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Role of Extratropical Cyclones in the Recently Observed Increase in Poleward Moisture Transport into the Arctic Ocean

doi: 10.1007/s00376-017-7116-0

  • Poleward atmospheric moisture transport (AMT) into the Arctic Ocean can change atmospheric moisture or water vapor content and cause cloud formation and redistribution, which may change downward longwave radiation and, in turn, surface energy budgets, air temperatures, and sea-ice production and melt. In this study, we found a consistently enhanced poleward AMT across 60°N since 1959 based on the NCAR-NCEP reanalysis. Regional analysis demonstrates that the poleward AMT predominantly occurs over the North Atlantic and North Pacific regions, contributing about 57% and 32%, respectively, to the total transport. To improve our understanding of the driving force for this enhanced poleward AMT, we explored the role that extratropical cyclone activity may play. Climatologically, about 207 extratropical cyclones move across 60°N into the Arctic Ocean each year, among which about 66 (32% of the total) and 47 (23%) originate from the North Atlantic and North Pacific Ocean, respectively. When analyzing the linear trends of the time series constructed by using a 20-year running window, we found a positive correlation of 0.70 between poleward yearly AMT and the integrated cyclone activity index (measurement of cyclone intensity, number, and duration). This shows the consistent multidecadal changes between these two parameters and may suggest cyclone activity plays a driving role in the enhanced poleward AMT. Furthermore, a composite analysis indicates that intensification and poleward extension of the Icelandic low and accompanying strengthened cyclone activity play an important role in enhancing poleward AMT over the North Atlantic region.
    摘要: 向极大气水汽输送(AMT)进入北冰洋后, 改变了该地区的大气湿度和水含量, 引起云的形成和重新分布. 这会改变向下长波辐射, 进而改变地表能量平衡, 地面气温, 海冰的生成和融化. 在本研究中, 我们基于NCAR-NCEP再分析资料发现, 自1959年以后, 跨过60°N的向极大气水汽输送持续增加. 进一步计算表明, 向极大气水汽输送主要发生在北大西洋和北太平洋, 它们分别占总输送的57%和32%. 为了深入理解增强向极大气水汽输送的驱动力, 我们研究了外热带气旋所起的作用. 在气候平均意义上, 每年有207个外热带气旋跨过60°N进入北冰洋, 其中有66个气旋起源北大西洋(占总数的32%), 43个气旋起源于北太平洋(占总数的23%). 我们使用20年滑动窗口对向极大气水汽输送时间序列和综合气旋活动指数(CAI)时间序列进行线性趋势分析, 发现两者线性趋势都为增加, 而且相关达到0.7. 这表明, 上述两个时间序列的多年代际变化是相互协调的. 同时, 这也表明气旋活动驱动向极大气水汽输送的增强. 进一步的合成分析表明, 冰岛低压在强化的同时向极地中央地区伸展, 与此相伴随的是外热带气旋活动的加强, 这对北大西洋地区向极大气水汽输送的增强起了重要的作用.
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    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, 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, 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, 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,<0923:TAHCOT>2.0.CO;2.10.1175/1520-0442(2000)013<0923:TAHCOT>2.0.CO;2
    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, 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,<2671: TAORTT>2.0.CO;2.10.1175/1520-0442(2000)013<2671:TAORTT>2.0.CO;
    Francis J. A., E. Hunter, 2006: New insight into the disappearing arctic sea ice,EOS,87,509-511, 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, 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, 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, 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.
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    Jakobson E., T. Vihma, 2010: Atmospheric moisture budget in the Arctic based on the ERA-40 reanalysis,International Journal of Climatology,30,2175-2194, 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
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    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, 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, 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, 022796.10.1002/ 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.
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    Sepp M., J. Jaagus, 2011: Changes in the activity and tracks of arctic cyclones,Climatic Change,105,577-595, 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,<0719:AWVCA>2.0.CO;2.10.1175/1520-0442(1995)0082.0.CO; 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, 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, 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.
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    Sorteberg A., B. Kvingedal, 2006: Atmospheric forcing on the Barents Sea winter ice extent,J. Climate,19,4772-4784, 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 ] 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, 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, 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,. 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, 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, 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, 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,<2300:CAIVOA>2.0.CO;2.10.1175/1520-0442(2004)017<2300:CAIVOA>2.0.CO;
    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, 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, 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; 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, 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.
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Manuscript received: 05 May 2017
Manuscript revised: 14 October 2017
Manuscript accepted: 17 October 2017
通讯作者: 陈斌,
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Role of Extratropical Cyclones in the Recently Observed Increase in Poleward Moisture Transport into the Arctic Ocean

