ERA40再分析资料揭示的北半球和东亚地区温带气旋生成频率变化

本文利用欧洲中心再分析数据ERA40的6小时间隔海平面气压场和一种改进的客观判定和追踪方法研究19582001年北半球和东亚地区温带气旋生成频率的气候态、年代际变化及可能原因。结果表明:(1)北半球温带气旋的源地主要位于北美东部(落基山下游地区)、西北大西洋地区、格陵兰至欧洲北部地区、蒙古地区和日本至西北太平洋地区。大洋的西岸和陡峭地形的背风坡有利于大气斜压性的增强和正涡度的发展,从而有利于地面气旋的形成。(2)年、冬季和春季30°～60°N气旋生成数目呈现减少的变化趋势,60°～90°N地区的气旋生成数呈增加的变化趋势。这在一定程度上支持了北半球风暴路径北移的观点。60°N以南和以北的温带气旋数目同北极涛动指数(AO)分别呈现负相关和正相关,这种相关性在年、春季和秋季最为显著。(3)1 958—2001年东亚地区的年气旋数目呈现明显的年代际变化。20世纪60年代至80年代中期40°～60°N、80°～140°E地区气旋数目呈增加趋势,而80年代中期之后温带气旋数目则锐减,主要原因是80年代以后该地区大气斜压性减弱,更高纬度地区的大气斜压性增强,从而导致了气旋源地的北移。在较低纬带的20°～40°N、110°～160°E地区气旋数目线性增加,这主要是由于位于40°～55°N的北太平洋风暴轴有向低纬度偏移的变化趋势造成的。     

北半球温带气旋的变化

许多学者对近半个世纪以来温带气旋的频数、强度和路径的年际、年代际变化等特征进行了研究,并探讨了温带气旋变化与大气环流的关系,试图揭示气候变暖背景下温带气旋变化的可能原因。较为一致的研究结论是：在全球变暖背景下,北半球气旋活动的变化显示出在中纬度明显减少,而在高纬度增加的趋势,意味着气旋的路径已经明显地北移。研究还表明,气旋活动的变化与对流层斜压性、急流以及北大西洋涛动、海温梯度等因素有关。     

北半球温带气旋的模拟和预估研究Ⅱ:6个CMIP5耦合模式预估的未来RCP4.5情景下的变化

利用 CMIP5（Coupled Model Intercomparison Project Phase 5）的6个气候耦合模式中等排放情景———RCP4?5（典型浓度路径4?5）下的模拟结果对北半球温带气旋数目、风暴路径和强度的未来变化进行了研究分析。结果表明：（1）相对于20世纪后半叶,RCP4?5情景下的2053—2100年,虽然各模式的模拟结果存在一定的区域性差异,但共同显示了至21世纪末北半球整体温带气旋生成将减少,较低纬度减少得更显著。（2）模式较一致地模拟出未来北半球温带气旋的中心气压有降低的趋势,涡度强度将线性减弱。大多数的模式模拟得到北大西洋风暴轴未来将继续向极地偏移,但强度主要将减弱;过半的模式显示北太平洋风暴轴也将向极地偏移,强度变化则随季节的不同而不同。（3）6个模式的模拟结果均显示对流层中高层斜压区未来将向高层和高纬度扩展,南半球的变化更为显著。斜压区的变化在某种程度上反映了风暴轴的类似变化,因此, 这也支持了北太平洋和北大西洋风暴路径未来可能向极地偏移的结论。RCP4?5情景下北半球整体温带气旋活动将显著减少,但斜压区和风暴轴向高纬度的偏移将使较低纬度未来温带气旋活动减少得更为显著。     

夏季进入北极温带气旋的活动特征及其影响因素北大核心

进入北极地区的气旋在移动过程中往往伴有大风、强降水等特征,对北极气候变化有深刻的影响。基于NCEP2再分析资料,识别并跟踪了北半球夏季(6—8月)从中纬度进入北极的温带气旋,考察了其年际变化特征和影响因素。结果表明:1979—2019年夏季进入北极的温带气旋共867个,其中消失在北极边缘区域和中心区域的数量分别为688个和161个,且后者平均强度更大、平均持续时间更长。分区域研究发现,夏季从陆地进入极区的气旋个数较多,而从海洋进入极区的气旋强度更大,活动更为剧烈。对进入北极的气旋年际时间序列进行分析发现,夏季进入北极的气旋个数和强度均存在年际变率,其中气旋个数的年际变率尤为显著。气旋个数年际变率主要周期为5 a,强度的主要周期约为2.7 a。进一步分析发现,引导气流是影响气旋向北移动的重要因素。此外,夏季北大西洋气旋强度与同期北大西洋涛动(North Atlantic Oscillation,NAO)指数存在较好相关。研究还表明,进入极区气旋活动的年际变化受大气斜压不稳定性的影响,在北太平洋地区区域平均的Eady增长率与气旋个数和强度的相关性均最强,相关系数分别为0.4和0.5。     

