (CGCM). In one of the experiments, the ocean model domain includes only the tropical Pacific Ocean (the Pacific Run). In the other experiment, the ocean model domain includes both the Indian and tropical Pacific Oceans (the Indo-Pacific Run). The experiment results show that the CGCM simulation of ENSO including both the Indian and tropical Pacific Oceans tends to be more realistic than that including the tropical Pacific Ocean only. In particular, the Indo-Pacific Run produces ENSO events with larger amplitude and greater variability on decadal time scales. The interactive Indian Ocean also affects the surface heat flux anomalies in the Indian Ocean during the ENSO cycle and surface wind stress anomalies in both the tropical Indian and Pacific Oceans. There are indications that both surface heat flux and wind stress are actively forcing a portion of the interannual variability in the Indian Ocean during the ENSO cycle.
    Yu, J.-Y . , 2005: Enhancement of ENSO's persistence barrier by biennial variability in a coupled atmosphere-ocean general circulation model. Geophys. Res. Lett., 32,L13707, doi: 10.1029/2005GL023406.10.1029/2005GL023406c1d9de2fd752cf5b05f9430fc5c97534http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2005GL023406%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2005GL023406/fullPossible causes of the spring persistence barrier in ENSO sea surface temperature anomalies are examined using a coupled atmosphere-ocean general circulation model (CGCM). Our study indicates that the persistence barrier is significantly enhanced when both Pacific and Indian Ocean couplings are included in the CGCM, compared to the simulation that includes only the Pacific Ocean coupling. ENSO's variance, phase locking to the annual cycle, and biennial variability are also increased in the Indo-Pacific Run. Further analysis reveals that the overall amplitude of ENSO is not a primary factor in determining the strength of the persistence barrier, rather, it is the amplitude of the biennial component of ENSO affecting the barrier the most. The persistence barrier is consistently strong (weak) when biennial ENSO variability is large (small). No such a clear relationship is found between the strength of the barrier and the amplitude of the low-frequency (3-5 years) component of ENSO. This modeling study demonstrates that the biennial component of ENSO is one major mechanism responsible for the spring persistence barrier and that interactions between the tropical Pacific Ocean and Indian Ocean-monsoon could enhance the biennial component of ENSO.
    Yu W. D., B. Q. Xiang, L. Liu, and N. Liu, 2005: Understanding the origins of interannual thermocline variations in the tropical Indian Ocean. Geophys. Res. Lett., 32,L24706, doi: 10.1029/2005GL024327.10.1029/2005GL024327ef22564f9e7d8e712c606f023ec71971http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2005GL024327%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2005GL024327/fullThe crystal structure of byakangelicin, one of furanocoumarin aldose reductase inhibitors, was determined by X-ray diffraction method. The crystal is triclinic, with a = 8.114(1), b = 10.194(1), c = 11.428(1)A, a = 111.50(1), beta= 95.57(1), gamma = 112.52(1) degrees , Dx = 1.41, Dm = 1.39 g/cm3, space group P1 and Z = 2. The intensity data were collected by omega-2theta scan method with CuK(a) radiations. The structure was solved by direct method and refined by full matrix least-squares procedure to the final R-value of 0.056. There are two molecules with different conformations in an asymmetric unit. The molecules are kept by two intermolecular O-HO type hydrogen bonds and van der Waal's forces in the crystal. The absolute configuration of the molecules was estimated to S-form by the 'Eta refinement' procedure.
    Yuan, D. L., Coauthors, 2011: Forcing of the Indian Ocean dipole on the interannual variations of the tropical Pacific Ocean: Roles of the Indonesian throughflow. J.Climate, 24, 3593- 3608.10.1175/2011JCLI3649.118bdb901c5e6ba0744a692a76dd9af9bhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2011JCli...24.3593Yhttp://adsabs.harvard.edu/abs/2011JCli...24.3593YControlled numerical experiments using ocean-only and ocean-atmosphere coupled general circulation models show that interannual sea level depression in the eastern Indian Ocean during the Indian Ocean dipole (IOD) events forces enhanced Indonesian Throughflow (ITF) to transport warm water from the upper-equatorial Pacific Ocean to the Indian Ocean. The enhanced transport produces elevation of the thermocline and cold subsurface temperature anomalies in the western equatorial Pacific Ocean, which propagate to the eastern equatorial Pacific to induce significant coupled evolution of the tropical Pacific oceanic and atmospheric circulation. Analyses suggest that the IOD-forced ITF transport anomalies are about the same amplitudes as those induced by the Pacific ENSO. Results of the coupled model experiments suggest that the anomalies induced by the IOD persist in the equatorial Pacific until the year following the IOD event, suggesting the importance of the oceanic channel in modulating the interannual climate variations of the tropical Pacific Ocean at the time lag beyond one year.
    Yuan D. L., H. Zhou, and X. Zhao, 2013: Interannual climate variability over the tropical Pacific Ocean induced by the Indian Ocean dipole through the Indonesian Throughflow. J. Climate,26, 2845-2861, doi: 10.1175/JCLI-D-12-00117.1.10.1175/JCLI-D-12-00117.122b98af0528d18616267ceabac463913http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2013jcli...26.2845yhttp://adsabs.harvard.edu/abs/2013jcli...26.2845yThe authors' previous dynamical study has suggested a link between the Indian and Pacific Ocean interannual climate variations through the transport variations of the Indonesian Throughflow. In this study, the consistency of this oceanic channel link with observations is investigated using correlation analyses of observed ocean temperature, sea surface height, and surface wind data. The analyses show significant lag correlations between the sea surface temperature anomalies (SSTA) in the southeastern tropical Indian Ocean in fall and those in the eastern Pacific cold tongue in the following summer through fall seasons, suggesting potential predictability of ENSO events beyond the period of 1 yr. The dynamics of this teleconnection seem not through the atmospheric bridge, because the wind anomalies in the far western equatorial Pacific in fall have insignificant correlations with the cold tongue anomalies at time lags beyond one season. Correlation analyses between the sea surface height anomalies (SSHA) in the southeastern tropical Indian Ocean and those over the Indo-Pacific basin suggest eastward propagation of the upwelling anomalies from the Indian Ocean into the equatorial Pacific Ocean through the Indonesian Seas. Correlations in the subsurface temperature in the equatorial vertical section of the Pacific Ocean confirm the propagation. In spite of the limitation of the short time series of observations available, the study seems to suggest that the ocean channel connection between the two basins is important for the evolution and predictability of ENSO.
    Zhou Q., W. S. Duan, M. Mu, and R. Feng, 2015: Influence of positive and negative Indian Ocean dipoles on ENSO via the Indonesian Throughflow: Results from sensitivity experiments. Adv. Atmos. Sci.,32, 783-793, doi: 10.1007/s00376-014-4141-0.10.1007/s00376-014-4141-09aa943831a9f7597b31ec7f0cb6b7f2chttp%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpmc%2Farticles%2FPMC1705594%2Fhttp://d.wanfangdata.com.cn/Periodical_dqkxjz-e201506005.aspxThe role of the Indonesian Throughflow (ITF) in the influence of the Indian Ocean Dipole (IOD) on ENSO is investigated using version 2 of the Parallel Ocean Program (POP2) ocean general circulation model. We demonstrate the results through sensitivity experiments on both positive and negative IOD events from observations and coupled general circulation model simulations. By shutting down the atmospheric bridge while maintaining the tropical oceanic channel, the IOD forcing is shown to influence the ENSO event in the following year, and the role of the ITF is emphasized. During positive IOD events, negative sea surface height anomalies (SSHAs) occur in the eastern Indian Ocean, indicating the existence of upwelling. These upwelling anomalies pass through the Indonesian seas and enter the western tropical Pacific, resulting in cold anomalies there. These cold temperature anomalies further propagate to the eastern equatorial Pacific, and ultimately induce a La Nia-like mode in the following year. In contrast, during negative IOD events, positive SSHAs are established in the eastern Indian Ocean, leading to downwelling anomalies that can also propagate into the subsurface of the western Pacific Ocean and travel further eastward. These downwelling anomalies induce negative ITF transport anomalies, and an El Nio-like mode in the tropical eastern Pacific Ocean that persists into the following year. The effects of negative and positive IOD events on ENSO via the ITF are symmetric. Finally, we also estimate the contribution of IOD forcing in explaining the Pacific variability associated with ENSO via ITF.
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Manuscript History