  • 1. Department of Physics, North Carolina A&T State University, Greensboro, North Carolina 27401, USA
  • 2. Department of Energy and Environmental Systems, North Carolina A&T State University, Greensboro, North Carolina 27401, USA
  • 3. Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
  • 4. Department of Atmospheric Science, University of Alaska Fairbanks, Fairbanks, Alaska 99775, USA

Abstract: Poleward atmospheric moisture transport (AMT) into the Arctic Ocean can change atmospheric moisture or water vapor content and cause cloud formation and redistribution, which may change downward longwave radiation and, in turn, surface energy budgets, air temperatures, and sea-ice production and melt. In this study, we found a consistently enhanced poleward AMT across 60°N since 1959 based on the NCAR-NCEP reanalysis. Regional analysis demonstrates that the poleward AMT predominantly occurs over the North Atlantic and North Pacific regions, contributing about 57% and 32%, respectively, to the total transport. To improve our understanding of the driving force for this enhanced poleward AMT, we explored the role that extratropical cyclone activity may play. Climatologically, about 207 extratropical cyclones move across 60°N into the Arctic Ocean each year, among which about 66 (32% of the total) and 47 (23%) originate from the North Atlantic and North Pacific Ocean, respectively. When analyzing the linear trends of the time series constructed by using a 20-year running window, we found a positive correlation of 0.70 between poleward yearly AMT and the integrated cyclone activity index (measurement of cyclone intensity, number, and duration). This shows the consistent multidecadal changes between these two parameters and may suggest cyclone activity plays a driving role in the enhanced poleward AMT. Furthermore, a composite analysis indicates that intensification and poleward extension of the Icelandic low and accompanying strengthened cyclone activity play an important role in enhancing poleward AMT over the North Atlantic region.

摘要: 向极大气水汽输送(AMT)进入北冰洋后, 改变了该地区的大气湿度和水含量, 引起云的形成和重新分布. 这会改变向下长波辐射, 进而改变地表能量平衡, 地面气温, 海冰的生成和融化. 在本研究中, 我们基于NCAR-NCEP再分析资料发现, 自1959年以后, 跨过60°N的向极大气水汽输送持续增加. 进一步计算表明, 向极大气水汽输送主要发生在北大西洋和北太平洋, 它们分别占总输送的57%和32%. 为了深入理解增强向极大气水汽输送的驱动力, 我们研究了外热带气旋所起的作用. 在气候平均意义上, 每年有207个外热带气旋跨过60°N进入北冰洋, 其中有66个气旋起源北大西洋(占总数的32%), 43个气旋起源于北太平洋(占总数的23%). 我们使用20年滑动窗口对向极大气水汽输送时间序列和综合气旋活动指数(CAI)时间序列进行线性趋势分析, 发现两者线性趋势都为增加, 而且相关达到0.7. 这表明, 上述两个时间序列的多年代际变化是相互协调的. 同时, 这也表明气旋活动驱动向极大气水汽输送的增强. 进一步的合成分析表明, 冰岛低压在强化的同时向极地中央地区伸展, 与此相伴随的是外热带气旋活动的加强, 这对北大西洋地区向极大气水汽输送的增强起了重要的作用.

