Baggett C., S. Lee, 2017: An identification of the mechanisms that lead to arctic warming during planetary-scale and synoptic-scale wave life cycles,J. Atmos. Sci.,74,1859-1877, http://dx.doi.org/10.1175/JAS-D-16-0156.1.10.1175/JAS-D-16-0156.16b096ffea05f1ebec6da0909125c6de4http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F315534926_An_identification_of_the_mechanisms_that_lead_to_Arctic_warming_during_planetary-scale_and_synoptic-scale_wave_life_cycleshttp://journals.ametsoc.org/doi/10.1175/JAS-D-16-0156.1Abstract The dynamical mechanisms that lead to wintertime Arctic warming during the planetary-scale wave (PSW) and synoptic-scale wave (SSW) life cycles are identified by performing a composite analysis of ERA-Interim data. The PSW life cycle is preceded by localized tropical convection over the western Pacific. Upon reaching the midlatitudes, the PSWs amplify as they undergo baroclinic conversion and constructively interfere with the climatological stationary waves. The PSWs flux large quantities of sensible and latent heat into the Arctic, which produces a regionally enhanced greenhouse effect that increases downward IR and warms the Arctic 2-m temperature. The SSW life cycle is also capable of increasing downward IR and warming the Arctic 2-m temperature, but the greatest warming is accomplished in the subset of SSW events with the most amplified PSWs. Consequently, during both the PSW and SSW life cycles, wintertime Arctic warming arises from the amplification of the PSWs.
Baggett C., S. Lee, and S. Feldstein, 2016: An investigation of the presence of atmospheric rivers over the North Pacific during planetary-scale wave life cycles and their role in Arctic warming,J. Atmos. Sci.,73,4329-4347, http://dx.doi.org/10.1175/JAS-D-16-0033.1.10.1175/JAS-D-16-0033.188b7a8cc6357e0905b6ceef8733f2c9dhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F306244253_An_investigation_of_the_presence_of_atmospheric_rivers_over_the_North_Pacific_during_planetary-scale_wave_life_cycles_and_their_role_in_Arctic_warminghttp://journals.ametsoc.org/doi/10.1175/JAS-D-16-0033.1Abstract Heretofore, the tropically excited Arctic warming (TEAM) mechanism put forward that localized tropical convection amplifies planetary-scale waves, which transport sensible and latent heat into the Arctic, leading to an enhancement of downward infrared radiation and Arctic surface warming. In this study, an investigation is made into the previously unexplored contribution of the synoptic-scale waves and their attendant atmospheric rivers to the TEAM mechanism. Reanalysis data are used to conduct a suite of observational analyses, trajectory calculations, and idealized model simulations. It is shown that localized tropical convection over the Maritime Continent precedes the peak of the planetary-scale wave life cycle by ~10-14 days. The Rossby wave source induced by the tropical convection excites a Rossby wave train over the North Pacific that amplifies the climatological December-March stationary waves. These amplified planetary-scale waves are baroclinic and transport sensible and latent heat poleward. During the planetary-scale wave life cycle, synoptic-scale waves are diverted northward over the central North Pacific. The warm conveyor belts associated with the synoptic-scale waves channel moisture from the subtropics into atmospheric rivers that ascend as they move poleward and penetrate into the Arctic near the Bering Strait. At this time, the synoptic-scale waves undergo cyclonic Rossby wave breaking, which further amplifies the planetary-scale waves. The planetary-scale wave life cycle ceases as ridging over Alaska retrogrades westward. The ridging blocks additional moisture transport into the Arctic. However, sensible and latent heat amounts remain elevated over the Arctic, which enhances downward infrared radiation and maintains warm surface temperatures.
Bender F. A.-M., V. Ramananthan, and G. Tselioudis, 2012: Changes in extratropical storm track cloudiness 1983-2008: Observational support for a poleward shift.Climate Dyn.,38,2037-2053, https://doi.org/10.1007/s00382-011-1065-6.10.1007/s00382-011-1065-606bb353d1246a02ad60dcdddb9de50c0http%3A%2F%2Fwww.springerlink.com%2Fcontent%2Fq0q837g3363q435g%2Fhttp://link.springer.com/10.1007/s00382-011-1065-6Climate model simulations suggest that the extratropical storm tracks will shift poleward as a consequence of global warming. In this study the northern and southern hemisphere storm tracks over the P
Comiso J. C., C. L. Parkinson, R. Gersten, and L. Stock, 2008: Accelerated decline in the Arctic sea ice cover,Geophys. Res. Lett.,35,L01703, https://doi.org/10.1029/2007GL031972.10.1029/2007GL031972343feb606e7415f45d25b40e917085b6http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2007GL031972%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1029/2007GL031972/pdfSatellite data reveal unusually low Arctic sea ice coverage during the summer of 2007, caused in part by anomalously high temperatures and southerly winds. The extent and area of the ice cover reached minima on 14 September 2007 at 4.1 10kmand 3.6 10km, respectively. These are 24% and 27% lower than the previous record lows, both reached on 21 September 2005, and 37% and 38% less than the climatological averages. Acceleration in the decline is evident as the extent and area trends of the entire ice cover (seasonal and perennial ice) have shifted from about -2.2 and -3.0% per decade in 1979-1996 to about -10.1 and -10.7% per decade in the last 10 years. The latter trends are now comparable to the high negative trends of -10.2 and -11.4% per decade for the perennial ice extent and area, 1979-2007.
Cullather R. I., D. H. Bromwich, and M. C. Serreze, 2000: The atmospheric hydrologic cycle over the Arctic basin from reanalyses,Part I: Comparison with observations and previous studies J. Climate,13,923-937, http://dx.doi.org/10.1175/1520-0442(2000)013<0923:TAHCOT>2.0.CO;2.10.1175/1520-0442(2000)013<0923:TAHCOT>2.0.CO;2http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%282000%29013%3C0923%3ATAHCOT%3E2.0.CO%3B2
Dacre H. F., P. A. Clark, O. Martinez-Alvarado, M. A. Stringer, and D. A. Lavers, 2015: How do atmospheric rivers form? Bull,Amer. Meteor. Soc.,96,1243-1255, http://dx.doi.org/10.1175/BAMS-D-14-00031.1.10.1175/BAMS-D-14-00031.107b34f70a28814e9212ebf58d9179d0chttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015BAMS...96.1243Dhttp://journals.ametsoc.org/doi/10.1175/BAMS-D-14-00031.1Identifying the source of atmospheric rivers: Are they rivers of moisture exported from the subtropics or footprints left behind by poleward travelling storms? The term atmospheric river is used to describe corridors of strong water vapor transport in the troposphere. Filaments of enhanced water vapor, commonly observed in satellite imagery extending from the subtropics to the extratropics, are routinely used as a proxy for identifying these regions of strong water vapor transport. The precipitation associated with these filaments of enhanced water vapor can lead to high impact flooding events. However, there remains some debate as to how these filaments form. In this paper we analyse the transport of water vapor within a climatology of wintertime North Atlantic extratropical cyclones. Results show that atmospheric rivers are formed by the cold front which sweeps up water vapor in the warm sector as it catches up with the warm front. This causes a narrow band of high water vapor content to form ahead of the cold front at the base of the warm conveyor belt airflow. Thus, water vapor in the cyclone's warm sector, and not long-distance transport of water vapor from the subtropics, is responsible for the generation of filaments of high water vapor content. A continuous cycle of evaporation and moisture convergence within the cyclone replenishes water vapor lost via precipitation. Thus, rather than representing a direct and continuous feed of moist air from the subtropics into the centre of a cyclone (as suggested by the term atmospheric river), these filaments are, in-fact, the result of water vapor exported from the cyclone and thus they represent the footprints left behind as cyclones travel polewards from subtropics.