Manuscript received: 26 February 2016
Manuscript revised: 24 June 2016
Manuscript accepted: 18 July 2016
通讯作者: 陈斌, bchen63@163.com
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Role of the Oceanic Channel in the Relationships between the Basin/Dipole Mode of SST Anomalies in the Tropical Indian Ocean and ENSO Transition

  • 1. Key Laboratory of Ocean Circulation and Waves, and Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
  • 2. Laboratory for Ocean Dynamics and Climate, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266235, China
  • 3. Center for Ocean and Climate Research, First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China

Abstract: The relationships between the tropical Indian Ocean basin (IOB)/dipole (IOD) mode of SST anomalies (SSTAs) and ENSO phase transition during the following year are examined and compared in observations for the period 1958-2008. Both partial correlation analysis and composite analysis show that both the positive (negative) phase of the IOB and IOD (independent of each other) in the tropical Indian Ocean are possible contributors to the El Niño (La Niña) decay and phase transition to La Niña (El Niño) about one year later. However, the influence on ENSO transition induced by the IOB is stronger than that by the IOD. The SSTAs in the equatorial central-eastern Pacific in the coming year originate from subsurface temperature anomalies in the equatorial eastern Indian and western Pacific Ocean, induced by the IOB and IOD through eastward and upward propagation to meet the surface. During this process, however the contribution of the oceanic channel process between the tropical Indian and Pacific oceans is totally different for the IOB and IOD. For the IOD, the influence of the Indonesian Throughflow transport anomalies could propagate to the eastern Pacific to induce the ENSO transition. For the IOB, the impact of the oceanic channel stays and disappears in the western Pacific without propagation to the eastern Pacific.