1. Introduction
  • Alteration of poleward atmospheric moisture transport (AMT) into the Arctic may have significant impacts on Arctic climate changes since moisture, or water vapor, represents one of the major greenhouse gases and critically determines cloud formation and distribution. Increased moisture transport into the Arctic Ocean may, therefore, enhance water-vapor- or cloud-forced downward longwave radiation, contributing to an increase in surface air temperature and summer sea-ice melt and a decrease in winter sea-ice production. (Wang and Key, 2005) found that increased water vapor content appears to be influential in the enhancement of downward longwave radiation in the Arctic, and (Zuidema et al., 2005) further emphasized the role of liquid-water-containing clouds. (Francis and Hunter, 2006) suggested that the location of the summer ice edge is strongly correlated to the variability in longwave energy emitted by the atmosphere. (Kay et al., 2008) found that changes in clouds are an increasingly important control in sea-ice extent minima. In particular, when studying the extreme event of Arctic sea-ice loss in 2007, (Kay and Gettelman, 2009) found that, while summer cloud reductions enhanced ice-albedo feedback, an increase in cloudiness in fall over newly open water helps trap heat by increasing the downwelling longwave radiation but also reduces net absorbed shortwave radiation. In addition, climate model projections have shown that an increase in poleward AMT caused by a warming planet likely creates a positive feedback on the Arctic system by increasing latent heat release and the emission of longwave radiation to the surface (Skific et al., 2009).

    The impact of intense poleward AMT across 70°N during boreal winter on the Arctic-mean downward longwave radiation was recently quantified by (Woods et al., 2013). They found that poleward moisture intrusions contribute to about 40% of the interannual variance in downward longwave radiation and about 30% of the interannual variance in skin temperature. Further evidence shows that the increase in downward longwave radiation associated with these moisture intrusions can impact the loss of sea ice (Woods and Caballero, 2016) and result in rapid surface air temperature increases (Kim et al., 2017). (Liu and Barnes, 2015) investigated these extreme moisture intrusions and found that they are linked to Rossby wave breaking. These planetary-scale waves are capable of diverting moisture, sensible, and latent heat fluxes into the Arctic, enhancing the downward infrared radiation and surface warming (Baggett et al., 2016; Baggett and Lee, 2017; Gong et al., 2017). These moisture intrusions also resemble atmospheric rivers (ARs; Newell et al., 1992; Ralph and Dettinger, 2011; Newman et al., 2012; Liu and Barnes, 2015; Baggett et al., 2016). ARs have been found to account for the majority of poleward moisture transport in the midlatitudes (Zhu and Newell, 1998; Guan and Waliser, 2015). Also, (Dacre et al., 2015) attributed the moisture in ARs to local evaporation and convergence within the warm sectors of cyclones, instead of long-range transport as the source.

    Synoptic cyclones constitute the major part of atmospheric transient eddies, which may also play an important role in moisture transport, in addition to the stationary and planetary waves (e.g., Peixoto and Oort, 1992; Baggett et al., 2016; Vihma et al., 2016; Baggett and Lee, 2017). (Groves and Francis, 2002) found that the movement of transient features into the Arctic account for 74% of the net precipitation in the Arctic basin. On the other hand, studies have suggested that cyclone activity plays a key role in the interaction between the surface and the atmosphere and has strong impacts on the Arctic sea ice (Murray and Simmonds, 1995; Sorteberg and Kvingedal, 2006). The strength, rather than the number, of cyclones in the Arctic basin, plays a more critical role in the decrease in September ice extent (Simmonds and Keay, 2009). The impacts of cyclones on the surface and sea ice may be affected through changes in atmospheric moisture content and resulting longwave radiation. In particular, it has been found that cyclone tracks have shifted poleward and cyclone activity has intensified in the Arctic (Zhang et al., 2004; Simmonds et al., 2008; Sepp and Jaagus, 2011; Bender et al., 2012), and the poleward shift may continue in the future as projected by climate models under global warming scenarios (Yin, 2005). So, to improve our understanding of the changes in Arctic atmospheric moisture and the water cycle, we decided to quantitatively investigate the linkages between poleward AMT and extratropical cyclone activity. In this study, we used 60°N as the boundary between the midlatitudes and the Arctic to investigate the impact of extratropical cyclones that move into the Arctic on the AMT. The 60°N line stands as the division between the "polar cap" and the midlatitudes, as it generally corresponds to the zero contour between the polar and midlatitude centers of action of the Arctic Oscillation (AO) (Thompson and Wallace, 1998).

