冬季风期间南海地区冷涌的动力学分析

利用一个部分非线性的二维经向重力内波动力模式，通过求行波解，对冬季风期间南海冷涌的传播形式，传播特征和传播速度进行了分析，并讨论了大尺度经向环流及其垂直结构、热力作用、层结稳定状况和初始扰动条件对冷涌传播的作用。主要结论是：冷涌的传播表现为具有非线性效应的无频散或弱频散的重力行波，其向南传播速度远大于平流速度而接近于重力内波波速；不同流场和位温的初始扰动状况决定了冷涌将以周期重力行波或者以孤立重力     

冷涌结构及冷涌期中低纬环流相互作用的数值试验

本文利用1982年12月下旬一次冷涌过程的数值试验结果,分析了冷涌的结构并初步讨论了冷涌期中低纬环流的相互作用。结果表明,冷涌仅在近地面反映清楚,水平流场上表现为由大地形激发的开尔文波引起的东北大风沿青藏高原东侧迅速南下,而冷涌前沿则是以重力波特征向南传播。Hadley环流随冷涌南进不断加强。冷涌在涡度场和散度场上分别为一条状北倾的正涡度区和辐合区,它对应于冷锋南下时造成的上升运动区。冷涌能引起赤道地区的扰动加强和中纬度对流层高层西风急流的增强。冷涌期中低纬地区能量增大,通过能量的频散作用把中低纬环流连结起来了。      

南海北部冷涌及其环流特征

利用中国南部沿海风向、地面气温及南海北部风速的变化确定南海北部冷涌的判别标准,构建了一个冷涌发生频率指数,对南海北部冷涌的年际变化特征及其环流异常进行讨论。结果表明,南海北部冷涌频率指数能够更加全面地描述冷涌系统的特征,与全国范围的冷空气爆发关系密切。南海北部冷涌与中高纬、中低纬环流具有密切的联系,是二者共同作用的结果。当贝加尔湖以西高压脊偏强,中高纬环流经向度加大,西太平洋副热带高压强度偏弱、位置偏东时,有利于大陆上冷空气南下入海,在南海北部形成冷涌。冷涌发生时,经向Hadley环流加强,调整中纬与低纬地区的大气质量,高层向北输送的大气在科氏力作用下导致东亚副热带西风急流加强,有利于冷涌的维持和加强。     

冬季风时期南海冷涌特征的初步分析

本文利用冬季风时期的资料,对在南海爆发的冷涌过程进行了统计分析,指出了南海冷涌的月际变化特征、话跃期及间隙期的周期特征。同时也讨论了冷涌过程中要素场、高低叠流场、温度场和逆温层的变化特征。所得结果,有助于对南海冷涌的认识。     

东亚冬季风的演变特征

文章主要讨论了东亚冬季风和冷涌的演变特征，并与南亚作了对比，发现在东亚地区，冬季风演变主要表现为10月中旬经向环流的突变及9月初、11月中旬和1月末对流层低层温度的3次突变；而在南亚地区，经向环流的变化不如东亚地区明显，而且高层要先于低层变化，对流层低层温度存在2次突变。在整个冬季，东亚地区冷涌的演变过程，主要表现为南海地区冷涌在12月份出现最高频率，而西太平洋冷涌在1月份出现最高频率；南亚地区冷涌在12月份出现最高频率，但远小于东亚地区且衰减速度很快。另一个不同点是东亚地区的冷涌强度是往上衰减的，而南亚地区的冷涌强度则是往上增强的。这说明东亚冬季风和南亚冬季风的性质有较大的区别。     

一次东亚寒潮爆发后冷涌发展的研究

分析了１９８１年１月２１—２８日一次东亚强寒潮爆发后南海和西太平洋地区冷涌的发展过程。冷涌的发展最先表现为低层强东北风沿东亚沿海地区和南海中北部的迅速建立。２—３ｄ之后，又在菲律宾以东洋面激起第二次冷涌。与冷涌相伴随的低层温度场变化也十分明显，同样具有先后两次降温过程。冷涌期高空风场表现有明显的脉动，南海北部高空西风迅速减弱，而在南海近赤道地区出现了很强的高空东风区。冷涌的发展与寒潮冷高压的南下有密切关系，它的强度在很大程度上取决于中纬度冷高压的强度。另一方面，冷涌发展后，通过Ｈａｄｌｅｙ环流可影响中纬西风急流和热带东风急流的变化。     