1. Introduction
  • ENSO is the most pronounced interannual variability in the tropics, contributing significantly to climate fluctuations in many regions of the globe. Several causal mechanisms of ENSO oscillation have been suggested, such as the delay oscillator (Suarez and Schopf, 1988), the recharge oscillator (Jin, 1997a, 1997b), and the Western Pacific Oscillator (Weisberg and Wang, 1997). These mechanisms emphasize processes in the Pacific Ocean basin, but they do not consider the cross-basin linkage between the tropical Pacific and Indian oceans. The Indian Ocean's variability may affect that in the Pacific Ocean, although most attention has been paid to the impact of the Pacific on the Indian Ocean (e.g., Ding and Li, 2012). There is evidence that SST variability in the Indian Ocean can modulate ENSO variability either through atmospheric wind variability (Yasunari, 1985, 1987; Yu et al., 2002; Behera and Yamagata, 2003; Wu and Kirtman, 2004; Kug and Kang, 2006; McPhaden, 2008; Xie et al., 2009; Izumo et al., 2010; Luo et al., 2010; Du et al., 2013) or Indonesian Throughflow (ITF) variability (Wyrtki, 1987; Wajsowicz and Schneider, 2001; Yuan et al., 2011, 2013). This provides hope for enhanced ENSO prediction skill (Izumo et al., 2010; Luo et al., 2010).

    It is well known that the SST anomalies (SSTAs) in the tropical Indian Ocean often co-occur with ENSO variation. Thus, studies tend to focus on the synchronous influence of the Indian Ocean on the Pacific (e.g., Annamalai et al., 2005, 2010). On the other hand, several studies have provided evidence that the Indian Ocean acts as a negative feedback mechanism to ENSO (Kug and Kang, 2006; Kug et al., 2006; Ohba and Ueda, 2007; Izumo et al., 2010; Yuan et al., 2011, 2013; Zhou et al., 2015). The suggestion is that the Indian Ocean SSTAs during El Niño (La Niña) lead to a relatively faster ENSO termination and phase transition to La Niña (El Niño). Such a lagged impact of the tropical Indian Ocean on the Pacific is the focus of the present paper.

    The most dominant SST variations in the tropical Indian Ocean are the basin-wide warming/cooling mode (hereafter, the IOB) (e.g., Yang et al., 2007) and the dipole mode (hereafter, the IOD) (Saji et al., 1999; Webster et al., 1999), which can be obtained via EOF analysis of the tropical Indian Ocean SST variability, i.e., EOF-1 and EOF-2, respectively (Fig. 1). The IOB involves a near uniform warming (cooling) over the entire basin. It reaches its maximum after the mature phase of ENSO. The IOD involves a weak warming (cooling) over the western Indian Ocean and a strong cooling (warming) in the east off Java and Sumatra (Saji et al., 1999; Webster et al., 1999). The IOD peaks in fall then quickly recedes. Results show that much of the IOB is caused by ENSO-induced surface heat flux anomalies (Klein et al., 1999). However, there are different views regarding the IOD, with some suggesting it is independent of ENSO (e.g., Saji and Yamagata, 2003; Yamagata et al., 2003) and others indicating it to be primarily forced by ENSO (Xie et al., 2002; Annamalai et al., 2005; Ohba and Ueda, 2005). Regardless of the origins of the IOB and IOD, they significantly affect ENSO evolution or transition when the Indian Ocean SSTAs appear.

    Figure 1.  The (a) first and (b) second EOF modes, with (c) their PCs, of the monthly SSTAs in the tropical Indian Ocean, from the ERSST data for the period 1958-2008. (d) Standard deviation of the PC of the first mode (PC1) and the PC of the second mode (PC2) as a function of calendar month. The red (black) curves in (c) and (d) denote PC1 (PC2).

    For the IOD, (Izumo et al., 2010) and Yuan et al. (2011, 2013) investigated its influence on the following year's El Niño, and they indicated that a positive (negative) phase of the IOD tends to co-occur with El Niño (La Niña) and favors La Niña (El Niño) about 1 year later. The former study concentrated on the impact of the atmospheric bridge between the Indian and Pacific oceans over the period 1981-2009. On the other hand, Yuan et al. (2011, 2013) focused on the oceanic channel and assigned a key role to the ITF for the period 1990-2009. The positive (negative) phase of the IOD is accompanied by a negative (positive) SSH anomalies (SSHAs) in the tropical eastern Indian Ocean, which forces upwelling (downwelling) Kelvin waves at the equator. The upwelling anomalies propagate eastward from the Indian Ocean into the western equatorial Pacific Ocean through the channels of the seas of Indonesia. They force an enhanced (weakened) ITF to transport more (less) warm water from the upper-equatorial Pacific Ocean to the Indian Ocean. This anomalous transport produces thermocline depth anomalies and cold (warm) subsurface temperature anomalies in the western equatorial Pacific Ocean. Then, these temperature anomalies propagate to the eastern equatorial Pacific, inducing a La Niña (El Niño) in the following year. Thus, their observational and numerical modeling results suggested that the ITF plays an important role in propagating the Indian Ocean anomalies into the equatorial Pacific Ocean, and this ocean channel between the two basins is important for the evolution and predictability of ENSO.

    For the IOB, (Kug and Kang, 2006) and (Kug et al., 2006) revealed that positive phases tend to favor phase transition of El Niño to La Niña through anomalous easterly in the western Pacific, paying attention to the atmospheric teleconnection over the period 1958-2000. However, it is unclear whether the modulation of the oceanic channel between the Indian and Pacific oceans plays a role in the relationship between Indian Ocean SSTAs and ENSO transition.