Dickson, R. R., Coauthors, 2000: The Arctic Ocean response to the North Atlantic oscillation,J. Climate,13,2671-2696, http://dx.doi.org/10.1175/1520-0442(2000)013<2671: TAORTT>2.0.CO;2.10.1175/1520-0442(2000)013<2671:TAORTT>2.0.CO;22cb74465f1cd05cffd6584b2c28a2b50http%3A%2F%2Ficesjms.oxfordjournals.org%2Fexternal-ref%3Faccess_num%3D10.1175%2F1520-0442%282000%29013%26lt%3B2671%3ATAORTT%26gt%3B2.0.CO%3B2%26amp%3Blink_type%3DDOIhttp://journals.ametsoc.org/doi/abs/10.1175/1520-0442%282000%29013%3C2671%3ATAORTT%3E2.0.CO%3B2
Francis J. A., E. Hunter, 2006: New insight into the disappearing arctic sea ice,EOS,87,509-511, http://dx.doi.org/10.1029/2006EO460001.10.1029/2006EO460001d6b349fc0f42dc1ed3fcb80200128015http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2006EO460001%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1029/2006EO460001/pdfThe dramatic loss of Arctic sea ice is ringing alarm bells in the minds of climate scientists, policy makers, and the public. The extent of perennial sea icece that has survived a summer melt seasonas declined 20% since the mid-1970s [Stroeue et al., 2005]. Its retreat varies regionally, driven by changes in winds and heating from the atmosphere and ocean.Limited data have hampered attempts to identify which culprits are to blame, but new satellite-derived information provides insight into the drivers of change. A clear message emerges. The location of the summer ice edge is strongly correlated to variability in longwave (infrared) energy emitted by the atmosphere (downward longwave flux; DLF), particularly during the most recent decade when losses have been most rapid. Increasing DLF, in turn, appears to be driven by more clouds and water vapor in spring over the Arctic.
Gong T. T., S. Feldstein, and S. Lee, 2017: The role of downward infrared radiation in the recent arctic winter warming trend,J. Climate,30,4937-4949, http://dx.doi.org/10.1175/JCLI-D-16-0180.1.10.1175/JCLI-D-16-0180.12fbbd597b3508f74aae10965045bc9cdhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F313022475_The_Role_of_Downward_Infrared_Radiation_in_the_Recent_Arctic_Winter_Warming_Trendhttp://journals.ametsoc.org/doi/10.1175/JCLI-D-16-0180.1react-text: 155 The Tropically Excited Arctic Warming (TEAM) mechanism ascribes warming of the Arctic surface to tropical convection, which excites poleward-propagating Rossby wave trains that transport water vapor and heat into the Arctic. A crucial component of the TEAM mechanism is the increase in downward infrared radiation (IR) that precedes the Arctic warming. Previous studies have examined the downward... /react-text react-text: 156 /react-text [Show full abstract]
Groves D. G., J. A. Francis, 2002: Moisture budget of the arctic atmosphere from TOVS satellite data,J. Geophys. Res.,107,ACL 11-1-ACL 11-21, http://dx.doi.org/10.1029/2001JD001191.10.1029/2001JD0011465a7d23256ae501c8231bfb5ee34114fbhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2001JD001146%2Fabstracthttp://doi.wiley.com/10.1029/2001JD001146Meteorological data from the United Kingdom Meteorological Office (UKMO) and constituent data from the Upper Atmospheric Research Satellite (UARS) are used to construct yearly zonal mean dynamical fields for the 1990s for use in the NASA/Goddard Space Flight Center (GSFC) two-dimensional (2-D) chemistry and transport model. This allows for interannual dynamical variability to be included in the model constituent simulations. In this study, we focus on the tropical stratosphere. We find that the phase of quasi-biennial oscillation (QBO) signals in equatorial CHand profile and total column Odata are resolved quite well using this empirically based 2-D model transport framework. However, the QBO amplitudes in the model constituents are systematically underestimated relative to the observations at most levels. This deficiency is probably due in part to the limited vertical resolutions of the 2-D model and the UKMO and UARS input data sets. We find that using different heating rate calculations in the model affects the interannual and QBO amplitudes in the constituent fields, but has little impact on the phase. Sensitivity tests reveal that the QBO in transport dominates the ozone interannual variability in the lower stratosphere, with the effect of the temperature QBO being dominant in the upper stratosphere via the strong temperature dependence of the ozone loss reaction rates. We also find that the QBO in odd nitrogen radicals, which is caused by the QBO modulated transport of NO, plays a significant but not dominant role in determining the ozone QBO variability in the middle stratosphere. The model mean age of air is in good overall agreement with that determined from tropical lower-middle stratospheric OMS balloon observations of SFand CO. The interannual variability of the equatorial mean age in the model increases with altitude and maximizes near 40 km, with a range of 4-5 years over the 1993-2000 time period.
Guan B., D. E. Waliser, 2015: Detection of atmospheric rivers: Evaluation and application of an algorithm for global studies,J. Geophys. Res.: Atmos.,120,12514-12535, http://dx.doi.org/10.1002/2015JD024257.10.1002/2015JD024257bae9432d688bd7125cab4b7c318162d4http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2015JD024257%2Fpdfhttp://doi.wiley.com/10.1002/2015JD024257Atmospheric rivers (ARs) are narrow, elongated, synoptic jets of water vapor that play important roles in the global water cycle and regional weather/hydrology. A technique is developed for objective detection of ARs on the global domain based on characteristics of the integrated water vapor transport (IVT). AR detection involves thresholding 6-hourly fields of ERA-Interim IVT based on the 85th percentile specific to each season and grid cell and a fixed lower limit of 100 kg msand checking for the geometry requirements of length >2000 km, length/width ratio >2, and other considerations indicative of AR conditions. Output of the detection includes the AR shape, axis, landfall location, and basic statistics of each detected AR. The performance of the technique is evaluated by comparison to AR detection in the western North America, Britain, and East Antarctica with three independently conducted studies using different techniques, with over ~90% agreement in AR dates. Among the parameters tested, AR detection shows the largest sensitivity to the length criterion in terms of changes in the resulting statistical distribution of AR intensity and geometry. Global distributions of key AR characteristics are examined, and the results highlight the global footprints of ARs and their potential importance on global and regional scales. Also examined are seasonal dependence of AR frequency and precipitation and their modulation by four prominent modes of large-scale climate variability. The results are in broad consistency with previous studies that focused on landfalling ARs in the west coasts of North America and Europe.