    The goal of this study is to understand the role that extratropical cyclone activity plays in the poleward AMT. We seek to achieve this goal by answering the following specific questions: First, do AMT and cyclone activity exhibit any similar regional and interannual variations? Second, which components (e.g., intensity, number, etc.) of cyclone activity correlate closely with the changes in poleward AMT? And finally, how does cyclone activity behave differently under the positive and negative phases of poleward AMT, and how does this difference impact the AMT? The paper is organized as follows: Section 2 describes the data and methods used to obtain the AMT and cyclone activity data. Sections 3 and 4 describe the results of the time series analysis of AMT and cyclone activity and the analysis of the relationship between them. Section 5 concludes the paper with a summary and discussion.

2. Data and methodology
  • Following (Zhang et al., 2012), AMT is calculated with the original six-hourly spectral output of the NCAR-NCEP reanalysis (Kalnay et al., 1996) from January 1948 to December 2016, using the formula $$ {\rm AMT}=\iint_{\rm region}\int_1^0\dfrac{p_{\rm s}qv}{\rho g}d\sigma dLdt , $$ where p s is the surface pressure, q the specific humidity, v the meridional component of the wind vector, g the gravity acceleration, ρ the density of water, and σ is the terrain-following coordinate of the data. \(\int_\rm region\) is the longitudinal integral over the region, and \(\int\) is the time integral for the selected time range.

    In this study, only the AMT across the latitude of 60°N is used to measure the amount of moisture transported meridionally, in which positive numbers mean poleward transport of moisture, and negative numbers mean equatorial transport. Monthly and yearly integrations of AMT are calculated to facilitate understanding how the AMT behaves through the entire study period globally, as well as in four regions comprising North America (120°-70°W), the North Atlantic (70°W-20°E), Eurasia (20°-140°E), and the North Pacific (140°E-120°W).

    The cyclone activity parameters used for this study are calculated with sea level pressure (SLP) from the same six-hourly NCEP-NCAR reanalysis for the period from January 1948 to December 2016. Then, the cyclone activity parameters between 55°N and 65°N are used to analyze and compare the relationship between cyclone activity and the AMT crossing 60°N. Selecting this area spanning 10° meridionally allows us to track, and therefore analyze, only those cyclones that developed below 60°N and crossed into the Arctic. Cyclones are identified and tracked using empirical criteria developed by (Zhang et al., 2004) after examining six-hourly SLP reanalysis snapshots. The criteria used for the identification and tracking of cyclones from (Zhang et al., 2004) include:

    (1) If the SLP at one grid point is lower than the surrounding eight grid points, a cyclone candidate is identified with its center at that grid point.

    (2) The minimum pressure gradient calculated between the candidate cyclone center and the adjacent eight grid points is required to be at least 0.15 hPa (100 km)-1.

    (3) The minimum SLP gradient between the cyclone candidate center and its four adjacent grid points must be negative inward for effectively removing any open lows from the data.

    (4) If two or more cyclone candidates are detected within a radius of 1200 km of each other at the same time, they are considered to be a single cyclone.

    (5) The cyclone candidate is required to have a minimum lifetime of 12 h. In addition, if the location of a cyclone is within 600 km of a cyclone center identified 6 h earlier, the center is considered to be a new location of the existing cyclone. Otherwise, a new cyclone is generated.

    To describe the variation characteristics for cyclone activity, we use the following three cyclone activity parameters, as defined by (Zhang et al., 2004):

    (1) Cyclone intensity: defined by averaging the differences between the central SLP of cyclones and the climatological monthly mean SLP at corresponding grid points for all cyclones in a particular region during the month.

    (2) Cyclone trajectory count (number): defined as the number of cyclones over the entire duration occurring in a particular region during the month.