    As reviewed above, the relationships between the IOB/ IOD over the tropical Indian Ocean and ENSO evolution have previously been examined somewhat in isolation and through using data with different time periods. We note that the impacts of the IOB and IOD have not been discussed and compared together through observational diagnostics. In particular, IOD events are always followed by IOB events, but previous studies do not consider the effect between these two modes. Moreover, numerical simulations are inconsistent with observational results. For instance, using a CGCM, (Ohba and Ueda, 2007) indicated that during boreal winter the IOB enhances surface easterlies over the equatorial western Pacific during the mature-decay phase of El Niño, which induces a transition to a La Niña phase through upwelling Kelvin waves. However, their model experiment did not reproduce the significant impact of the IOD on the Pacific. Thus, the first purpose of the present paper is to compare the relationships between the IOB/IOD and ENSO phase transition using observational data, after excluding the effect of the preceding IOD and following IOB through partial correlation analysis and composite analysis. In addition, for the IOB, previous studies focused only on the atmospheric bridge process, neglecting the role of the oceanic channel process in the relationships with ENSO transition. Therefore, the second purpose of the present paper is to discuss whether the relative role of the oceanic channel is similar for these two leading SSTAs modes.

    The present study investigates the impact of the external forcing of the tropical Indian Ocean on the Pacific. We attempt to reveal and compare the relationships between the two major SSTAs patterns in the tropical Indian Ocean and the decay and phase transition of ENSO in the following year using observational data from 1958 to 2008, as well as discuss the dynamics associated with the oceanic channel, especially after separating the effect of the IOD and IOB. Our comparison of the impacts of the IOB and IOD will provide a more comprehensive understanding of the role of the Indian Ocean in ENSO variability, and confirm previous model results. Note that the present paper is not to discuss whether or not ENSO triggers the SSTAs in the tropical Indian Ocean. We only study the impacts of the Indian Ocean on the Pacific, and consider how they influence ENSO decay and transition in the following year when the Indian Ocean SSTAs patterns appear.

2. Data
  • The latest version of ERSST, i.e., ERSST.v3b, is obtained from http://www.esrl.noaa.gov/psd/. The field is on a 2°× 2° global latitude-longitude grid (Xue et al., 2003; Smith et al., 2008). The other SST data used in this study are from the HADISST dataset (Rayner et al., 2003), compiled on a 1°× 1° latitude-longitude grid.

    The subsurface temperature, salinity, sea level and horizontal velocity data are obtained from the monthly means of the SODA (version 2.1.6) product for 1958-2008 (Carton and Giese, 2008). This dataset is available at a 0.5°× 0.5° horizontal resolution and has 40 vertical levels with 10-m spacing near the surface. (Xie et al., 2002) indicated that SODA agrees well with expendable bathythermograph data; plus, many previous studies have used SODA data to calculate ITF transport (e.g., England and Huang, 2005; Potemra, 2005; Tillinger and Gordon, 2009). Another reanalysis dataset used is the ECMWF's ORAS4 (Ocean Reanalysis System 4). These data (Mogensen et al., 2012; Balmaseda et al., 2013) are available at a horizontal resolution of 1°× 1°, with 42 vertical levels and an approximate 10-m level thickness in the upper 200 m. Compared with conventional observational datasets, these two reanalysis datasets provide a longer time record (1958-2008) to investigate the relationship between the ITF and Indo-Pacific variability on the interannual to decadal timescales.

    The analysis period for this study is 51 years, from January 1958 to December 2008, because the SODA data are only available from 1958 to 2008. The annual cycle of each variable is removed by subtracting the monthly mean at each grid point. And the anomalies used in the present paper are detrended after removing the annual cycle.

3. Results
  • The IOB and IOD over the tropical Indian Ocean are derived from EOF-1 and EOF-2 of the ERSST data, respectively (Figs. 1a and 1b). EOF-1 and EOF-2 account for 32% and 14% of the total variance, respectively. Note that the anomalies used in this study are detrended after removing the annual cycle. Through analyzing the principal component (PC) of EOF (Fig. 1c), it can be seen that the seasonal variability of the IOB (PC1) and IOD (PC2) has an obvious phase-locking feature (Fig. 1d). The IOB is strongest in boreal winter (February to March). The maximum of the IOD occurs in fall (July to October) in the ERSST data, which shows a maximum in September to October in the HADISST data (not shown). In order to investigate the influences of the IOB and IOD on the temporal evolution of ENSO, PC1 during February-March is chosen to define the boreal winter IOB index (WBI), and we use PC2 during September-October as the boreal fall IOD index (FDI). Note that different months are used to accord the seasonal dependence of the IOB and IOD. Moreover, the following results associated with the WBI and FDI are not sensitive to the months used for the definition of these two indexes. PC1 (PC2) during January-March (August-October) shows similar results.