Hurrell J. W., 2015: Climate variability: North Atlantic and Arctic oscillation. Encyclopedia of Atmospheric Sciences. 2nd ed., G. R. North et al., Eds., Elsevier Ltd., 47- 60.
Jakobson E., T. Vihma, 2010: Atmospheric moisture budget in the Arctic based on the ERA-40 reanalysis,International Journal of Climatology,30,2175-2194, http://dx.doi.org/10.1002/joc.2039.10.1002/joc.2039dbbf63028b7016ffcc2f3f8debc0d64ehttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fjoc.2039%2Ffullhttp://doi.wiley.com/10.1002/joc.v30%3A14The atmospheric moisture budget in the Arctic in 1979�2001 was analysed on the basis of the ERA-40 reanalysis. Zonal variations in the profiles of specific humidity mainly occur at altitudes below 5 km. The moisture transport peaks at altitudes lower than previously suggested; the median peak level of meridional moisture flux (MMF) across 70ºN is in winter at 930 hPa pressure level and in other seasons at 970�990 hPa level. Mean precipitable water for the polar cap (70�90ºN) ranges from 2.4 mm in winter to 12.3 mm in summer. Transient eddies (TE) are responsible for most of the water vapour transport across 70ºN by providing from 81% of MMF in winter to 92% of MMF in summer. The contribution by stationary eddies (SE) ranges from 5 to 9%, whereas the contribution of mean meridional circulation (MMC) ranges from 1% in summer to 12% in winter. Relative inter-annual variation in MMF components is highest for SE (standard deviation/mean = 133%), second highest for the MMC (61%) and smallest for TE (4%). The MMF across 70ºN accounts for 59% of the annual precipitation. Averaged for the polar cap, the mean annual moisture flux convergence (192 mm) and net precipitation (179 mm) are close to each other, but local differences exceeding 200 mm occur at several places. Over the open ocean, the moisture flux convergence is considered more reliable. The Arctic Oscillation (AO) index correlates with MMF in spring and winter (correlation coefficient r = 0.75) and with net precipitation in spring (r = 0.61) and winter (r = 0.50). The AO and precipitable water correlate in Canada and Greenland in winter and spring (r = 61 0.7) and in Europe in winter (r = 0.8). Copyright 08 2009 Royal Meteorological Society
Kalnay, E., Coauthors, 1996: The NCEP/NCAR 40-year reanalysis project,Bull. Amer. Meteor. Soc.,77,437-470, http://dx.doi.org/10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2.10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;29bfeacc7ab553b364e43408563ad850bhttp%3A%2F%2Fintl-icb.oxfordjournals.org%2Fexternal-ref%3Faccess_num%3D10.1175%2F1520-0477%281996%290772.0.CO%3B2%26amp%3Blink_type%3DDOIhttp://journals.ametsoc.org/doi/abs/10.1175/1520-0477%281996%29077%3C0437%3ATNYRP%3E2.0.CO%3B2
Kay J. E., A. Gettelman, 2009: Cloud influence on and response to seasonal Arctic sea ice loss,J. Geophys. Res.,114,D18204, http://dx.doi.org/10.1029/2009JD011773.10.1029/2009JD011773445f9e6f1149926fa32316947d97ea04http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2009JD011773%2Fpdfhttp://doi.wiley.com/10.1029/2009JD011773Recent declines in Arctic sea ice extent provide new opportunities to assess cloud influence on and response to seasonal sea ice loss. This study combines unique satellite observations with complementary data sets to document Arctic cloud and atmospheric structure during summer and early fall. The analysis focuses on 2006-2008, a period over which ice extent plummeted to record levels, substantial variability in atmospheric circulation patterns occurred, and spaceborne radar and lidar observations of vertical cloud structure became available. The observations show that large-scale atmospheric circulation patterns, near-surface static stability, and surface conditions control Arctic cloud cover during the melt season. While no summer cloud response to sea ice loss was found, low clouds did form over newly open water during early fall. This seasonal variation in the cloud response to sea ice loss can be explained by near-surface static stability and air-sea temperature gradients. During summer, temperature inversions and weak air-sea temperature gradients limit atmosphere-ocean coupling. In contrast, relatively low static stability and strong air-sea gradients during early fall permit upward turbulent fluxes of moisture and heat and increased low cloud formation over newly open water. Because of their seasonal timing, cloud changes resulting from sea ice loss play a minor role in regulating ice-albedo feedbacks during summer, but may contribute to a cloud-ice feedback during early fall.
Kay J. E., T. L'Ecuyer A. Gettelman, G. Stephens, and C. O'Dell, 2008: The contribution of cloud and radiation anomalies to the 2007 Arctic sea ice extent minimum,Geophys. Res. Lett.,35,L08503, http://dx.doi.org/10.1029/2008GL033451.10.1029/2008GL0334516a372a66671eaa9ae3d3f2ecaaa559bdhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2008GL033451%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2008GL033451/fullReduced cloudiness and enhanced downwelling radiation are associated with the unprecedented 2007 Arctic sea ice loss. Over the Western Arctic Ocean, total summertime cloud cover estimated from spaceborne radar and lidar data decreased by 16% from 2006 to 2007. The clearer skies led to downwelling shortwave (longwave) radiative fluxes increases of +32 Wm(-4 Wm) from 2006 to 2007. Over three months, simple calculations show that these radiation differences alone could enhance surface ice melt by 0.3 m, or warm the surface ocean by 2.4 K, which enhances basal ice melt. Increased air temperatures and decreased relative humidity associated with an anti-cyclonic atmospheric circulation pattern explain the reduced cloudiness. Longer-term observations show that the 2007 cloudiness is anomalous in the recent past, but is not unprecedented. Thus, in a warmer world with thinner ice, natural summertime circulation and cloud variability is an increasingly important control on sea ice extent minima.