    (3) Cyclone Activity Index (CAI): defined as the sum of the differences between the cyclone central SLP and the climatological monthly mean SLP at corresponding grid points over all cyclone centers in a particular region during the month. Thus, CAI is an integrated parameter measuring the intensity, number and duration of cyclone activity.

3. Climatology of AMT and cyclone activity across 60°N
  • Performing a change-point analysis on the values of AMT that have been integrated for each year along 60°N allows us to determine whether any changes in the behavior of the data are present (Taylor, 2015). With this analysis, we identify three distinct periods at a 99% significance level by the Student's t-test (Fig. 1). The first period is denoted by a downward trend of -26.95 km3 yr-1 of poleward yearly AMT in the first 11 years. The second period in the yearly AMT trend is identified from 1959 to 1982 where an upward trend of 6.08 km3 yr-1 is present. The third and last period is defined to span from 1982 up to the end of the study period and continues the upward trend seen in the second period, albeit at a smaller magnitude of 4.46 km3 yr-1. The year 1982 marks the year in which the average of yearly AMT after 1982 is higher than the average of yearly AMT across the entire period of study, whereas the average of yearly AMT before 1982 is lower than the average yearly AMT for the period of study. Poleward yearly AMT is the largest in the North Atlantic region, followed by the North Pacific and Eurasia regions. Yearly AMT in the North America region is mainly equatorward, which was also found by (Cullather et al., 2000) and (Rogers et al., 2001). Annually, the yearly AMT across 60°N is 5464 km3, in which 89% is from the oceanic regions (57% from North Atlantic and 32% from North Pacific) and 11% is from the continental regions. The Eurasia region accounts for 14% of the total poleward yearly AMT, balanced by a 3% equatorward transport in the North America region.

    Using the same three periods found in the global yearly AMT (Fig. 1a) for each region (Figs. 1b-e) helps to explain the region's contributions to the trends of global yearly AMT observed in each period. During the first period, all regions except the North Atlantic present downward trends, with the North Pacific contributing to 70% of the global downward trend. For the second period, the contributions to the global upward trend are mainly provided by the North Atlantic and Eurasia. Finally, all regions except Eurasia have upward trends in the last period. Eurasia exhibiting a downward trend during this last period explains why the global upward trend is smaller than that of the second period.

    Figure 1.  Annual variation of yearly AMT (units: km3) across 60°N (a) globally and (b-e) for the four regions of (b) the North Atlantic, (c) the North Pacific, (d) Eurasia, and (e) North America. Positive values indicate a poleward flux of moisture and negative values an equatorward flux. The thin solid straight lines are trend lines that indicate upward/downward trends for each of the three periods determined through a change-point analysis.

    Measuring the number of cyclones across 55°-65°N also demonstrates a clear regional distribution (Table 1). Annually, an average of 207 cyclones cross the latitudes of 55°-65°N, of which 66 (32%) cyclones are from the North Atlantic region and 57 (27%) from Eurasia. North America presents the smallest number with 37 (18%) cyclones, and the remaining 47 (23%) are from the North Pacific. The total cyclone number across 55°-65°N demonstrates an upward trend before 1971 and a downward trend after 1971, as indicated by the change-point analysis (Fig. 2). Similar to (Zhang et al., 2004), the cyclone number in the North Pacific region exhibits a downward trend throughout both periods, which helps promote Eurasia to become a bigger contributor than the North Pacific (Table 1). In addition, Eurasia also presents the largest upward trend in cyclone number amongst all the regions for the first period.

    Figure 2.  Global yearly cyclone number (top), cyclone intensity (units: hPa) (middle), and CAI (units: hPa) (bottom) that crossed 60°N. The thin solid straight lines are trend lines that indicate upward/downward trends for each period determined through a change-point analysis.

    Globally, the intensity of extratropical cyclones crossing 55°-65°N is around 17 hPa stronger than the mean SLP in that area (Table 1). Again, similar to (Zhang et al., 2004), cyclone intensity exhibits an upward trend during the entire study period, though the change-point analysis indicates a slight change in the upward trend behavior in 1955 (Fig. 2). Of the various regions, we find that the North Atlantic presents the highest intensity, and the trends in each period are the closest to the global trends, which again illustrates the importance of this region to global cyclone activity (Table 1). In contrast to the trend in cyclone number, cyclone intensity in the North Pacific shows an upward trend for both periods.