    Figure 2.  Left-hand panels: lagged correlations between the WBI and equatorial SSTAs (2°S-2°N) from September (year 0) to June (year 2). Right panels: lagged correlations between the FDI and equatorial SSTAs. In the bottom panels the partial influence of the IOD (left-hand panel) and IOB (right-hand panel) on the equatorial SSTAs is removed using the partial correlation. The color shading indicates positive (warm color) and negative correlations (cold color) above the 90% significance level.

    Figure 3.  Time series of the WBI (red), FDI (blue) and Niño3.4 SSTAs during December-February (black).

    We denote the ENSO-developing year as year 0, the decaying year as year 1, and the following year as year 2. The top panels of Fig. 2 give the lagged correlations between the WBI/FDI and the equatorial (2°S-2°N) SSTAs from September (year 0) to June (year 2) using the ERSST dataset. The longitude-time sections of the lagged correlation are used to describe the influence of the IOB and IOD in the tropical Indian Ocean on the temporal evolution of ENSO, separately. It can be seen that the WBI is closely correlated with the following equatorial eastern Pacific SSTAs. In boreal winter (year 0), the correlation is positive in the Indian Ocean, showing the effect of a positive IOB during the peak phase. In addition, positive correlation in the equatorial eastern Pacific indicates that a positive IOB tends to co-occur with El Niño. In the following spring to summer (year 1), the positive correlation in the Indian Ocean begins to weaken, which means that the IOB decays. In the meantime, the positive correlation in the equatorial eastern Pacific diminishes and rapidly changes to an opposite sign, suggesting that a positive IOB of the Indian Ocean leads to a relatively faster termination of El Niño and phase transition. The La Niña then begins from July (year 1) and persists until the next year's spring. The situation is similar for the FDI, except for a negative correlation over the eastern Indian Ocean and western Pacific Ocean during the first winter (year 0), showing the effect of a peak positive IOD phase. For negative phases of the IOB and IOD, the conditions are the same but with anomalies of opposite sign. The relationship between the WBI/FDI and ENSO evolution is also obtained from the HADISST dataset. The above results show that both the IOB and IOD in the tropical Indian Ocean possibly feed back negatively on ENSO evolution.

    Because IOD events are always followed by IOB events, one may imagine that the correlation between the tropical Indian SSTAs and the ENSO phase transition during the following year is induced only by the IOD. Furthermore, we use partial correlation (Cohen and Cohen, 1983) to remove the partial influence of the preceding IOD and following IOB, respectively. This method has been used in many previous studies (e.g., Saji and Yamagata, 2003; Yu et al., 2005; Kug and Kang, 2006; Izumo et al., 2014). As shown in the bottom panels of Fig. 2, in the following year, the significant negative lag correlations in the equatorial central-eastern Pacific still exist for both the IOB and IOD after excluding each other's effects. Therefore, the lagged teleconnection between the IOB/IOD and ENSO transition is not interdependent. Compared to the IOB, however, the negative lagged correlation in the equatorial eastern Pacific during summer to winter (year 1) is obviously weaker for the IOD.

    Figure 4.  Composites of tropical SSTAs for the difference between independent positive and negative IOB events (left-hand panels) and the difference between independent positive and negative IOD events (right-hand panels) in different seasons over the period 1958-2008: (a) fall (year 0) [SON(0)]; (b) winter (year 0) [D(0)JF(1)]; (c) spring (year 1) [MAM(1)]; (d) summer (year 1) [JJA(1)]; (e) fall (year 1) [SON(1)]; (f) winter (year 1) [D(1)JF(2)]; and (g) spring (year 2) [MAM(2)]. Color shading indicates that the positive (negative) SSTAs (°C) greater (less) than 0.2.

    One may doubt that the above lagged correlation may not imply a cause-effect relationship. To clarify this problem, composite analysis is further conducted. Figure 3 shows the time series of the WBI, FDI and Niño3.4 SSTAs during December-February. The Niño3.4 SSTAs are averaged over (5°S-5°N, 170°-120°W). It can be seen that some positive (negative) IOB and IOD events concur with El Niño (La Niña) events. In addition, in the following year, the tropical Pacific is mostly in its cold (warm) phase. Similarly, it is necessary to separate the effect of the IOD and IOB due to some of IOD events coinciding with IOB events. Ultimately, eight independent IOB events over the period 1958-2008, each greater than one standard deviation and not coincident with IOB events (positive events: 1958, 1968, 1969, 1987; negative events: 1967, 1970, 1983, 1985), are chosen to carry out the composite analysis. For the composite IOD, there are 14 independent events greater than one standard deviation and not coincident with IOB events (positive events: 1961, 1967, 1977, 1978, 1986, 1994, 2006; negative events: 1958, 1960, 1971, 1974, 1988, 1996, 1998).

