2015年初北极极端气旋对中国寒潮的影响

从北大西洋中高纬度进入北极的极端气旋会引起北极异常增暖,与中高纬度极端天气事件关系密切,危害极大。利用ERA-Interim再分析资料和中国地面气象站观测资料,探讨了2015年1—2月两个极端气旋（C1,C2）影响中国天气的物理过程和机制。结果表明:当极端气旋生成并北移,附近大气低层和高层均出现异常增暖,中高纬度大气环流表现为乌拉尔阻塞形势形成,极涡断裂,低压槽加深南压,我国发生寒潮天气;且极端气旋伴随的异常增暖加强Rossby波能量频散,使中高纬度的槽和脊发展。对比发现,C1和C2的生成地和路径均存在差异,相比于C2,C1生成纬度较高且路径偏东,对应低温寒潮天气范围更大,但强度比C2略弱。这些结果均表明,极端气旋的生成和移动是中国寒潮天气发生的重要原因之一。     

Extreme midlatitude cyclones and their implications for precipitation and wind speed extremes in simulations of the Maunder Minimum versus present day conditions

Extreme midlatitude cyclone characteristics, precipitation, wind speed events, their inter-relationships, and the connection to large-scale atmospheric patterns are investigated in simulations of a prolonged cold period, known as the Maunder Minimum from 1640 to 1715 and compared with today. An ensemble of six simulations for the Maunder Minimum as well as a control simulation for perpetual 1990 conditions are carried out with a coupled atmosphere-ocean general circulation model, i.e., the Climate Community System Model (CCSM). The comparison of the simulations shows that in a climate state colder than today the occurrence of cyclones, the extreme events of precipitation and wind speed shift southward in all seasons in the North Atlantic and the North Pacific. The extremes of cyclone intensity increases significantly in winter in almost all regions, which is related to a stronger meridional temperature gradient and an increase in lower tropospheric baroclinicity. Extremes of cyclone intensity in subregions of the North Atlantic are related to extremes in precipitation and in wind speed during winter. Moreover, extremes of cyclone intensity are also connected to distinct large-scale atmospheric patterns for the different subregions, but these relationships vanish during summer. Analyzing the mean 1,000 hPa geopotential height change of the Maunder Minimum simulations compared with the control simulation, we find a similar pattern as the correlation pattern with the cyclone intensity index of the southern Europe cyclones. This illustrates that changes in the atmospheric high-frequency, i.e., the simulated southward shift of cyclones in the North Atlantic and the related increase of extreme precipitation and wind speed in particular in the Mediterranean in winter, are associated with large-scale atmospheric circulation changes.     

夏季进入北极温带气旋的活动特征及其影响因素

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。     

北半球温带气旋活动和风暴路径的年代际变化

基于欧洲中心再分析数据ERA40的海平面气压场和高度场,本文分别采用拉格朗日和欧拉方法研究分析了1958～2001年北半球的不同季节温带气旋活动和风暴路径的年代际变化,以及可能的原因.以客观判定和追踪温带气旋为基础的拉格朗日方法得到了北半球的两个温带气旋主要活动中心,即北太平洋地区和北大西洋/北美地区,同时以500 hPa位势高度天气尺度滤波方差为基础的欧拉方法得到了同主要气旋活动中心相吻合的两条风暴轴.研究表明,44年中北大西洋/北美地区温带气旋活动北移加强,以春季最为显著.风暴轴也同样存在着向极移动并加强的特征,并且温带气旋和风暴路径两者移动趋势的相关性很高.作为一个典型地区,北大西洋/北美地区的气旋活动体现了风暴路径的北移,以及温带地区向极地的扩展.但有意思的是北太平洋的情况完全不同,即北太平洋地区的温带气旋活动和风暴轴向低纬度偏移并加强,以春季的南移趋势最为显著.对于此结论,两种方法也有很高的统计相关性.虽然大量研究表明北半球整体上呈现出风暴路径北移的变化特征,但对于具体地区情况有明显差异.另外,400 hPa最大Eady增长率和气旋活动频率的经验正交展开函数 (EOF) 第一模态的空间分布和时间序列非常相似,北太平洋地区和北大西洋地区风暴路径相反的变化趋势很可能同其大气斜压性的同位相的变化有着密切的关系.这也从另一个方面支持了本文对温带气旋和风暴路径年代际变化的分析.     