两次冷涌过程的分析

本文利用两次船舶航行实测资料及部分常规观测资料研究了两次冷涌过程的时—空结构。除进一步证实了冷涌具有重力波特征的论点外,还得出了几点初步结论:(1)这两次过程前缘的传擂速度为20—2米秒-1,很接近重力内波波速的动力气象理论值;(2)某海区冷涌到达之前的大气层结决定了冷涌是否具有重力波特征:(3)以重力波传播的冷涌比平流移动的更有效地把能量传至低纬洋面。      

东亚冬季风的统计研究

利用ECMWFl979—1984年5个冬季(12—2月)的资料对东亚和西太平洋地区的冬季风进行了统计研究。得到的主要结果有: (1)东亚和西太平洋地区有两个冷涌区;从东海到南海等东亚沿岸地区和菲律宾以东西太平洋地区。其中以前者最明显。冷涌主要是700hPa以下的现象,近地面层北风最强,随高度迅速减弱。 (2)伴随着冷涌向南扩展,冷空气经华南沿中南半岛沿岸区一直向南扩展到近赤道地区; (3)在近地面层,西伯利亚高压的活动和强度与冷涌有密切的联系; (4)东亚的冬季风或冷涌有明显的年际变化,这与西伯利亚高压和南支槽活动的年际变化有关。      

一次冷通过过程的静止卫星水汽图像分析

对一次冷涌过程的静止气象卫星水汽图像分析表明，冷涌爆发前，从蒙古地区有斧形干区向南伸展到长江流域，且斧形干区南伸过程中范围扩大、强度增大。冷涌爆发后，斧形干区南压过程中演变化为例T形干区，之后倒T形干式转变为西南东北向的带状干区进入南海海面。     

东亚冷涌期间低纬环流和降水形成的数值试验

应用数值试验的方法，研究了东亚冷涌期间低纬环流和降水的形成。结果表明：低纬积云对流作用加强了东北季风和东北信风，但阻滞了热带辐合带进一步南移。印尼和澳大利亚北部季风降水可以独立存在。南海冷涌以及越赤道的西太平洋东北信风都可以是澳大利亚北部夏季风及降水形成的重要因子。数值研究还指出；印尼加里曼丹的强季风降水及常驻性低层扰动形成的基本因子是南海冷涌所触发的积云对流发展。印尼、加里曼丹地形及东北信风扰动可影响该处降水的强度。     

南半球冷空气入侵与热带气旋的形成

文中采用NCAR/PSU研制的非静力中尺度模式MM 5 ,研究了南半球冷空气入侵在热带气旋形成中的作用。初始场为纬向平均场 ,不含任何扰动 ,但为热带气旋的发生提供了基本条件 ;通过改变设在赤道的南边界条件 ,设计系列数值试验反映南半球不同强度冷空气的入侵。数值试验结果表明 :南半球冷空气侵袭后 ,在菲律宾以东洋面上形成热带气旋 ;没有冷空气入侵时 ,只有扰动产生 ,没有热带气旋形成。在对流不稳定的背景场中 ,即使没有冷空气入侵 ,低层小尺度辐合引起强上升运动 ,产生的非绝热加热 ,在热带洋面上也能形成扰动。但是非绝热加热使得稳定度增加 ,没有低层强辐合的支持 ,对流不能持续 ,扰动不能发展成为热带气旋。南半球冷空气的入侵 ,一方面气温降低 ,使得中低层层结稳定度降低 ;另一方面 ,冷空气形成向北的气压梯度 ,在低纬度产生南风 ,导致低层强辐合。稳定度因子和低层辐合的共同作用 ,驱动深厚的垂直环流 ,产生十分显著的非绝热加热 ,形成了暖心的热带气旋。上述研究结果一定程度上肯定了存有疑义的冷空气学说     