北极涛动对ENSO影响的研究进展

北极涛动(AO)是北半球中高纬地区大气环流年际变率的第一主导模态,厄尔尼诺-南方涛动(ENSO)则是热带太平洋地区年际变化的最强信号。这两个气候系统对全球大范围的天气和气候都能产生显著的影响。总结了近年来关于AO影响ENSO的最新研究进展,试图促进对中高纬度大气系统影响热带系统物理过程的理解。研究表明,春季AO对随后冬季ENSO的爆发存在显著的影响,其中北太平洋中纬度地区的天气尺度波与平均流之间的相互作用及其相关的涡度输送对赤道西太平洋异常西风的形成起着重要的作用。在年代际时间尺度上,研究发现,春季AO对随后冬季ENSO的影响在20世纪70年代初发生了一次显著的年代际变化,该年代际变化与北太平洋地区风暴轴强度的年代际变化紧密相联。研究还揭示了春季AO对ENSO影响的不对称性,即只有当春季AO处于正位相时,才能对随后冬季赤道中东太平洋海温产生显著影响。研究发现春季AO位相变化对前冬NPO与后一个冬季ENSO之间的关系具有显著的调制作用。此外,研究还发现,除春季AO外,11月AO对随后春季和夏季赤道中东太平洋海温也存在显著的影响,这主要和北太平洋地区风暴轴气候态强度的年循环有关。     

辽宁春季透雨的环流背景及与海温相关分析

利用1961—2008年4—5月辽宁14个站逐日降水资料,分析了春季第一场透雨出现日期与500 hPa的环流背景及海温的相关关系。结果表明：中国辽宁春季透雨出现日期与前一年9—11月500 hPa高度场有较好的相关性,与北半球同年4月500 hPa环流年代际变化有较好的对应关系;透雨出现日期与北太平洋前期海温呈显著的负相关,当北太平洋地区前期海温偏高时,中国辽宁春季透雨出现日期偏早;当北太平洋地区前期海温偏低时,中国辽宁春季透雨出现日期偏迟。     

冬季北太平洋风暴轴的年际变化及其与500hPa高度以及热带和北太平洋海温的联系

文中研究了冬季北太平洋风暴轴的年际异常及其与500hPa高度以及热带和北太平洋海温的联系。结果发现,各年冬季北太平洋风暴轴的中心强度和位置具有显著的年际差异。对15个冬季北太平洋风暴轴区域500hPa天气尺度滤波位势高度方差与热带和北太平洋海温的SVD分析表明,第一对空间典型分布反映了赤道中、东太平洋区域海温异常对风暴轴年际变化的影响,而第二对空间典型分布反映了黑潮区域海温异常对风暴轴年际变化的影响。进一步的合成分析显示,赤道中、东太平洋区域海温异常主要影响冬季北太平洋风暴轴的东西摆动和中、东端的强度变化,而黑潮区域海温异常则主要影响冬季北太平洋风暴轴中、西端的强度变化和南北位移。并且这种影响分别与500hPa高度场上的PNA遥相关型和WP遥相关型有密切联系。     

太平洋年代际振荡对冬季北半球两大洋风暴轴协同变化的可能影响

利用NCEP/NCAR提供的再分析资料和NOAA提供的海温资料分析太平洋年代际振荡(Pacific Decadal Oscillation,PDO)不同位相的年代际背景下北半球海气耦合关系的异常与风暴轴协同变化的联系,主要结果如下:1)冬季太平洋年代际振荡与北半球两大洋风暴轴协同变化之间存在显著的相关关系,当PDO暖位相时,对应两大洋风暴轴南北位置反向的异常变化,其中北太平洋风暴轴偏南且中东部减弱,北大西洋风暴轴偏北且中东部增强,PDO冷位相时相反。2)PDO为暖位相时,对应El Niňo型海温异常,北大西洋海温呈三极型,平均槽脊加强,经向环流增强,极涡收缩,北太平洋风暴轴南压,大西洋风暴轴则北抬,此时欧亚大陆北部和北美大陆大部分地区温度异常升高,亚洲南部、非洲北部及巴伦支海以北的高纬温度异常降低,北美西南部和格陵兰岛附近温度也为异常降低,PDO冷位相时相反。     