Factors contributing to the development of extreme North Atlantic cyclones and their relationship with the NAO

The occurrence of extreme cyclones is analysed in terms of their relationship to the NAO phase and the dominating environmental variables controlling their intensification. These are latent energy (equivalent potential temperature 850 hPa is used as an indicator), upper-air baroclinicity, horizontal divergence and jet stream strength. Cyclones over the North Atlantic are identified and tracked using a numerical algorithm, permitting a detailed analysis of their life cycles. Extreme cyclones are selected as the 10% most severe in terms of intensity. Investigations focus on the main strengthening phase of each cyclone. The environmental factors are related to the NAO, which affects the location and orientation of the cyclone tracks, thus explaining why extreme cyclones occur more (less) frequently during strong positive (negative) NAO phases. The enhanced number of extreme cyclones in positive NAO phases can be explained by the larger area with suitable growth conditions, which is better aligned with the cyclone tracks and is associated with increased cyclone life time and intensity. Moreover, strong intensification of cyclones is frequently linked to the occurrence of extreme values of growth factors in the immediate vicinity of the cyclone centre. Similar results are found for ECHAM5/OM1 for present day conditions, demonstrating that relationships between the environment factors and cyclones are also valid in the GCM. For future climate conditions (following the SRES A1B scenario), the results are similar, but a small increase of the frequency of extreme values is detected near the cyclone cores. On the other hand, total cyclone numbers decrease by 10% over the North Atlantic. An exception is the region near the British Isles, which features increased track density and intensity of extreme cyclones irrespective of the NAO phase. These changes are associated with an intensified jet stream close to Europe. Moreover, an enhanced frequency of explosive developments over the British Isles is found, leading to more frequent windstorms affecting Europe.

2020年夏季海洋天气评述

2020年夏季（6—8月）,北半球极涡呈现明显的单极型分布,极涡主体位于北极圈内,中心偏向东半球,中高纬环流呈现4波型分布。6—7月,西太平洋副热带高压较常年平均偏强,且位置偏西偏南,不利于热带气旋活动。2020年夏季共有8个热带气旋在西北太平洋和南海生成,其中7月没有热带气旋生成。除西北太平洋和南海之外,其他热带洋面另有20个热带气旋生成,其中北大西洋11个,东太平洋8个,北印度洋1个。受偏南暖湿气流的影响,我国北方海域多海雾天气。同时受入海气旋活动影响,多海上大风过程。夏季近海海域共出现了7次比较明显的海雾过程,其中6月3次,7月1次,8月3次。大风过程出现了10次, 2次由热带气旋影响,7次与入海气旋活动有关。发生2 m以上的大浪过程12次,6—8月分别出现了4次、5次和3次。     

春季影响中国北方地区的蒙古气旋及其背景环流

基于1948~2013年NCEP/NCAR逐日再分析资料,采用850 hPa相对涡度场气旋追踪方法,统计了春季蒙古气旋的活动特征,包括其源地、盛期位置、消亡地以及路径的分布。在此基础上,根据蒙古气旋的不同移动路径,界定了蒙古气旋的两种路径,即向东路径和东南路径,并统计了这两类气旋的频数与强度的年际和年代际变化。同时还揭示了这两类蒙古气旋的低频背景环流特征。本文主要结论如下:(1)蒙古气旋主要生成于贝加尔湖南侧和东侧的山脉背风坡,并在蒙古东部地区以及我国东北地区达到盛期,多数气旋消亡于东北亚及其临海区域;(2)两类气旋的生成个数均有显著的年代际变化。向东路径气旋在1950年代的个数偏少,1970年代至1990年代的个数整体偏多,之后有所减少,但进入21世纪后气旋有个数呈现增多的趋势。东南路径气旋在1970年代以前一直处于偏少阶段,1970年代至1980年代中期处于偏多阶段,2005年以后东南路径气旋个数有减少的趋势;(3)向东路径蒙古气旋的背景低频环流由斯堪的纳维亚半岛正异常中心、贝加尔湖及西侧的负异常中心以及环日本海地区的正异常中心所组成的正位相斯堪的纳维亚环流型为主要特征。东南路径蒙古气旋则以俄罗斯西部的负异常中心、拉普捷夫海附近延伸到我国西北地区的正高度异常区以及东北低涡环流为主要特征。     

Objective cyclone climatologies of the North Atlantic – a comparison between the ECMWF and NCEP Reanalyses