Kim, B.-M., Coauthors, 2017: Major cause of unprecedented arctic warming in January 2016: Critical role of an Atlantic windstorm,Scientific Reports,7,40051, http://dx.doi.org/10.1038/srep40051.10.1038/srep4005152097050f83e82b57ad64399928d1fa73d034cfhttp%3A%2F%2Fwww.nature.com%2Farticles%2Fsrep40051http://www.nature.com/articles/srep40051In January 2016, the Arctic experienced an extremely anomalous warming event after an extraordinary increase in air temperature at the end of 2015. During this event, a strong intrusion of warm and moist air and an increase in downward longwave radiation, as well as a loss of sea ice in the Barents and Kara seas, were observed. Observational analyses revealed that the abrupt warming was triggered by the entry of a strong Atlantic windstorm into the Arctic in late December 2015, which brought enormous moist and warm air masses to the Arctic. Although the storm terminated at the eastern coast of Greenland in late December, it was followed by a prolonged blocking period in early 2016 that sustained the extreme Arctic warming. Numerical experiments indicate that the warming effect of sea ice loss and associated upward turbulent heat fluxes are relatively minor in this event. This result suggests the importance of the synoptically driven warm and moist air intrusion into the Arctic as a primary contributing factor of this extreme Arctic warming event.
Liu C. J., E. A. Barnes, 2015: Extreme moisture transport into the Arctic linked to Rossby wave breaking,J. Geophys. Res.: Atmos.,120,3774-3788, http://dx.doi.org/10.1002/2014JD 022796.10.1002/2014JD0227965b8ecb8ebbb062f38c6a41d3b1d2be9ehttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2014JD022796%2Fpdfhttp://doi.wiley.com/10.1002/2014JD022796Abstract The transport of moisture into the Arctic is tightly connected to midlatitude dynamics. We show that the bulk of the transient poleward moisture transport across 60ºN is driven by extreme transport (fluxes greater than the 90th percentile) events. We demonstrate that these events are closely related to the two types of Rossby wave breaking (RWB)攁nticyclonic wave breaking (AWB) and cyclonic wave breaking (CWB). Using a RWB tracking algorithm, we determine that RWB can account for approximately 68% of the extreme poleward moisture transport by transients across 60ºN in winter and 56% in summer. Additional analysis suggests that the seasonality of such RWB-related moisture transport is determined approximately equally by (1) the magnitude of transport (which is largely a function of the background moisture gradient) and (2) the frequency of RWB occurrence. The seasonality of RWB occurrence is, in turn, tied to the seasonal variation of the latitude of the jet streams擜WB-related (CWB-related) transport occurs more frequently when the jet is shifted poleward (equatorward). The interannual variability of RWB-related transport across 60ºN in winter is shown to be strongly influenced by climate variability captured by the El Ni09o/Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). In the positive (negative) phase of ENSO, AWB transports less (more) moisture through the Bering Strait and CWB transports more (less) through western Canada. In the positive (negative) phase of the NAO, AWB transports more (less) moisture through the Norwegian Sea and CWB transports less (more) along the west coast of Greenland. These results highlight that low-frequency climate variability outside of the polar regions can influence the Arctic water vapor by modulating extreme synoptic transport events.
Murray R. J., I. Simmonds, 1995: Responses of climate and cyclones to reductions in Arctic winter sea ice,J. Geophys. Res.,100,4791-4806, http://dx.doi.org/10.1029/94JC02206.10.1029/94JC0220652f8cbba4e7afa8118c28718d7639b89http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F94JC02206%2Fabstracthttp://doi.wiley.com/10.1029/94JC02206A perpetual January simulation of the global atmosphere has been used to study the climatology of the northern hemisphere winter, with particular reference to its synoptic characteristics and to changes induced by reductions in the concentration of Arctic sea ice. Lows were tracked using an objective scheme and were analyzed for a number measures of cyclone behavior. The qualitative features of the cyclone density and flux distributions compare well with observations, except in certain respects noted. Cyclone velocities and intensities are also presented and discussed. Decreases in sea ice concentration produced a monotonic but nonlinear warming in the lower troposphere and weakening and southward contraction of the midlatitude westerlies. Mean sea level pressure reductions were biased toward the western Arctic. There was a significant decrease in the speeds and intensities of cyclonic systems north of 45鎺砃 but little overall change in areal densities or in the arrangement of the major storm tracks. Some density maxima were displaced toward regions of opened up sea ice or in response to circulation changes. A number of aspects of the response are interpreted as being due to changes in thermal steering and baroclinicity or to nonlinear effects. Surprisingly, no significant expansion of cyclonic activity occurred in the central Arctic. The relative constancy of cyclone numbers and storm tracks is a response very different from that found in a similar study of the southern hemisphere.
Newell R. E., N. E. Newell, Y. Zhu, and C. Scott, 1992: Tropospheric rivers? pilot study,Geophys. Res. Lett.,19,2401-2404, http://dx.doi.org/10.1029/92GL02916.10.1029/92GL02916491888a714cf19f9aff3f87f7451c288http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F92GL02916%2Fpdfhttp://doi.wiley.com/10.1029/92GL02916Computations of daily global tropospheric water vapor flux values show the presence of a filamentary structure. The filaments, here called rivers, have lengths many times their widths and persist for many days while being translated through the atmosphere. They are present in data analysed for both 1984 and 1991. The water vapor flux maxima coincide quite closely to reflectivity features (averaged from wavelengths of 380 and 360 nm) as revealed by the Total Ozone Mapping Spectrometer (TOMS). It is suggested that the filamentary structure may also be present in other trace constituents.
Newman M., G. N. Kiladis, K. M. Weickmann, F. M. Ralph, and P. D. Sardeshmukh, 2012: Relative contributions of synoptic and low-frequency eddies to time-mean atmospheric moisture transport,including the role of atmospheric rivers. J. Climate,25,7341-7361,http://dx.doi.org/10.1175/JCLI-D-11-00665.1.
Peixoto J. P., A. H. Oort, 1992: Physics of Climate. American Institute of Physics, 520 pp.
Ralph F. M., M. D. Dettinger, 2011: Storms,floods,and the science of atmospheric rivers. EOS,92, 265-266,http://dx.doi.org/10.1029/2011EO320001.