Revisiting wintertime cold air intrusions at the east of the Andes: propagating features from subtropical Argentina to Peruvian Amazon and relationship with large-scale circulation patterns

This study investigates the spatial and temporal characteristics of cold surges that propagates northward along the eastern flank of the Andes from subtropical to tropical South America analysing wintertime in situ daily minimum temperature observations from Argentina, Bolivia and Peru and ERA-40 reanalysis over the 1975–2001 period. Cold surges usually last 2 or 3 days but are generally less persistent in the southern La Plata basin compared to tropical regions. On average, three to four cold surges are reported each year. Our analysis reveals that 52 % of cold episodes registered in the south of La Plata basin propagate northward to the northern Peruvian Amazon at a speed of around 20 m s^?1. In comparison to cold surges that do not reach the tropical region, we demonstrate that these cold surges are characterized, before they reach the tropical region, by a higher occurrence of a specific circulation pattern associated to southern low-level winds progression toward low latitudes combined with subsidence and dry condition in the middle and low troposphere that reinforce the cold episode through a radiative effect. Finally, the relationship between cold surges and atmosphere dynamics is illustrated for the two most severe cold intrusions that reached the Peruvian and Bolivian Amazon in the last 20 years.     

Long-term variability of cold surges in Korea

Cold surge statistics have been analyzed from the 52-year (1961–2012) winter temperature data archived at Korea Meteorological Administration. Despite a significant winter warming in South Korea, there is no statistically affirmable sign of the occurrence frequency, duration and total days of cold surges to have changed in the record; there is little abatement in the frequency, duration, and total days of cold surges. Thermal advection anomalies were also derived from the NCEP/NCAR and ERA interim reanalysis datasets. Cold surges defined in terms of thermal advection anomalies do not exhibit any statistically significant change either. The increased mean and the decreased variance of thermal advection both indicate that cold advection from north has, in fact, decreased gradually in South Korea. It appears that cold surges are statistically rare enough to be affected by increased mean and decreased variance of thermal advection. Polar warming is often interpreted as weakening of jet stream and increasing southward flux of cold air. Analysis of thermal advection, on the other hand, does not show increased advection of cold air from north in South Korea.     

East Asian winter monsoon: results from eight AMIP models

 This study evaluates simulations of the East Asian winter monsoon in eight GCMs that participated in the Atmospheric Model Intercomparison Project (AMIP). In addition to validating the mean state of the winter monsoon, the cold surge and its transient properties, which includes the frequency, intensity, preferred propagation tracks, and the evolution patterns of the surges, are examined. GCM simulated temporal distribution of the Siberian high and cold surges is also discussed. Finally, the forcing of the cold surges on the tropical surface wind and convection, along with their interannual variation is analyzed. The mean state of the winter monsoon is generally portrayed well in most of the models. These include the climatological position of the Siberian high, the 200 hPa divergent center, and the large-scale wind patterns at the surface and the 200 hPa. Models display a wide range of skill in simulating the cold surge and its transient properties. In some of the models, the simulated cold surge trajectory, intensity, frequency, propagation patterns and source regions are in general agreement with those from the observed. While in others, the models cannot adequately capture these observed characteristics. The temporal distribution of the Siberian high and cold surges were realistically reproduced in most GCMs. Most models were able to simulate the effect of the cold surges on the tropical surface wind, although a few models unrealistically generated subtropical southerly wind in the mid-winter. The relationship between cold surges and the tropical convection was not satisfactorily simulated in most models. The common discrepancies in the winter monsoon simulation can be attributed to many factors. In some models, the reason is directly related to the improper location of the large-scale convective center near the western Pacific. The satisfactory simulations of the monsoon circulation and the cold surges are partly due to the topographical characteristics of the East Asian continent, i.e., the Tibetan Plateau to the west and the oceans to the east. The correct simulation of the interannual variation of the surface wind near the South China Sea (SCS) and the maritime continent is a demanding task for most of the models. This will require adequate simulations of many aspects, including tropical convection, the Siberian cold dome, the extratropical-tropical linkage, and the air-sea interaction. The discrepancies noted here furnish a guide for the continuing improvement of the winter monsoon simulations. Improved simulations will lead to an adequate delineation of the surface wind and convection near the maritime continent, which is essential for portraying the winter monsoon forcing in a coupled model. Received: 10 March 1997/Accepted: 4 June 1997     