Simple and easily reproducible techniques have been used to construct two objective cyclone climatologies of the North Atlantic-European sector. The goal of this study is to increase understanding of cyclones with the potential to cause damage, in particular, those reaching Beaufort category 7 and above. The two climatologies constructed here span the period 1979–2000 and have been developed from reanalysis mean sea level pressure data from the ECMWF (European Centre for Medium Range Weather Forecasts) and NCEP (National Centres for Environmental Prediction). The ECMWF reanalysis data are only available for 15 years, and have been extended from 1994 using operational analyses. The major temporal and spatial characteristics of North Atlantic cyclones are examined and a comparison between the climatologies developed from the two data sets is carried out. The well-known cyclogenesis regions along the east coast of the United States and to the southeast of Greenland are replicated by both reanalyses, as is the characteristic southwest/northeast orientation of the dominant cyclone track across the Atlantic basin. However, only weak correlations are found between the time series of cyclone frequency produced from the two reanalyses, and this is particularly true for the lower intensity Beaufort Scale category 0–6 cyclones. This result, together with the large differences in the spatial distribution of cyclones over Greenland for Beaufort Scale 0–6 cyclones, indicates the NCEP reanalyses generates fewer systems than the ECMWF reanalyses. The overall conclusion is that the ECMWF mean sea level pressure data produce a more comprehensive climatology of North Atlantic cyclones at all scales.     

20世纪90年代以后华北初春低温增强和北大西洋海温关系

本文利用ECMWF再分析资料及Hadley中心提供的海温数据分析了20世纪90年代以后华北地区初春低温增强的原因,并通过数值模拟结果予以验证。结果表明,北大西洋“马蹄型”海温模态与影响我国华北地区的欧亚波列存在显著的相关关系。同时该海温模态与1997年以后北大西洋关键区垂直波作用通量有着较密切的相关关系,1997年以后北大西洋地区的500 hPa环流模态,整体呈现出东移南撤的趋势。1997年以后格陵兰岛东侧表面温度受异常热力强迫导致正值区增多,同时此处西风急流加大,有利于Rossby波向下游传播,导致其下游欧洲大陆地区形成暖脊。通过局地多尺度能量涡度分析法（Localized Multiscale Energy and Vorticity Analysis,简称MS-EVA）证明格陵兰岛东侧关键区表面温度的异常热力强迫作用与气压梯度力在对流层整层做正功,导致高层动能的增加并向外辐散,使得脊加强向北伸展。通过欧亚波列致使下游华北地区上空气旋式异常加强,促使亚洲极涡加强和稳定维持,华北地区温度下降剧烈,极端低温事件增多。最后通过CAM5.1模式模拟研究了北大西洋“马蹄型”海温模态对大气环流异常及华北地区极端低温的影响。模拟结果很好地验证了观测结果,进一步表明该海温模态可以通过激发出欧亚波列,影响欧亚大陆大气环流异常,进而导致我国华北地区气旋性加强和经向环流加大,极端低温事件增多。     

Projected future changes in tropical cyclone-related wave climate in the North Atlantic

Tropical cyclones are a major hazard for numerous countries surrounding the tropical-to-subtropical North Atlantic sub-basin including the Caribbean Sea and Gulf of Mexico. Their intense winds, which can exceed 300 km h⁻¹, can cause serious damage, particularly along coastlines where the combined action of waves, currents and low atmospheric pressure leads to storm surge and coastal flooding. This work presents future projections of North Atlantic tropical cyclone-related wave climate. A new configuration of the ARPEGE-Climat global atmospheric model on a stretched grid reaching ~ 14 km resolution to the north-east of the eastern Caribbean is able to reproduce the distribution of tropical cyclone winds, including Category 5 hurricanes. Historical (1984–2013, 5 members) and future (2051–2080, 5 members) simulations with the IPCC RCP8.5 scenario are used to drive the MFWAM (Météo-France Wave Action Model) spectral wave model over the Atlantic basin during the hurricane season. An intermediate 50-km resolution grid is used to propagate mid-latitude swells into a higher 10-km resolution grid over the tropical cyclone main development region. Wave model performance is evaluated over the historical period with the ERA5 reanalysis and satellite altimetry data. Future projections exhibit a modest but widespread reduction in seasonal mean wave heights in response to weakening subtropical anticyclone, yet marked increases in tropical cyclone-related wind sea and extreme wave heights within a large region extending from the African coasts to the North American continent.