Rogers A. N., D. H. Bromwich, E. N. Sinclair, and R. I. Cullather, 2001: The atmospheric hydrologic cycle over the Arctic basin from reanalyses,Part II: Interannual variability J. Climate,14,2414-2429, http://dx.doi.org/10.1175/1520-0442(2001)014<2414:TAHCOT>2.0.CO;2.10.1175/1520-0442(2001)014<2414:TAHCOT>2.0.CO;2http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%282001%29014%3C2414%3ATAHCOT%3E2.0.CO%3B2
Sepp M., J. Jaagus, 2011: Changes in the activity and tracks of arctic cyclones,Climatic Change,105,577-595, http://dx.doi.org/10.1007/s10584-010-9893-7.10.1007/s10584-010-9893-71fb7da4102256e6d8cbdd9e6716ae872http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs10584-010-9893-7http://link.springer.com/10.1007/s10584-010-9893-7Changes in the frequency and air pressure of cyclones that enter or are formed within the Arctic basin are herein examined by applying the database of cyclones created using NCEP/NCAR re-analysis. The Arctic basin is defined as the area north of latitude 68º N. Deep cyclones with a mean sea level pressure (SLP) of below 1,00002hPa, were analysed separately from shallow cyclones. Changes in the variables in the first, last, deepest and northernmost points of cyclones were studied. The cyclones were grouped into sectors by using the point on latitude 68º N at which the cyclone entered the Arctic region. The analysis described herein shows that the frequency of incoming cyclones, i.e. those that entered the Arctic basin, increased significantly during the period 1948�2002, but that the frequency of Arctic cyclones formed within the Arctic basin did not. The frequency of deep cyclones that entered the Arctic basin, as well as the frequency of cyclones that formed within it, clearly increased, while the frequency of shallow Arctic cyclones decreased. The most significant changes in the seasonal parameters associated with the cyclones occurred in winter. The mean annual SLP of deep cyclones decreased significantly, particularly for deep Arctic cyclones. The frequency of incoming cyclones showed an increase in the Bering Strait, Alaskan, Baffin Sea, and East Siberian sectors.
Serreze M. C., R. G. Barry, and J. E. Walsh, 1995: Atmospheric water vapor characteristics at 70N. J. Climate,8, 719-731, http://dx.doi.org/10.1175/1520-0442(1995)008<0719:AWVCA>2.0.CO;2.10.1175/1520-0442(1995)0082.0.CO;28aed39fc29365914d653a93dc9ffc45dhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1995JCli....8..719Shttp://adsabs.harvard.edu/abs/1995JCli....8..719SUsing an extensive rawinsonde archive, characteristics of Arctic water vapor and its transports at 70ºN are examined for the period 1974-1991. Monthly-mean profiles and vertically integrated values of specific humidity and meridional vapor fluxes are computed for land stations north of 65ºN for the surface up to 300 mb using once to twice daily soundings. Mean values at 70ºN for these and other variables, including temperature and meridional winds, are obtained through an objective analysis of the monthly station means.The annual zonal mean specific humidity at 70ºN ranges from 2.4 g kgat the surface to 0.02 g kgat 300 mb. Zonal-mean precipitable water ranges from 2.9 mm in February and March to 16.2 mm in July. For all months, over 95% of water vapor is found below 500 mb. Although mean winds are equatorward up to about 400 mb, the tendency for poleward winds to transport more water vapor results in a poleward annual-mean flux at all levels except at the surface, peaking at 1.5 g kgm sat 850 mb. Whereas over 85% of the integrated zonal-mean meridional flux is found below 500 mb for all months, a smaller percentage is found at lower levels during summer due to stronger equatorward winds. The flux convergence across 70ºN is positive in all months, peaking in September at an equivalent monthly water depth of 22.1 mm averaged over the region north of 70ºN. Aerological estimates of precipitation minus evaporation (P - E) for the area north of 70ºN that account for changes in water storage also peak in September (26.1 mm), with the annual total of 163 mm larger than previous estimates by up to 36%. Integrated vapor transports exhibit marked longitudinal variations, with maximum annual poleward transports of 16-25 kg smfound over the Norwegian Sea and Baffin Bay. The Canadian Arctic archipelago is the only sector where mean integrated transports are equatorward, ranging from 1 to 10 kg smdepending on longitude. The September peak in P - E results from a circulation shift yielding poleward fluxes along a broad zone from near the prime meridian to 15OºE.
Simmonds I., K. Keay, 2009: Extraordinary September Arctic sea ice reductions and their relationships with storm behavior over 1979-2008,Geophys. Res. Let.,36,L19715, http://dx.doi.org/10.1029/2009GL039810.10.1029/2009GL039810e3ebe6a465cbce1f7b79d809c58567c8http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2009GL039810%2Fpdfhttp://doi.wiley.com/10.1029/2009GL039810Dramatic changes have been observed in Arctic sea ice, cyclone behavior and atmospheric circulation in recent decades. Decreases in September ice extent have been remarkable over the last 30 years, and particularly so in very recent times. The analysis reveals that the trends and variability in September ice coverage and mean cyclone characteristics are related, and that the strength (rather than the number) of cyclones in the Arctic basin is playing a central role in the changes observed in that region, especially in the last few years. The findings reinforce suggestions that the decline in the extent and thickness of Arctic ice has started to render it particularly vulnerable to future anomalous cyclonic activity and atmospheric forcing.
Simmonds I., C. Burke, and K. Keay, 2008: Arctic climate change as manifest in cyclone behavior,J. Climate,21,5777-5796, http://dx.doi.org/10.1175/2008JCLI2366.1.10.1175/2008JCLI2366.1536cf2dad82ad59fd7f29896a4fb4c4fhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FXREF%3Fid%3D10.1175%2F2008JCLI2366.1http://journals.ametsoc.org/doi/abs/10.1175/2008JCLI2366.1The Arctic region has exhibited dramatic changes in recent times. Many of these are intimately tied up with synoptic activity, but little research has been undertaken on how the characteristics of Arctic cyclones have changed. This paper presents a comprehensive analysis of Arctic (here defined as the domain north of 70鎺砃) cyclones diagnosed with the Melbourne University cyclone tracking scheme applied to the 40-yr ECMWF Re-Analysis (ERA-40) and the NCEP-NCAR (NCEP1) and NCEP-Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP)-II (NCEP2) reanalysis sets (the last two extending to the end of 2006). A wide variety of cyclone characteristics is presented as befits these complex features. In winter the highest density of cyclones is found between Norway and Svalbard and to the east to the Barents and Kara Seas, and significant numbers are found in the central Arctic. In summer the greatest frequencies are found in the central Arctic. The total number of cyclones identified in the ERA-40 record exceeds those in the two NCEP compilations. The mean size of cyclones shows similar maxima in the central Arctic in both winter and summer. By contrast, the greatest mean system depth in winter (in excess of 8 hPa) is found to the southeast of Greenland, although average depths exceed 6 hPa over a considerable portion of the basin. In summer the deepest cyclones are found in the central portion of the Arctic. The analysis shows that the total number of cyclones in winter exceeds that in summer, a result in contrast to earlier studies. This difference comes about primarily due to the greater numbers of "open strong" systems in winter in all reanalyses. Cyclones in this category are associated with very active synoptic situations; it is of importance that they be included in cyclone counts but would not be considered in many cyclone identification schemes. Since 1979 neither the ERA-40 nor the NCEP2 sets show significant trends in any of the cyclone variables considered. However, over the entire record starting in 1958 the NCEP1 reanalysis exhibits a significant increase in summer cyclone frequency (due mainly to the increase in closed strong systems). Both NCEP1 and ERA-40 also reveal significant increases in the number of summer closed strong cyclones, as well as in their mean depth and intensity in that season. Interannual variations in Arctic cyclone numbers are closely related to the Arctic Oscillation (AO) index in the full reanalyses records. An even stronger relationship is found between the AO and the number of deep cyclones. These relationships have still held in the last decade when the AO has returned to more normal values but the summer and fall sea ice extent has continued to decrease.