    Variations in cyclone number and intensity can have opposite phases; therefore, an integrated index (i.e., the CAI) is also used here to evaluate the overall variation in cyclone activity across 55°-65°N (Fig. 2). The change-point analysis is also performed for the CAI and results in a downward trend before 1956 and an upward trend after 1956. Globally, the CAI has an average of 2573 hPa across 55°-65°N, from which the most significant contributions are provided by the North Atlantic and Eurasia regions with 922 hPa (36%) and 592 hPa (23%) respectively (Table 1). The region that contributes the least to the global CAI is North America, with 512 hPa (20%). The North Pacific provides the remaining amount of 547 mb (21%), in accordance with the lower values of cyclone number and intensity when compared to the other ocean basin. Because the biggest contribution of the CAI comes from the North Atlantic region, the changes in the North Atlantic CAI throughout the study period are reflected in the global CAI. The CAI behavior during the downward trend period can be attributed to the North Atlantic and North America regions, while the upward trend period arises from the North Atlantic, Eurasia, and North America regions (Table 1). The CAI trends in the North Pacific and North America mostly cancel each other out, especially during the global downward trend period.

4. Relationship between cyclone activity and enhanced poleward AMT across 60°N
  • With the observed increases in both AMT and cyclone activity across 60°N and the similarity of their regional distributions, we further explore the relationship between them in this section. To understand if the trends that have been observed in AMT, as well as cyclone activity, have any relationship between one another, we calculate the trends within the distinct time series of AMT, cyclone number, cyclone intensity, and CAI. Because the observed trends are inherently long-term, a 20-yr running window is used to calculate these trends. Figures 3a-c illustrate how these trend time series behave over time. The time series of the yearly AMT trend exhibits a negative trend during 1958-67, followed by a positive trend during 1967-99. During the positive trend period, a maximum is reached around 1982, the year that the change-point analysis denotes as the beginning of the third period in the yearly AMT. The trends in cyclone number, intensity, and CAI behave differently. For example, cyclone number has a positive trend throughout the majority of the time series before 1999, while cyclone intensity begins with a relatively flat trend, then becomes negative during 1961-65, and remains positive till 1995. When the trend correlations of yearly AMT to cyclone number, intensity, and CAI are calculated, we have -0.01, 0.46 and 0.70, respectively, of which only the latter two are significant according to the Student's t-test at the greater than 99% confidence level. The correlation results reveal that when the relations between cyclone activity and yearly AMT are explored, one cannot focus on simply the amount of storms or the intensity. Instead, an index such as the CAI that encompasses most major characteristics of cyclone activity including quantity, intensity, and duration is required. The trends in CAI display a similar pattern as the trends in yearly AMT, except at the beginning of the examined period when the CAI trend is relatively flat, and during 1995-2005 when the CAI trend is negative. Therefore, the overall cyclone activity, not intensity or number only, seems to be closely related to the change in yearly AMT.

    Figure 3.  Comparison of trend time series for AMT and (a) cyclone number, (b) cyclone intensity, and (c) CAI, from 1948 to 2016, using a 20-yr running window. Correlations between each variable set are shown in the plot.

    Figure 4.  Monthly AMT for (a-l) January-December, respectively, with one standard deviation above and below the mean (dotted) illustrated by dashed lines. The trend line from 1959 to 2016 is shown by a bold line.

    Cyclone activity, particularly in the North Atlantic, is closely tied to the AO or North Atlantic Oscillation (NAO) index. For example, a positive AO/NAO index is related to an increase in storminess in the vicinity of Iceland and the Norwegian Sea, provided by a higher amount of storms in the North Atlantic (Zhang et al., 2004; Hurrell, 2015). The variability of AMT is also well related to the AO index, mainly due to changes in the mean flow and storminess changing the moisture transport pattern (Dickson et al., 2000; Jakobson and Vihma, 2010; Hurrell, 2015). However, in recent decades, the linkage between AMT and AO, as well as the cyclone activity, have become complicated, which might be attributable to the climate pattern change from a conventional tripolar AO to an anomalous dipolar leading pattern, also known as the Arctic Rapid-change Pattern (ARP) discussed by (Zhang et al., 2008).