     

Long-term precipitation variability in Morocco and the link to the large-scale circulation in recent and future climates

Summary ?Monthly precipitation data from the Global Historical Climatology Network for 42 stations in Morocco and its vicinity are investigated with respect to baroclinicity, storm track and cyclone activity, moisture transports, North Atlantic Oscillation (NAO) variations, and different circulation types by means of correlation and composite studies. The results are related to a climate change scenario from an ECHAM4/OPYC3 transient greenhouse gas only (GHG) simulation. Precipitation in northwestern Morocco shows a clear link to the baroclinic activity over the North Atlantic during boreal winter (DJF). In large precipitation months the North Atlantic storm track is shifted southward, more westerly and northwesterly circulation situations occur and moisture transports from the Atlantic are enhanced. The occurrence of local cyclones and upper-level troughs is more frequent than in low precipitation months. The negative correlation to the NAO is relatively strong, especially with Gibraltar as a southern pole (−0.71). The northward shift of the storm track and eastward shift of the Azores High predicted by the ECHAM model for increasing GHG concentrations would therefore be associated with decreasing precipitation and potentially serious impacts for the future water supply for parts of Morocco. In the region south of the Atlas mountains, moisture transports from the Atlantic along the southern flank of the Atlas Mountains associated with cyclones west of Morocco and the Iberian Peninsula can be identified as a decisive factor for precipitation. Northeastern Morocco and Northwestern Algeria, however, is rather dominated by the influence of cyclones over the Western Mediterranean that are associated with a strong northwesterly moisture transport. As both regions appear to be less dependent on the North Atlantic storm track and more on local processes, a straight forward interpretation of the large-scale changes predicted by the ECHAM4/OPYC3 cannot be done without the application of down-scaling methods in the future. Received July 19, 2001; revised May 31, 2002     

2018年夏季海洋天气评述

2018年夏季（6—8月）大气环流特征为：北半球极涡呈单极型分布,主体位于北冰洋上空,中高纬西风带呈4波型分布。受大尺度环流影响,6—7月,我国南方海域多热带气旋活动,北方海域多海雾;7—8月副热带高压位置较常年偏北,影响华东的台风明显偏多。2018年夏季,共有18个台风命名,比常年平均偏多7个。我国近海夏季的大风、大浪与台风活动关系密切。有20次8级以上大风过程：其中热带气旋过程大风有13次,两次伴随冷空气活动;温带气旋、准静止锋引发大风5次;另外,有2次过程主要由季风引起。有7次比较明显的海雾过程出现,其中6月出现4次,7月3次。发生13次2 m以上的大浪过程,其中,6月3次,7月和8月各5次。其他各大洋共有18个命名热带气旋生成,分别为：大西洋4个、东太平洋14个。     

Cyclone activity and circulation oscillations in the North Atlantic

Carried out is the comparative analysis of the cyclone activity for different combinations of positive and negative values of the North Atlantic Oscillation (NAO) and East Atlantic Oscillation (EA) indices. The integral characteristics of the cyclone activity (density and intensity of cyclones) are computed on the basis of the method of automatic indication of cyclone centers from the sea-level pressure data. It is demonstrated that the NAO index is really the major indicator of cyclone activity anomaly formation in the North Atlantic, however, the variations of cyclone activity in the European region, of the number of cyclones and their integral intensity are better characterized by the EA index.     

Storm cyclones in the North Atlantic

Considered are long-term features and characteristics of storm activity variability in the North Atlantic based on the method of automatic identification of cyclone centers using the data on sea level pressure as well as the estimates of the maximum speed of the surface wind in the area of cyclones. Integral regional parameters of the variability of storm cyclone frequency for the gradations of the surface wind speed are presented for some areas of the North Atlantic. Investigated is the interrelation between extremely deep and storm cyclones.     

On the contribution of Rossby waves driven by surface buoyancy fluxes to low-frequency North Atlantic steric sea surface height variations

Previous studies have shown that wind-forced baroclinic Rossby waves can capture a large portion of low-frequency steric sea surface height (SSH) variations in the North Atlantic. In this paper, the classical wind-driven Rossby wave model derived in a 1.5-layer ocean is extended to include surface buoyancy forcing, and the new model is then used to assess the contribution from buoyancy-forced Rossby waves to low-frequency North Atlantic steric SSH variations. Buoyancy forcing is determined from surface heating as freshwater fluxes are negligible. It is found that buoyancy-forced Rossby waves are important in only a few regions belonging to the subtropical-to-midlatitude and eastern subpolar North Atlantic. In these regions, the new Rossby wave model accounts for 25%–70% of low-frequency steric SSH variations. Furthermore, as part of the analysis it is also shown that a simple static model driven by local surface heat fluxes captures 60%–75% of low-frequency steric SSH variations in the Labrador Sea, which is a region where Rossby waves are found to have no influence on the steric SSH.     