    Figure 4 shows the composites of the tropical SSTAs for the difference between independent positive and negative IOB events (left panels) and the difference between independent positive and negative IOD events (right panels) from the boreal fall (year 0) season [SON(0), September-November(0)] through the spring (year 2) season [MAM(2), March-May(2)]. For the IOB (left panels of Fig. 4), the SSTAs are positive in the Indian Ocean in the winter (year 0) [D(0)JF(1), December(0)-February(1)], which is accompanied by warming in the central-eastern Pacific. However, the positive SSTAs in the central-eastern Pacific become weak in the coming spring (year 1) [MAM(1), March-May(1)] and changes to negative in the following summer (year 1) [JJA(1), June-August(1)]. These negative SSTAs develop in the following fall (year 1) and winter (year 1) seasons. For the IOD (right panels of Fig. 4), the SSTAs are negative in the tropical eastern Indian Ocean and positive in the western and central Indian Ocean in the late fall (year 0), which reflects the impact of the peak positive IOD phase. At the same time, a significant teleconnection is demonstrated by the positive SSTAs in the eastern Pacific and the negative values in the western Pacific. The SSTAs over the equatorial central-eastern Pacific change to an opposite sign in the following spring and persists in the following summer to winter seasons. The SSTA's evolution shows some differences between the IOB and IOD. The anomalies in the central-eastern Pacific during the second year induced by the IOB are obviously stronger than those induced by the IOD, and the former is located in the equatorial central-eastern Pacific while the latter is situated in the eastern Pacific. Thus, similar to the result of the lagged correlation analysis, composite analysis supports our conclusion.

    In previous studies, the relationships between the IOB/ IOD and ENSO evolution have been examined in isolation. In particular, the effect between these two modes has not been considered. Moreover, the model experiment of (Ohba and Ueda, 2007) did not reproduce this significant connection between the IOD and the tropical central-eastern Pacific. Our result, using partial correlation and composite analysis and the same data, indicates that both the IOB and IOD could lead to ENSO phase transition. The difference is that the influence induced by the IOB is stronger than that by the IOD.

  • (Izumo et al., 2014) indicated that a positive IOD in fall and a positive IOB in winter both promote a transition of ENSO events during the coming year, through the wind anomalies over the western Pacific promoted by both IOD and IOB events (e.g., Kug and Kang, 2006; Ohba and Ueda, 2007). In addition to the atmospheric bridge, some studies have indicated an influence of the tropical Indian on ENSO transition through modulation of the ITF (Sprintall et al., 2000; Wijffels and Meyers, 2004; Kandaga et al., 2009; Drushka et al., 2010; Yuan et al., 2011, 2013). Yuan et al. (2011, 2013) suggested that the IOD could induce ENSO phase transition through transport variations of the ITF, using observational data and numerical experiments. However, the role of the ITF in the relationship between the IOB and SSTAs evolution in the equatorial Pacific has not been discussed. Therefore, it is necessary to compare the relative roles of the oceanic channel process in the relationship between the IOB/IOD and ENSO evolution in the following year.

    Figure 5.  Left-hand panels: partial correlations between the WBI and subsurface temperature anomalies in the Indian-Pacific equatorial vertical section from SODA data in different seasons, in which the partial influence of the IOD on the equatorial SSTAs is removed using the partial correlation. Right-hand panels: as in the left-hand panels, but for the FDI, in which the partial influence of the IOB is removed. Color shading indicates positive and negative correlations above the 90% significance level.

    Figure 6.  Composites of tropical subsurface temperature anomalies (°C) in the Indian-Pacific equatorial vertical section from the SODA data for the difference between independent positive and negative IOB events (left-hand panels) and the difference between independent positive and negative IOD events (right-hand panels). Color shading indicates that the positive (negative) subsurface temperature anomalies greater (less) than 0.5.

    Figure 7.  Low-pass-filtered time series of the monthly ITF transport anomalies from the SODA data (top panel) and ECMWF ORAS4 data (bottom panel).

    From the above analysis in section 3.1, it can be seen that the SSTAs in the equatorial central-eastern Pacific about 1 year later do not come from the horizontal propagation of the SST signal elsewhere. For the IOD, (Yuan et al., 2013) indicated that lagged correlations in the subsurface temperature in a vertical section of the equatorial Pacific Ocean suggest eastward propagation of the upwelling anomalies from the Indian Ocean into the equatorial Pacific Ocean through the seas of Indonesia. This result seems to suggest that the ocean channel connection between the two basins is important for the evolution and predictability of ENSO. Following the work of (Yuan et al., 2013), Fig. 5 shows the partial correlations between the WBI (FDI) and subsurface temperature anomalies in a vertical section of the equatorial Indian and Pacific oceans from the SODA data, in which the partial influence of the IOD (IOB) on the equatorial SSTAs is removed using the partial correlation. Similar to the result of the IOD (right-hand panels), for the IOB (left-hand panels) the cold SSTAs in the equatorial central-eastern Pacific in the following year come from cold subsurface temperature anomalies in the equatorial eastern Indian and western Pacific oceans, which propagate eastward and upward along the thermocline to arrive at the surface. The result from the ECMWF ORAS4 data (not shown) is consistent with that of the SODA data. Furthermore, composites of tropical subsurface temperature anomalies for the difference between independent positive and negative IOB (IOD) events also support the result of the partial correlation analysis (Fig. 6).

    However, (Kug and Kang, 2006) and (Ohba and Ueda, 2007) indicated that the generation and propagation of subsurface temperature anomalies in the Pacific possibly comes from the atmospheric bridge process associated with the IOB/IOD. Anomalous easterlies (westerlies) in the equatorial western Pacific during the mature phase of El Niño (La Niña), exciting the equatorial oceanic Kelvin wave, plays a role in ENSO transition. Therefore, in addition to the oceanic channel, Figs. 5 and 6 also include the effect of the atmospheric bridge process related to the IOB/IOD. It is necessary to clarify the relative role of the oceanic channel process in the relationship between the IOB/IOD and ENSO transition.