Skific N., J. A. Francis, and J. J. Cassano, 2009: Attribution of projected changes in atmospheric moisture transport in the Arctic: A self-organizing map perspective,J. Climate,22,4135-4153, http://dx.doi.org/10.1175/2009JCLI2645.1.10.1175/2009JCLI2645.170a854dba2f4e97666e6b72b40d4e40fhttp%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20093240158.htmlhttp://journals.ametsoc.org/doi/abs/10.1175/2009JCLI2645.1Meridonal moisture transport into the Arctic derived from one simulation of the National Center for Atmospheric Research Community Climate System Model (CCSM3), spanning the periods of 1960-99, 2010-30, and 2070-89, is analyzed. The twenty-first-century simulation incorporates the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios (SRES) A2 scenario for CO2 an...
Sorteberg A., B. Kvingedal, 2006: Atmospheric forcing on the Barents Sea winter ice extent,J. Climate,19,4772-4784, http://dx.doi.org/10.1175/JCLI3885.1.10.1175/JCLI3885.1428f2ab34b73dd896d70663949044725http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2006JCli...19.4772Shttp://journals.ametsoc.org/doi/abs/10.1175/JCLI3885.1The atmospheric forcing on the Barents Sea ice extent during winter [December09ebruary (DJF)] has been investigated for the period 196709�2002. The time series for the sea ice extent is updated and includes the winter of 2005, which marks a new record low in the wintertime Barents Sea ice extent, and a linear trend of -3.5% decade-1 in the ice extent was found. Covariability between the Barents Sea ice extent and the atmospheric mean seasonal flow and the synoptic cyclones has been discussed separately. For the mean flow, linear correlations and regression analysis reveal that anomalous northerly (southerly) winds prevail in the Nordic Seas during winters with extensive (sparse) Barents Sea ice extent. Some of the variability in the mean flow is captured by the North Atlantic Oscillation (NAO); however, the wintertime link between the Barents Sea ice extent and the NAO is moderate. By studying the cyclone activity in the high-latitude Northern Hemisphere using a dataset of individual cyclones, two regions that influence the wintertime Barents Sea ice extent were identified. The variability in the northward-moving cyclones traveling into the Arctic over East Siberia was found to covary strongly with the Barents Sea ice extent. The main mechanism is believed to be the change in the Arctic winds and in ice advection connected to the cyclones. In addition, cyclone activity of northward-moving cyclones over the western Nordic Seas was identified to strongly influence the Barents Sea ice extent. This relationship was particularly strong on decadal time scales and when the ice extent lagged the cyclone variability by 109�2 yr. The lag indicates that the mechanism is related to the cyclones0964 ability to modulate the inflow of Atlantic water into the Nordic Seas and the transport time of oceanic heat anomalies from the Nordic Seas into the Barents Sea. Multiple regression indicates that the two mechanisms may explain (or at least covary with) 46% of the wintertime Barents Sea variance over the 196709�2002 period and that 79% of the decadal part of the ice variability may be predicted 2 yr ahead using information about the decadal cyclone variability in the Nordic Seas.
Taylor W. A., 2015: Change-Point Analysis: A powerful new tool for detecting changes. Taylor Enterprises., [Available online from ]http://www.variation.com/cpa/tech/changepoint.html12ab2b0fbb727765aed142b76c377a26http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F2245702_Change-Point_Analysis_A_Powerful_New_Tool_For_Detecting_Changeshttp://www.researchgate.net/publication/2245702_Change-Point_Analysis_A_Powerful_New_Tool_For_Detecting_ChangesABSTRACT This article describes how to perform a change-point analysis and
Thompson D. J. W., J. M. Wallace, 1998: The Arctic oscillation signature in the wintertime geopotential height and temperature fields,Geophys. Res. Lett.,25,1297-1300, http://dx.doi.org/10.1029/98GL00950.10.1029/98GL00950adf244c1165dc0c5b3e8ecc1d4c5e7fehttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F98GL00950%2Fpdfhttp://doi.wiley.com/10.1029/98GL00950The leading empirical orthogonal function of the wintertime sea-level pressure field is more strongly coupled to surface air temperature fluctuations over the Eurasian continent than the North Atlantic Oscillation (NAO). It resembles the NAO in many respects; but its primary center of action covers more of the Arctic, giving it a more zonally symmetric appearance. Coupled to strong fluctuations at the 50-hPa level on the intraseasonal, interannual, and interdecadal time scales, this 閳ユ穾rctic Oscillation閳� (AO) can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft. It is proposed that the zonally asymmetric surface air temperature and mid-tropospheric circulation anomalies observed in association with the AO may be secondary baroclinic features induced by the land-sea contrasts. The same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature, sea-level pressure, and 50-hPa height over the past 30 years: parts of Eurasia have warmed by as much as several K, sea-level pressure over parts of the Arctic has fallen by 4 hPa, and the core of the lower stratospheric polar vortex has cooled by several K. These trends can be interpreted as the development of a systematic bias in one of the atmosphere's dominant, naturally occurring modes of variability.
Vihma, T., Coauthors, 2016: The atmospheric role in the Arctic water cycle: A review on processes,past and future changes,and their impacts. J. Geophys. Res.: Biogeosci.,121, 586-620,http://dx.doi.org/10.1002/2015JG003132.10.1002/2015JG00313231924bfea98fcc96de5b7543081a8f20http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2015JG003132%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1002/2015JG003132/pdfAtmospheric humidity, clouds, precipitation, and evapotranspiration are essential components of the Arctic climate system. During recent decades, specific humidity and precipitation have generally increased in the Arctic, but changes in evapotranspiration are poorly known. Trends in clouds vary depending on the region and season. Climate model experiments suggest that increases in precipitation are related to global warming. In turn, feedbacks associated with the increase in atmospheric moisture and decrease in sea ice and snow cover have contributed to the Arctic amplification of global warming. Climate models have captured the overall wetting trend but have limited success in reproducing regional details. For the rest of the 21st century, climate models project strong warming and increasing precipitation, but different models yield different results for changes in cloud cover. The model differences are largest in months of minimum sea ice cover. Evapotranspiration is projected to increase in winter but in summer to decrease over the oceans and increase over land. Increasing net precipitation increases river discharge to the Arctic Ocean. Over sea ice in summer, projected increase in rain and decrease in snowfall decrease the surface albedo and, hence, further amplify snow/ice surface melt. With reducing sea ice, wind forcing on the Arctic Ocean increases with impacts on ocean currents and freshwater transport out of the Arctic. Improvements in observations, process understanding, and modeling capabilities are needed to better quantify the atmospheric role in the Arctic water cycle and its changes.