冬季太平洋SSTA对北太平洋风暴轴年际变化的影响

研究了冬季北太平洋风暴轴的年际异常及其与500hPa高度以及热带和北太平洋海温的联系。结果发现，冬季北太平洋风暴轴中心有线性增强、偏北、偏东的趋势。对15个冬季北太平洋风暴轴区域 500 hPa天气尺度滤波位势高度方差与同期热带和北太平洋海温的 SVD分析表明，第一对空间典型分布反映了 ENSO区海温异常对风暴轴年际变化的影响，而第二对空间典型分布反映了黑潮区域海温异常对风暴轴年际变化的影响。进一步的合成分析显示，ENSO区海温异常可以通过激发500hPa高度场上的PNA遥相关型影响冬季北太平洋风暴轴的东西摆动和中、东端的强度变化，而黑潮区域海温异常则通过激发 500 hPa高度场上的 WP遥相关型，主要影响冬季北太平洋风暴轴中、西端的强度变化和南北位移。     

近50 a春季东亚温带气旋活动频数的气候特征及其变化

根据1948-2000年共53 a的NCEP/NCAR逐日海平面气压再分析资料,分析了春季(3、4、5月)东亚内陆和沿海地区气旋活动频数、气旋移动路径等气候特征及其年际、年代际变化,结果表明在春季的3、4、5月份中,内陆存在三个明显的气旋活动频数较大的地区,分别位于101°E、45°N附近的蒙古高压南缘、贝加尔湖以东的115°E、53°N附近地区和126°E、53°N我国东北北部附近地区.蒙古地区春季气旋活动频数存在明显的年代际变化,50年代气旋活动频数较少,60年代开始到70年代后期气旋活动频数较多,从70年代末至今又进入一个气旋活动频数较少的时期.贝加尔湖东部地区在50年代初气旋活动频数较多,50年代中期到60年代中期气旋活动频数较少,而60年代后期到70年代后期气旋活动频数又较多,此后气旋活动频数逐渐减少.我国东北地区春季各月气旋活动频数没有明显的年代际变化,只有80年代以来气旋活动频数的振幅较大.东亚沿海春季气旋活动频数较高的地区主要位于我国东北到俄罗斯远东和我国长江中下游到日本一带.从气旋活动路径来看,我国长江中下游到日本一带气旋活动路径在各个年代基本相同,没有明显的年代际变化,而我国东北到俄罗斯远东地区气旋活动路径在不同年代略有差异.进一步分析发现,气旋活动频数与北方地区春季降水量及沙尘暴发生次数具有一定关系.     

Interannual variations and future change of wintertime extratropical cyclone activity over North America in CCSM3

Climatology and interannual variations of wintertime extratropical cyclone frequency in CCSM3 twentieth century simulation are compared with the NCEP/NCAR reanalysis during 1950–1999. CCSM3 can simulate the storm tracks reasonably well, although the model produces slightly less cyclones at the beginning of the Pacific and Atlantic storm tracks and weaker poleward deflection over the Pacific. As in the reanalysis, frequency of cyclones stronger than 980 hPa shows significant correlation with the Pacific/North America (PNA) teleconnection pattern over the Pacific region and with the North Atlantic Oscillation (NAO) in the Atlantic sector. Composite maps are constructed for opposite phases of El Nino-Southern Oscillation (ENSO) and the NAO and all anomalous patterns coincide with observed. One CCSM3 twenty-first century A1B scenario realization indicates there is significant increase in the extratropical cyclone frequency on the US west coast and decrease in Alaska. Meanwhile, cyclone frequency increases from the Great Lakes region to Quebec and decreases over the US east coast, suggesting a possible northward shift of the Atlantic storm tracks under the warmer climate. The cyclone frequency anomalies are closely linked to changes in seasonal mean states of the upper-troposphere zonal wind and baroclinicity in the lower troposphere. Due to lack of 6-hourly outputs, we cannot apply the cyclone-tracking algorithm to the other eight CCSM3 realizations. Based on the linkage between the mean state change and the cyclone frequency anomalies, it is likely a common feature among the other ensemble members that cyclone activity is reduced on the East Coast and in Alaska as a result of global warming.     