    Figure 7 shows the time series of the monthly transport anomalies of the ITF dealing with the low-pass-filter. The anomalies of the ITF volume transport are defined as the depth-integrated northward velocity in reference to the 729-m level of no motion through a zonal chokepoint section at 8.25°S, based on the SODA data. The time series are filtered by a Gaussian filter using a cutoff period at 13 months. Negative values indicate transports from the Pacific to the Indian Ocean. England and Huang (2004) suggested that the ITF from the SODA reanalysis data is a reasonably accurate reconstruction of the observed ITF, and the ITF transport anomalies derived from the geostrophic flow show broadly similar behavior. The ECMWF ORAS4 data show a similar temporal evolution.

    Figure 8 shows the lagged correlations between the ITF transport anomalies during the peak phase of the IOB/IOD and subsurface temperature anomalies in the equatorial Indian and Pacific vertical section during the following seasons from the SODA data. Clearly, for the IOD (right-hand panels), the cold subsurface temperature anomalies in the eastern Indian Ocean and western Pacific Ocean, associated with the ITF transports anomalies, could propagate eastward and upward along the thermocline to arrive at the surface in the central-eastern Pacific during the following winter (year 1). For the IOB (left-hand panels), however, the negative anomalies only stay in the tropical western Pacific and the seas of Indonesia in winter (year 0), which weakens in the following summer and disappears in the following fall (year 1). The ITF transport anomalies during the peak phase of the IOB are not significantly correlated with subsurface temperature anomalies in the central-eastern Pacific in the following summer (year 1) to winter (year 1). Thus, the ITF transport anomalies related to the IOD might induce the SSHAs and SSTAs in the central-eastern Pacific during the following seasons through the eastward propagation of upwelling Kelvin waves. However, the ITF influence induced by the IOB only stays and disappears in the tropical western Pacific without propagation to the equatorial eastern Pacific to affect the temperature in the cold tongue. The result from the ECMWF ORAS4 data is consistent with that of the SODA data (not shown).

    To clearly show the contribution of the ITF transport anomalies induced by the IOB/IOD on ENSO phase transition, we repeat the composites of tropical subsurface temperature anomalies in Fig. 6 after removing the influence of ITF transport anomalies during the peak phase of the IOD/IOB (Fig. 9). For the IOB (left-hand panels), the influence of the ITF anomalies on the equatorial Pacific subsurface temperature is very small, because the Pacific subsurface temperature anomalies still have similar values after removing the ITF signal. For the IOD (right-hand panels), the negative subsurface temperature anomalies are weakened in the equatorial Pacific in the following seasons, especially in the eastern Pacific, after the removal of the ITF signal induced by the IOD. This suggests that the oceanic channel between the tropical Indian and Pacific oceans contributes to the dynamics of the lagged teleconnection between the IOD and the ENSO evolution in the following year, but the ITF transport anomalies is ineffective as a link between the IOB signals and ENSO phase transition.

    Figure 8.  Left-hand panels: lagged correlations between the ITF transport anomalies during the peak phase of the IOB (February to March) and subsurface temperature anomalies in the Indian-Pacific equatorial vertical section from the SODA data in different seasons. Right-hand panels: as in the left-hand panels, but for the ITF transport anomalies during the peak phase of the IOD (September to October). Color shading indicates positive and negative correlations above the 90% significance level.

    Figure 9.  As in Fig. 6, but with the influence of the ITF transport anomalies removed.

4. Discussion
  • This paper only investigates the impact of external forcing from the tropical Indian Ocean. The internal dynamics over the tropical Pacific is still vital during ENSO evolution. The external forcing from the Indian Ocean may reinforce local processes in the Pacific, which could affect ENSO evolution in the coming year. It is well known that ENSO has a typical period of roughly 3-7 years and a biennial (2 yr) component. However, the quasi-biennial oscillation is not strong. Although lagged correlations between the Nino3.4 SSTAs index during the mature phase of ENSO and the equatorial Pacific SSTAs reverse to being negative during the following summer (year 1) to winter (year 1), the negative values are not significant (not shown). However, because of the influence of the tropical Indian Ocean, the ENSO phase reversal seems to complete in one year after the mature phase of ENSO (Fig. 2). This suggests that the ENSO cycle shows enhanced variability for periods of about 2 years. Also, previous modeling studies indicate that ENSO's biennial component significantly increases in the Indo-Pacific run, as compared to simulations only including Pacific coupling (e.g., Yu, 2005). (Izumo et al., 2014) also indicated that the interactions between ENSO, the IOD and the IOB operate on a biennial timescale.

    Many recent studies (e.g., Ohba and Ueda, 2009; Ohba et al., 2010; Okumura et al., 2011; Dommenget et al., 2013; Ohba, 2013) show that the transition process of ENSO is asymmetric. (Ohba and Watanabe, 2012) showed asymmetric impacts of the IOB on ENSO transition between the warming and cooling phase of ENSO. Moreover, it is found that the asymmetric impact on El Niño and La Niña also exist for the IOD when we separate the analysis for the positive and negative phase of the IOD. The details and relative roles of the atmospheric bridge and oceanic channel will be discussed in another paper.