Wang X. J., J. R. Key, 2005: Arctic surface,cloud,and radiation properties based on the AVHRR polar pathfinder dataset. Part II. Recent trends. J. Climate,18 ,2575-2593,.https://doi.org/10.1175/JCLI3439.110.1175/JCLI3439.1db6ca159cc3f71acce98f7f17be25292http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2005JCli...18.2575Whttp://journals.ametsoc.org/doi/abs/10.1175/JCLI3439.1Over the past 20 yr, some Arctic surface and cloud properties have changed significantly. Results of an analysis of satellite data show that the Arctic has warmed and become cloudier in spring and summer but has cooled and become less cloudy in winter. The annual rate of surface temperature change is 0.057ºC for the Arctic region north of 60ºN. The surface broadband albedo has decreased significantly in autumn, especially over the Arctic Ocean, indicating a later freeze-up and snowfall. The surface albedo has decreased at an annual rate of -0.15% (absolute). Cloud fraction has decreased at an annual rate of -0.6% (absolute) in winter and increased at annual rates of 0.32% and 0.16% in spring and summer, respectively. On an annual time scale, there is no trend in cloud fraction. During spring and summer, changes in sea ice albedo that result from surface warming tend to modulate the radiative effect of increasing cloud cover. On an annual time scale, the all-wave cloud forcing at the surface has decreased at an annual rate of -0.335 W m, indicating an increased cooling by clouds. There are large correlations between surface temperature anomalies and climate indices such as the Arctic Oscillation (AO) index for some areas, implying linkages between global climate change and Arctic climate change.
Woods C., R. Caballero, 2016: The role of moist intrusions in winter Arctic warming and sea ice decline,J. Climate,29,4473-4485, http://dx.doi.org/10.1175/JCLI-D-15-0773.1.10.1175/JCLI-D-15-0773.1e3c1f0fc4753f0533c62b7133d3caeebhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F298331032_The_role_of_moist_intrusions_in_winter_Arctic_warming_and_sea_ice_declinehttp://journals.ametsoc.org/doi/10.1175/JCLI-D-15-0773.1This paper examines the trajectories followed by intense intrusions of moist air into the Arctic polar region during autumn and winter and their impact on local temperature and sea ice concentration. It is found that the vertical structure of the warming associated with moist intrusions is bottom amplified, corresponding to a transition of local conditions from a 渃old clear� state with a strong inversion to a 渨arm opaque� state with a weaker inversion. In the marginal sea ice zone of the Barents Sea, the passage of an intrusion also causes a retreat of the ice margin, which persists for many days after the intrusion has passed. The authors find that there is a positive trend in the number of intrusion events crossing 70ºN during December and January that can explain roughly 45% of the surface air temperature and 30% of the sea ice concentration trends observed in the Barents Sea during the past two decades.
Woods C., R. Caballero, and G. Svensson, 2013: Large-scale circulation associated with moisture intrusions into the Arctic during winter,Geophys. Res. Lett.,40,4717-4721, http://dx.doi.org/10.1002/grl.50912.10.1002/grl.50912184c9667bf6d9ccad4e5c604af081158http%3A%2F%2Fwww.diva-portal.org%2Fsmash%2Frecord.jsf%3Fpid%3Ddiva2%253A664000http://doi.wiley.com/10.1002/grl.50912We examine the poleward transport of water vapor across 70 degrees N during boreal winter in the ERA-Interim reanalysis product, focusing on intense moisture intrusion events. We analyze the large-scale circulation patterns associated with these intrusions and the impacts they have at the surface. A total of 298 events are identified between 1990 and 2010, an average of 14 per season, accounting for 28% of the total poleward transport of moisture across 70 degrees N. They are concentrated over the main ocean basins at that latitude in the Labrador Sea, North Atlantic, Barents/Kara Sea, and Pacific. Composites of sea level pressure and potential temperature on the 2 potential vorticity unit surface during intrusions show a large-scale blocking pattern to the east of each basin, deflecting midlatitude cyclones and their associated moisture poleward. The interannual variability of intrusions is strongly correlated with variability in winter-mean surface downward longwave radiation and skin temperature averaged over the Arctic.
Yin J. H., 2005: A consistent poleward shift of the storm tracks in simulations of 21st century climate,Geophys. Res. Lett.,32,L18701, http://dx.doi.org/10.1029/2005GL023684.10.1029/2005GL023684783307940fb9a138e380a47de077b376http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2005GL023684%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2005GL023684/fullA consistent poleward and upward shift and intensification of the storm tracks is found in an ensemble of 21st century climate simulations performed by 15 coupled climate models. The shift of the storm tracks is accompanied by a poleward shift and upward expansion of the midlatitude baroclinic regions associated with enhanced warming in the tropical upper troposphere and increased tropopause height. The poleward shift in baroclinicity is augmented in the Southern Hemisphere and partially offset in the Northern Hemisphere by changes in the surface meridional temperature gradient. The poleward shift of the storm tracks also tends to be accompanied by poleward shifts in surface wind stress and precipitation, and a shift towards the high index state of the annular modes. These results highlight the integral role that the storm tracks play in the climate system, and the importance of understanding how and why they will change in the future.