Extratropical cyclone variability in the Northern Hemisphere winter from the NCEP/NCAR reanalysis data

 The winter climatology of Northern Hemisphere cyclone activity was derived from 6-hourly NCEP/NCAR reanalysis data for the period from 1958 to 1999, using software which provides improved accuracy in cyclone identification in comparison to numerical tracking schemes. Cyclone characteristics over the Kuroshio and Gulfstream are very different to those over continental North America and the Arctic. Analysis of Northern Hemisphere cyclones shows secular and decadal-scale changes in cyclone frequency, intensity, lifetime and deepening rates. The western Pacific and Atlantic are characterized by an increase in cyclone intensity and deepening during the 42-year period, although the eastern Pacific and continental North America demonstrate opposite tendencies in most cyclone characteristics. There is an increase of the number of cyclones in the Arctic and in the western Pacific and a downward tendency over the Gulf Stream and subpolar Pacific. Decadal scale variability in cyclone activity over the Atlantic and Pacific exhibits south-north dipole-like patterns. Atlantic and Pacific cyclone activity associated with the NAO and PNA is analyzed. Atlantic cyclone frequency demonstrates a high correlation with NAO and reflects the NAO shift in the mid 1970s, associated with considerable changes in European storm tracks. The PNA is largely linked to the eastern Pacific cyclone frequencies, and controls cyclone activity over the Gulf region and the North American coast during the last two decades. Assessment of the accuracy of the results and comparison with those derived using numerical algorithms, shows that biases inherent in numerical procedures are not negligible. Received: 7 July 2000 / Accepted: 30 November 2000     

Response of Northern Hemisphere storm tracks to Indian-western Pacific Ocean warming in atmospheric general circulation models

With 40 years integration output of two atmospheric general circulation models (GAMIL/IAP and HadAM3/UKMO) forced with identical prescribed seasonally-varying sea surface temperature, this study examines the effect of the observed Indian-western Pacific Ocean (IWP) warming on the Northern Hemisphere storm tracks. Both models indicate that the observed IWP warming tends to cause both the North Pacific storm track (NPST) and the North Atlantic storm track (NAST) to move northward. Such a consistent effect on the two storm tracks is closely associated with the changes in the low-level atmospheric baroclinicity, high-level jet stream and upper-level geopotential height. The IWP warming can excite a wavelike circum-global teleconnection in the geopotential height that gives rise to an anticyclonic anomaly over the midlatitude North Pacific and a positive-phase NAO anomaly over the North Atlantic. These geopotential height anomalies tend to enhance upper-level zonal westerly winds north of the climatological jet axes and increase low-level baroclinicity and eddy growth rates, thus favoring transient eddy more active north of the climatological storm track axes, responsible for the northward shift of the both storm tracks. The IWP warming-induced northward shift of the NAST is quite similar to the observed, suggesting that the IWP warming can be one of the key factors to cause decadal northward shift of the NAST since the 1980s. However, the IWP warming-induced northward shift of the NPST is completely opposite to the observed, implying that the observed southward shift of the NPST since the 1980s would be primarily attributed to other reasons, although the IWP warming can have a cancelling effect against those reasons.     

Modelled atmospheric response to changes in Northern Hemisphere snow cover

The surface boundary conditions are altered in a numerical simulation of January climate by prescribing (a) higher and (b) lower than average snow extent over Northern Hemisphere land masses. The anomalies in snow cover are shown to have quite a strong impact on the mean climatic state. Associated with an increase in the areal extent of the snow, there is a significant reduction in temperature throughout the lower troposphere. There are also large increases in sea-level pressure over most land areas. Significant responses in the mass field are also seen at 500 hPa where reductions in atmospheric thickness lead to significant negative anomalies in the height field. Responses are also seen non-locally, over both the North Pacific and North Atlantic basins. The impact of increased snow on cyclone tracks is also examined. A reduction in cyclones is noted over both continents and over the western sectors of both ocean basins. Over the North Atlantic basin this reduction extends across over Europe, significantly weakening the storm track. In the North Pacific, cyclone density is reduced in the west while in the east, there is actually a strengthening of the storm tracks. There are corresponding changes in the genesis of cyclones in both of these regions. The change in cyclogenesis, intensity and density is demonstrated to be associated with changes in baroclinicity between the two experiments. The anomalous snow boundary conditions lead to significant changes in the meridional temperature gradients over both ocean basins which impact on the baroclinic zones. Received: 5 January 1996 / Accepted: 4 May 1996     

Climatology of extratropical cyclones over the South American–southern oceans sector