    The relationship between the tropical Indian Ocean SSTAs and ENSO transition is influenced by multiple factors. Many studies have documented interdecadal variability in ENSO or the Indian Ocean SSTAs. (Izumo et al., 2014) explored the interdecadal robustness of the influence of the IOD and Pacific recharge on the following year's El Niño over the period 1872-2008, with a focus on the atmospheric bridge process. (Yuan et al., 2013) concluded that the dynamics are not related to the atmospheric bridge over the period 1990-2009, which supports the ITF connection. Thus, the relative roles of the atmospheric bridge and oceanic channel may be different in this lagged remote relationship over different periods. In addition to the FDI, the 1-year lagged correlations of the WBI with the following year's ENSO also possess significant interdecadal variability (not shown). Therefore, the lagged relationship between the tropical Indian Ocean SSTAs and ENSO transition on interdecadal timescales, and the associated dynamical processes, still need further investigation. In addition, tropical Atlantic warming during El Niño is also clear, and some of the differences between the two Indian Ocean SSTAs modes might originate from the Atlantic SST variability. Therefore, the role of Atlantic SST variability in Indian Ocean SST variability needs further investigation.

    Using a novel method based on information flow, (Liang, 2014) discussed the relationship between the IOD and ENSO. The study indicated that the IOD and ENSO are mutually causal, but the causality is asymmetric: the IOD functions to make ENSO more uncertain, while ENSO tends to stabilize the IOD. This new method has not been applied to investigate the influence of the IOB on ENSO. It would be of interest to discuss the relationship between the IOB and ENSO, including ENSO's phase transition.

5. Conclusion
  • The tropical Indian and Pacific oceans can affect one another. Rather than discussing whether or not ENSO triggers the SSTAs in the tropical Indian Ocean, the present paper focuses on the effects of the leading modes of the SSTAs in the tropical Indian Ocean on the variability of ENSO about 1 year later, which is not a synchronous influence. The SSTAs in the tropical Indian Ocean have two major modes: the IOB and IOD. Their occurrence influences the variation in the tropical Pacific Ocean.

    However, the influence on ENSO transition associated with the IOB and IOD has previously only been examined in isolation and using data with different time periods. In particular, whilst IOD events are always followed by IOB events, previous studies have not considered the effect between these two modes. Moreover, the model experiment of (Ohba and Ueda, 2007) did not reproduce this significant connection between the IOD and the tropical central-eastern Pacific. The present study compares the relationship between the IOB/IOD and ENSO transition in the following year using observational data for 1958-2008, through partial correlation analysis and composite analysis to separate the effect between the preceding IOD and following IOB. Our results indicate that both the positive (negative) phase of the IOB and IOD (independent of each other) in the tropical Indian Ocean are possible contributors to the El Niño (La Niña) decay and phase transition to La Niña (El Niño) about 1 year later. The difference is that the influence induced by the IOB is stronger than that by the IOD.

    The cold (warm) SSTAs in the equatorial central-eastern Pacific in the coming year originate from cold (warm) subsurface temperature anomalies in the equatorial eastern Indian and western Pacific Ocean. This signal propagates eastward and upward along the thermocline to arrive at the surface. Some studies have indicated that the generation and propagation of subsurface temperature anomalies might come from the modulation of the oceanic channel, in addition to the atmospheric bridge. However, for the IOB, previous studies only focused on the atmospheric bridge process, without discussion of the role of the oceanic channel process in the relationships with ENSO transition. Our results show that the relative contributions of the oceanic channel are different for the IOB and IOD. For the IOD, the Kelvin waves induced by the IOD could penetrate from the Indian Ocean into the western Pacific through the ITF, which further propagate eastward and upward to induce ENSO decay and transition in the following year. However, for the IOB, the associated Kelvin waves propagate to the equatorial western Pacific without propagation to the eastern Pacific, which disappear in the western Pacific during the coming summer-fall. The indication is that the ITF transport anomalies are ineffective as a link between the IOB signals and ENSO phase transition. However, why the Kelvin waves induced by the IOB are unable to propagate further to the eastern Pacific remains an open question.

    Figure 10.  Composites of tropical wind anomalies (m s-1) at 850 hPa in different seasons for the difference between independent positive and negative IOB events (left-hand panels) and the difference between independent positive and negative IOD events (right-hand panels).

    Furthermore, it can be seen from Fig. 10 that the atmospheric bridge process plays a dominant role in the lagged teleconnection between the IOB and ENSO phase transition. For the IOB, significant easterly anomalies over the western Pacific occur during D(0)JF(1), which sustain to the following summer [JJA(1)]. Through the baroclinic atmosphere Kelvin wave, the IOB in the Indian Ocean can affect the development of the anomalous easterlies in the western Pacific (e.g., Xie et al., 2009; Du et al., 2013), which then influence El Niño's transition. However, the easterly anomalies over the western Pacific associated with the IOD are very weak throughout. Thus, the effect of the IOD on ENSO transition through the atmospheric bridge is not dominant.

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