1990年代和2000年代华北寒潮的对比及其与北极海冰的联系

基于华北区域均一化站点气温资料、NCEP/NCAR大气再分析资料、CFSR大气再分析资料以及NOAA高分辨率北极海冰密集度资料,对比研究了1990年代与2000年代华北寒潮的差异及其与秋、冬季北极海冰的联系。结果表明:影响华北的区域性寒潮事件在1990年代（2000年代）发生的频次为1957~2011年最多（少）,但寒潮平均强度偏弱（强）,冷空气大多源自格陵兰岛（新地岛）附近,取偏西（偏北）路径进入华北。秋、冬季新地岛附近以及巴芬湾附近海冰在2000年代初显著减少,导致1990年代与2000年代出现显著环流差异:1990年代（2000年代）极区位势高度存在负异常（正异常）,欧亚大陆大型槽脊波动较弱（强）,绕极区西风较强（弱）,东亚大槽较弱（强）,西伯利亚高压较弱（强）,使得华北寒潮在两段时间出现上述差异。     

Impact of cold air surges on rainfall variability in the Sahel and wet African tropics: a multi-scale analysis

Satellite-derived rainfall estimates and the ERA-Interim reanalysis are used to better understand cold air surge/precipitation interactions and to identify the implications for rainfall variability in the Sahel and tropical Africa on synoptic to seasonal timescales. At the synoptic timescale, cold air surges are associated with cold conditions over the eastern Sahara throughout the year due to the eastward passage of surface low pressure systems over the Mediterranean and the subsequent ridging over northern Africa. Rainfall decreases over central and eastern Africa approximately 4–5 days after the cold air first arrives in northeastern Africa. These precipitation anomalies persist for 4 or more days. At the seasonal timescale, a significant relationship between eastern Saharan low-level temperatures and rainfall in the Sahel and tropical Africa is identified, with colder conditions associated with reduced convection on the northern flank of the primary convergence zone, and vice versa. During boreal winter, the anomalous rainfall occurs over tropical Africa (0°N–8°N). During the summer, rainfall anomalies associated with cold air surges occur over the Sahel (10°N–16°N). These relationships are mediated by anomalous anticyclonic flow over northwestern Africa and western Europe. The analysis shows that cold air surges are significantly associated with summertime cooling over the Sahara, but less so during the winter.     

The 2020 Summer Floods and 2020/21 Winter Extreme Cold Surges in China and the 2020 Typhoon Season in the Western North Pacific

China experienced significant flooding in the summer of 2020 and multiple extreme cold surges during the winter of 2020/21. Additionally, the 2020 typhoon season had below average activity with especially quiet activity during the first half of the season in the western North Pacific(WNP). Sea surface temperature changes in the Pacific, Indian, and Atlantic Oceans all contributed to the heavy rainfall in China, but the Atlantic and Indian Oceans seem to have played dominant roles. Enhancement and movement of the Siberian High caused a wavier pattern in the jet stream that allowed cold polar air to reach southward, inducing cold surges in China. Large vertical wind shear and low humidity in the WNP were responsible for fewer typhoons in the first half of the typhoon season. Although it is known that global warming can increase the frequency of extreme weather and climate events, its influences on individual events still need to be quantified.Additionally, the extreme cold surge during 16–18 February 2021 in the United States shares similar mechanisms with the winter 2020/21 extreme cold surges in China.     

1988年2—4月冷涌影响南沙海区的特点

本文是对1988年南沙群岛海区2—4月的船舶观测资料进行分析,目的是揭示冷涌对南沙海区的影响特点。一、资料及规定本文所用的船舶观测资料是对洋面上定点观测得到的。观测点在北纬9°32′23″,东经112°52′56″,位于开阔洋面,周围无高出水面的障碍物。观测时间为08—20时每三小时一