    To better understand how the atmospheric circulation pattern and associated cyclone activity impact the poleward AMT, we perform a composite analysis of SLP associated with positive and negative AMT phases. To construct the composites of SLP, plus or minus one standard deviation of monthly AMT for the month analyzed is used to group the years into when strong or weak poleward AMT is present. Figure 4 depicts the interannual variations of monthly AMT in each month. Here, we only focus on the time when the increased poleward AMT occurs, so the period with upward trends, i.e., the second and third periods (1959-2016) in the yearly AMT change-point analysis, are used to calculate the trend of monthly AMT in each month. The monthly AMT varies from month to month with greater monthly AMT (400-500 km3) from August to November, and weaker monthly AMT (up to 350 km3) from February to June. Of special interest is that from 1956 through 2016, all months show upward trends with the highest trends occurring in August (1.824 km3 yr-1), September (1.297 km3 yr-1), and November (1.015 km3 yr-1). This suggests that any changes that affect poleward monthly AMT might be more visible in these months. Also of interest is that October, the month with the second highest mean monthly AMT (497.74 km3), is also one of the months with a relatively flat trend (0.495 km3 yr-1). The presence of this flat trend indicates that a relatively stable poleward monthly AMT is present in October.

    Figure 5.  Composite SLP for AMT larger than 1 standard deviation (left column) and smaller than 1 standard deviation (middle column) and their difference [right column; only statistically significant (99% confidence level) grid points plotted] for (a-c) January, (d-f) April, (g-i) July, and (j-l) October. Poleward (equatorward) moisture transport at 60°N is represented by red (blue) dots, whose sizes represent the magnitude of the transport.

    The composites of SLP and monthly AMT across 60°N associated with the positive and negative AMT phases for January, April, July, and October, representative of each season, are shown in Fig. 5. During the winter month of January, we observe significant planetary-scale systems including the Siberian high, North America high, Aleutian low, and Icelandic low along the belt of 60°N (Figs. 5a and b). The differences in the SLP composites between the positive and negative phases (significant at the 99% confidence level based on the Student's t-test) indicate that the Icelandic low is much stronger and extends poleward into the Arctic during the positive phase. Cyclone activity crossing 60°N over the North Atlantic region, as represented by the CAI in Table 2, is also stronger during the positive phase. Enhanced poleward AMT in the North Atlantic region co-occurs with stronger cyclone activity there (Fig. 5a and Table 2).

    During the spring month of April, similar differences in the SLP composite to that seen in winter are present. Again, the Icelandic low (though much weaker when compared to winter) extends poleward during the positive phase (Figs. 5d and e). Similarly, there is stronger cyclone activity crossing 60°N through the North Atlantic region co-occurring with enhanced poleward AMT in the region (Table 2).

    Over summer (July), the differences in the SLP composite between the positive and negative phases are much smaller (Figs. 5g-i). The low pressure over the Arctic is slightly stronger during the positive phase. Also, the low pressure over Eurasia is slightly stronger during the negative phase. Cyclone activity crossing 60°N in the North Atlantic region is much weaker in July but still relatively strong during the positive phase (Table 2).

    During fall (October), a pattern of differences in SLP composites reminiscent of that in spring and winter is again observed (Figs. 5j and k). The Icelandic low does not extend as far south as it did during the positive phase. Cyclone activity crossing 60°N measured by the CAI (Table 2) is similar to that in April, even though the entire low system is much stronger in October. During the negative phase, the Icelandic low is centered around 60°N, suggesting significant cyclone activity across there, and a relatively large CAI is observed (Table 2).