Zhang X. D., J. E. Walsh, J. Zhang, U. S. Bhatt, and M. Ikeda, 2004: Climatology and interannual variability of Arctic cyclone activity: 1948-2002,J. Climate,17,2300-2317, http://dx.doi.org/10.1175/1520-0442(2004)017<2300:CAIVOA>2.0.CO;2.10.1175/1520-0442(2004)017<2300:CAIVOA>2.0.CO;2f1aa7469e03cc0b2ec4f374af61fb70dhttp%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2004JCli...17.2300Z%26amp%3Bdb_key%3DPHY%26amp%3Blink_type%3DABSTRACT%26amp%3Bhigh%3D01095http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%282004%29017%3C2300%3ACAIVOA%3E2.0.CO%3B2
Zhang X. D., A. Sorteberg, J. Zhang, R. Gerdes, and J. C. Comiso, 2008: Recent radical shifts of atmospheric circulations and rapid changes in Arctic climate system,Geophys. Res. Lett.,35,L22701, http://dx.doi.org/10.1029/2008GL035607.10.1029/2008GL0356076cc495fa3921c951fdf9ebde40b2ff41http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2008GL035607%2Ffullhttp://doi.wiley.com/10.1029/2008GL035607Arctic climate system change has accelerated tremendously since the beginning of this century, and a strikingly extreme sea-ice loss occurred in summer 2007. However, the greenhouse-gas-emissions forcing has only increased gradually and the driving role in Arctic climate change of the positively-polarized Arctic/North Atlantic Oscillation (AO/NAO) trend has been substantially weakened. Although various contributing factors have been examined, the fundamental physical process, which orchestrates these contributors to drive the acceleration and the latest extreme event, remains unknown. We report on drastic, systematic spatial changes in atmospheric circulations, showing a sudden jump from the conventional tri-polar AO/NAO to an unprecedented dipolar leading pattern, following accelerated northeastward shifts of the AO/NAO centers of action. These shifts provide an accelerating impetus for the recent rapid Arctic climate system changes, perhaps shedding light on recent arguments about a tipping point of global-warming-forced climate change in the Arctic. The radical spatial shift is a precursor to the observed extreme change event, demonstrating skilful information for future prediction.
Zhang X. D., J. X. He, J. Zhang, I. Polyakov, R. Gerdes, J. Inoue, and P. L. Wu, 2012: Enhanced poleward moisture transport and amplified northern high-latitude wetting trend,Nat. Clim. Change,3,47-51, http://dx.doi.org/10.1038/nclimate1631.10.1038/NCLIMATE16316e358e18b021f6eeda2a5d532b42060ahttp%3A%2F%2Fwww.nature.com%2Fnclimate%2Fjournal%2Fv3%2Fn1%2Fabs%2Fnclimate1631.htmlhttp://www.nature.com/doifinder/10.1038/nclimate1631Observations and climate change projections forced by greenhouse gas emissions have indicated a wetting trend in northern high latitudes, evidenced by increasing Eurasian Arctic river discharges(1-3). The increase in river discharge has accelerated in the latest decade and an unprecedented, record high discharge occurred in 2007 along with an extreme loss of Arctic summer sea-ice cover(4-6). Studies have ascribed this increasing discharge to various factors attributable to local global warming effects, including intensifying precipitation minus evaporation, thawing permafrost, increasing greenness and reduced plant transpiration(7-11). However, no agreement has been reached and causal physical processes remain unclear. Here we show that enhancement of poleward atmospheric moisture transport (AMT) decisively contributes to increased Eurasian Arctic river discharges. Net AMT into the Eurasian Arctic river basins captures 98% of the gauged climatological river discharges. The trend of 2.6% net AMT increase per decade accounts well for the 1.8% per decade increase in gauged discharges and also suggests an increase in underlying soil moisture. A radical shift of the atmospheric circulation pattern induced an unusually large AMT and warm surface in 2006-2007 over Eurasia, resulting in the record high discharge.
Zhu Y., R. E. Newell, 1998: A proposed algorithm for moisture fluxes from atmospheric rivers,Mon. Wea. Rev.,126,725-735, org/10.1175/1520-0493(1998)126<0725:APAFMF>2.0.CO;210.1175/1520-0493(1998)1262.0.CO;274c9ade35643a3f59b34b58b4ac9b3f8http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1998MWRv..126..725Zhttp://journals.ametsoc.org/doi/abs/10.1175/1520-0493%281998%29126%3C0725%3AAPAFMF%3E2.0.CO%3B2ABSTRACT A new algorithm is applied to study water vapor fluxes in the troposphere using wind and moisture data from the European Centre for Medium-Range Weather Forecasts. The fluxes are divided into filamentary structures known as tropospheric rivers and what are termed here broad fields. The results show that the tropospheric rivers may carry essentially the total meridional transport observed in the extratropical atmosphere but may occupy only about 10% of the total longitudinal length at a given latitude. The transient fluxes in traditional studies do not catch the filamentary structures completely and may therefore underestimate the fraction of transport assigned to moving systems, as well as omitting the geographical concentration. The mean flow and eddy fluxes evaluated by the new algorithm are considered to be more physically realistic.
Zuidema, P., Coauthors, 2005: An Arctic springtime mixed-phase cloudy boundary layer observed during SHEBA,J. Atmos. Sci.,62,160-176, http://dx.doi.org/10.1175/JAS-3368.1.10.1175/JAS-3368.1d3851b955c7b563c25b228b5219a1b67http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2005JAtS...62..160Zhttp://journals.ametsoc.org/doi/abs/10.1175/JAS-3368.1The microphysical characteristics, radiative impact, and life cycle of a long-lived, surface-based mixed-layer, mixed-phase cloud with an average temperature of approximately 6120ºC are presented and discussed. The cloud was observed during the Surface Heat Budget of the Arctic experiment (SHEBA) from 1 to 10 May 1998. Vertically resolved properties of the liquid and ice phases are retrieved using surface-based remote sensors, utilize the adiabatic assumption for the liquid component, and are aided by and validated with aircraft measurements from 4 and 7 May. The cloud radar ice microphysical retrievals, originally developed for all-ice clouds, compare well with aircraft measurements despite the presence of much greater liquid water contents than ice water contents. The retrieved time-mean liquid cloud optical depth of 10.1 7.8 far surpasses the mean ice cloud optical depth of 0.2, so that the liquid phase is primarily responsible for the cloud radiative (flux) impact. The ice phase, in turn, regulates the overall cloud optical depth through two mechanisms: sedimentation from a thin upper ice cloud, and a local ice production mechanism with a time scale of a few hours, thought to reflect a preferred freezing of the larger liquid drops. The liquid water paths replenish within half a day or less after their uptake by ice, attesting to strong water vapor fluxes. Deeper boundary layer depths and higher cloud optical depths coincide with large-scale rising motion at 850 hPa, but the synoptic activity is also associated with upper-level ice clouds. Interestingly, the local ice formation mechanism appears to be more active when the large-scale subsidence rate implies increased cloud-top entrainment. Strong cloud-top radiative cooling rates promote cloud longevity when the cloud is optically thick. The radiative impact of the cloud upon the surface is significant: a time-mean positive net cloud forcing of 41 W m612 with a diurnal amplitude of 20 W m612. This is primarily because a high surface reflectance (0.86) reduces the solar cooling influence. The net cloud forcing is primarily sensitive to cloud optical depth for the low-optical-depth cloudy columns and to the surface reflectance for the high-optical-depth cloudy columns. Any projected increase in the springtime cloud optical depth at this location (76ºN, 165ºW) is not expected to significantly alter the surface radiation budget, because clouds were almost always present, and almost 60% of the cloudy columns had optical depths >6.