A climatology of extratropical cyclones is presented. Extratropical cyclones, their main characteristics and their predominant tracks, as well as their interannual variability, affect weather in South America. For that purpose, a storm track database has been compiled by applying a cyclone tracking scheme to six-hourly sea level pressure fields, available from the National Center for Environmental Prediction–National Center for Atmospheric Research reanalyses II for the 1979–2003 period. The spatial distribution of the cyclogenesis frequency shows two main centers: one around Northern Argentina, Uruguay, and Southern Brazil in all seasons and the other near to the North Antarctic Peninsula. The lifetime of extratropical cyclones in the South American sector exhibits small seasonality, being typically of the order of 3.0 days during most of the year and slightly higher (3.5 days) in austral summer. The distance travelled by the cyclones formed in the South American sector tends to be smaller than the total paths found in other areas of the Southern Hemisphere. A k-mean clustering technique is used to summarize the analysis of the 25-year climatology of cyclone tracks. Three clusters were found: one storm-track cluster in Northeast Argentina; a second one west of the Andes Cordillera; and a third cluster located to the north of the Antarctic Peninsula (around the Weddell Sea). The influence of the Antarctic Oscillation (AAO) in the variability of extratropical cyclones is explored, and some signals of the impacts of the variability of the AAO can be observed in the position of the extratropical cyclones around 40°S, while the impacts on the intensity is detected around 55°S.     

Three-Dimensional Structure and Low-Level Features of the North Pacific Storm Track from 1999 to 2005

A storm track is a region in which synoptic eddy activities are statistically most prevalent and intense. At daily weather charts, it roughly corresponds to the mean trajectories of cyclones and anticyclones. In this paper, the recent QuikSCAT （Quick Scatterometer） satellite sea winds data with a 0.5°×0.5° horizontal resolution, and the NCEP （National Centers for Environmental Prediction） 10-m height Gaussian grid wind data and pressure-level reanalysis data, are employed to document the spatial structure of the North Pacific storm track in winter （January） and summer （July） from 1999 to 2005. The results show that in winter the North Pacific storm track is stronger, and is located in lower latitudes with a distinct zonal distribution. In summer, it is weaker, and is located in higher latitudes. Based on the horizontal distributions of geopotential height variance at various levels, three-dimensional schematic diagrams of the North Pacific storm track in winter and summer are extracted and presented. Analyses of the QuikSCAT wind data indicate that this dataset can depict the low-level storm track features in detail. The double storm tracks over the Southern Oceans found by Nakamura and Shimpo are confirmed. More significantly, two new pairs of low-level storm tracks over the North Pacific and the North Atlantic are identified by using this high-resolution dataset. The pair over the North Pacific is focused in this paper, and is named as the ＂subtropical storm track＂ and the ＂subpolar storm track＂, respectively. Moreover, statistical analyses of cyclone and anticyclone trajectories in the winters of 1999 to 2005 reveal as well the existence of the low-level double storm tracks over the North Pacific.     

冬季北半球风暴轴的多尺度能量变化特征及其可能机制

基于1971—2016年NCEP/NCAR(美国环境预报中心和国家大气研究中心)的逐日再分析资料及NCPC(美国国家海洋和大气管理局气候预报中心)的海温、大气环流及海洋指数等资料通过多尺度能量分析(MS-EVA)等方法,把冬季北半球风暴轴看做一整体,分析了风暴轴区域多尺度的能量变化特征及其可能机制。主要结论概括如下:(1)多年气候平均状态下,风暴轴的动能来源主要表现为在风暴轴中上游先由低频尺度向天气尺度输送有效位能,随后在风暴轴主体区再由天气尺度有效位能转换为天气尺度动能,其中风暴轴西端可直接由低频尺度向天气尺度输送动能。(2)北半球三大风暴轴联合EOF结果表明:第一模态下,主要体现了北西伯利亚风暴轴与北太平洋风暴轴强度的减弱(增强),同时伴随着北大西洋风暴轴位置北抬(南压);第二模态下,主要体现了北西伯利亚风暴轴强度减弱(增强),同时北太平洋风暴轴位置北抬(南压)中东部强度增强(减弱),而北大西洋风暴轴位置南压(北抬)。(3)回归分析表明:北半球风暴轴异常在不同模态下与低频尺度环流联系密切。低频尺度波动可通过海温及西风急流等异常变化先影响风暴轴区域多尺度间的能量转换,进而影响风暴轴整体的异常变化。