    Through the composite analysis performed here, it is possible to examine the relationship between the circulation pattern and associated cyclone activity and the poleward monthly AMT. Focus here is given to the North Atlantic region where the contribution to global monthly AMT is the greatest and the differences of SLP composites are significant at the 99% confidence level. A consistent relationship is found insofar as stronger cyclone activity across 60°N measured by the CAI there generally co-occurs with enhanced poleward monthly AMT in each representative seasonal month examined. This further supports why a strong correlation exists between the trends of the CAI and poleward yearly AMT.

5. Summary and discussion
  • The poleward AMT calculated from the NCAR-NCEP reanalysis data set has been analyzed along 60°N as a general boundary between the midlatitudes and the Arctic. An upward trend of poleward yearly AMT across 60°N since 1959 has been found, which has then consistently enhanced since 1982. The poleward yearly AMT demonstrates a distinct regional distribution, in which the maximum poleward yearly AMT occurs in the North Atlantic region. Poleward AMT also shows apparent seasonal variations, with greater values during August to November and smaller ones from February to June. An upward trend of monthly AMT is identified in all months since 1959, with the highest trend occurring in August, September, and November. It is also shown that greater poleward monthly AMT, as well as a greater upward trend, generally occurs during late summer and early fall, suggesting that the trend identified previously has continued and even amplified along with the rapidly changing Arctic system (Serreze et al., 1995; Comiso et al., 2008; Zhang et al., 2008; Skific et al., 2009).

    A poleward shift in cyclone tracks has been identified by several studies (Zhang et al., 2004; Simmonds et al., 2008; Sepp and Jaagus, 2011; Bender et al., 2012), which provides an incentive to study the relationship between extratropical cyclone activity and enhanced poleward AMT. The activity of cyclones across 55°-65°N measured with cyclone number, intensity and the integrated CAI is analyzed. Similar to the upward trend in poleward yearly AMT, cyclone intensity and the CAI also demonstrate a clear upward trend since 1956, with the biggest contribution to the CAI trend coming from the North Atlantic and Eurasia regions.

    The similarity of the trends observed in both the poleward yearly AMT and the activity of cyclones across 60°N motivated us to further explore the relationships between them. From the time series of linear trends constructed by using a 20-yr running window, we find a positive correlation of 0.70 between the poleward yearly AMT and CAI, an integrated measurement of cyclone activity including intensity, number, and duration. These consistent multidecadal variations between yearly AMT and the CAI may suggest a driving role played by the overall cyclone activity in the poleward AMT, considering the fundamental nature of cyclones in holding and transporting moisture and energy.

    Furthermore, a composite analysis of SLP and cyclone activity measured by the CAI and based on the positive and negative AMT phases reveals the impact of changed atmospheric circulation and cyclone activity on the poleward AMT. It is found that the poleward intrusions of the Icelandic low and accompanying enhancement of cyclone activity across 60°N would play an important role in driving the enhanced poleward AMT. The Icelandic low is generally defined on a monthly or seasonal time scale, as an integral representation of daily-scale storm activities. So, enhanced storm activity would consequently intensify the Icelandic low, perhaps through the wave breaking process. The poleward intrusion is consistent with the changes in the large-scale atmospheric circulation, which are characterized by a spatial structure shift from the AO to ARP (Zhang et al., 2008, 2012), and have decisively driven the rapid changes in the Arctic climate.

    Poleward AMT impacts the Arctic climate and environment in many different ways. Since water vapor content is a major greenhouse gas, an increase in poleward AMT can thus divert moisture in the Arctic, enhancing the downward infrared radiation and, in turn, causing surface warming (e.g., Woods et al., 2013; Baggett et al., 2016; Baggett and Lee, 2017; Gong et al., 2017). In addition, a greater amount of water vapor content may lead to greater amounts of condensation, which in turn warms the atmosphere even more due to latent heat release. This may generate a positive feedback between latent heat release and the emission of atmospheric longwave radiation to the surface, further enhancing the warming and sea-ice decrease, and leading to enlarged open water and increased local evaporation. Details and quantitative analysis of this feedback process associated with cyclone activity and poleward AMT would be an appropriate avenue for follow-up research.




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