Analysis of tropical cyclone intensification trends and variability in the North Atlantic Basin over the period 1970–2003

Summary Over the past three decades, the sea-surface temperatures of the lower latitudes of the North Atlantic basin have increased while the lower-tropospheric temperatures show no upward trend. This differential warming of the atmosphere may have a destabilizing effect that could influence the development and intensification of tropical cyclones (TCs). In this investigation, we find that in general, TC intensification (a) is higher during the daytime period and during the later months of the storm season, (b) tends to be higher in the western portion of the North Atlantic basin, and (c) is not explained by current month or antecedent SSTs. Any changes associated with warming of the surface compared to a smaller temperature rise in the lower-troposphere (and resultant changes in atmospheric stability) have not produced detectable impacts on intensification rates of tropical cyclones in the North Atlantic basin.     

Dangerous wind waves in the North Atlantic at different regimes of atmospheric circulation

Studied are the formation processes of dangerous wind waves in the water area of the North Atlantic during three cold periods (October–March) from 2007 to 2010. Obtained are the estimates of variability of cyclone trajectories, wind fields, and wave height in the North Atlantic at the intensification of zonal or meridional atmospheric circulation.     

The Impacts of Climate Change on the Autumn North Atlantic Wave Climate

Abstract

In this study, we investigate the impact of global warming induced by possible climate change on the autumn winds, the related storm climate, and the wave climate over the North Atlantic Ocean. These analyses are based on a third-generation wave model, WAVEWATCHIII? and dynamically downscaled winds, obtained from the Canadian Regional Climate Model driven by the third version of the Coupled Global Climate Model (T47) from the Canadian Centre for Climate Modelling and Analysis following the A1B climate change scenario of the Special Report on Emission Scenarios from the Intergovernmental Panel on Climate Change. Compared with the present wave climate, represented as 1970–1999, the significant wave heights in the northeast North Atlantic will increase, whereas in other areas, such as the mid-latitudes, they will decrease, with associated changes in winds in the future climate (2040–2069). An analysis of inverse wave ages is used to suggest that wind-driven wave regimes tend to occur more frequently in the northeast North Atlantic and decrease in the mid-latitudes in the climate change scenario. The dominant North Atlantic storm-track region is estimated to shift northward, especially over the northern Northeast Atlantic, where the frequency of occurrence of the most intense cyclones is estimated to increase. We suggest that changes in storm densities are related to changes in the upper level steering flow in the atmosphere, which are the precursor to changes in the winds and ocean waves.     

南海登陆热带气旋对长江三角洲地区降水的影响

利用热带气旋年鉴资料统计了1949~2000年南海热带气旋影响期间我国长三角地区的最大降水量，计算有南海热带气旋影响时最大降水量大于50 mm的条件概率为60%。并把历年引起长三角地区50 mm以上降水的南海气旋频数序列作小波变换，发现其具有周期性振荡的特点，并且振荡周期在频率的分布上也有一定的规律。把南海热带气旋频数序列与夏季3个月副高面积指数之和的序列做交叉谱分析，发现两者具有准2年和准5年的耦合周期，气旋频数序列在位相上超前约半年。最后用1951~1999年的500 hPa月平均高度资料统计频数异常年的500 hPa高度场的距平和t统计量，发现在西太平洋、大西洋和极地都有显著的异常。     

A 28-Year Climatological Analysis of Size Parameters for Northwestern Pacific Tropical Cyclones

A 28-year best track dataset containing size parameters that include the radii of the 15.4 m s^-1 winds （R15） and the 25.7 m s^-1 winds （R26） of tropical cyclones （TCs） in the Northwestern Pacific, the NCEP/ NCAR reanalysis dataset and the Extended Reconstructed Sea Surface Temperature （ERSST） dataset are employed in this study. The climatology of size parameters for the tropical cyclones in the Northwestern Pacific from 1977 to 2004 is investigated in terms of the spatial and temporal distributions. The results show that the major activity of TCs in the Northwestern Pacific is from July to October. A majority of TCs lie over the ocean west of 150°E, and a few TCs can intensify to the Saffir-Simpson （S-S） categories 4, 5. Both R15 and R26 tend to increase as the tropical cyclones intensify. The values of R15 and R26 are larger for intense TCs in the Northwestern Pacific than in the North Atlantic generally. Both R15 and R26 peak in October, and before and after October, R15 and R26 decrease, which is different from the case in the North Atlantic. The smaller R15s and R26s occur in a large range over the Northwestern Pacific, while the larger R15s and R26s mainly lie in the eastern ocean from Taiwan Island to the Philippine Islands where many tropical cyclones develop in intense systems. The tropical cyclones with size parameters of R15 or R26 on average take a longer time to intensify than to weaken, and the weak tropical cyclones have faster weakening rates than intensification rates. From 1977 to 2004, the annual mean values of R15 increase basically with year; during the 28-year period, the value of R15 increases by 52.7 kin, but R26 does not change with year obviously.