北极区近30年环流的变化及对中国强冷事件的影响

利用1971-2000年NCEP再分析资料和中央气象台提供的我国强冷空气过程统计资料，分析了近30年冬半年北极区及中高纬度地区大气环流形势的气候变化以及对中国强冷事件的影响。结果表明，20世纪70-90年代北极区大气温度发生了明显变化，极涡系统也随之改变。80年代中期前，极涡范围偏大；之后面积减小。从长期趋势来看，70年代中期至90年代中期，极涡强度与面积呈反位相变化；同期，亚洲中高纬地区大气环流也相应调整，总体趋势由低指数向高指数模态转变，环流经向度减小；东亚大槽东迁，槽线90年代比70年代东移了近1个经距，冬季则更为突出，平均东移2．2个经距；西伯利亚冷高压也发生了年代变迁，70年代高压强度弱、控制范围偏小，80年代增强，范围明显扩大，90年代高压中心强度变化不大，但高压主体向南扩展的纬度较80年代偏北1～2个纬距，主体面积减小了5％。研究表明，正是由于极区、近极区环流系统的改变，造成近30年我国强冷空气爆发的事件特性发生了年代际变化。70年代强冷事件最为频繁，以西路冷空气为主，80年代频次明显减少，以偏西北路冷空气为主，但多源自新地岛东部，极端强冷事件降温距平达到最大，单次冷空气势力最强；90年代强冷事件频次又有所增加，但冷空气强度明显减弱。究其原因，主要与气候增暖、积温偏高，易产生降温有关。而上述近30年冷事件频次和强度的变化特征符合极端天气事件与气候变化的关系，因此我国强冷空气爆发事件特性的改变可能也是气候增暖的结果和反映。

北极偶极子型大气环流对北极海冰影响的数值模拟研究

利用美国冰雪数据中心提供的海冰范围和由ERA-Interim提供的月均海平面气压提取的北极偶极子型大气环流(AD)指数分析得出,12~2月的AD平均指数与2月北极海冰范围正相关,5~8月的平均AD指数与9月北极海冰范围负相关。为了区分冬季/夏季AD型大气环流影响下热力、动力强迫对海冰密集度的影响,本文利用CICE海冰模式进行了一系列数值模拟试验。研究表明,模式能够再现正/负位相的AD型大气环流下海冰密集度分布的主要特点,对北极海冰的模拟能力满足研究需求。冬季,北极海冰密集度对AD型风场的变化不敏感,负位相的AD型大气环流通过热力作用或风驱动以外的强迫作用造成大西洋扇区边缘海海冰密集度减小;夏季,正位相的AD型大气环流影响下,东格陵兰海之外的海区海冰密集度减小,这种影响主要来自于风场强迫,风场以外的强迫作用对大西洋扇区边缘海和巴伦支海海冰密集度偏小也有重要贡献。

2016~2017年秋冬季北极海冰异常与大气环流的关系

2016~2017冬季,年北极海冰范围创下自1979年以来最低记录。本文利用美国冰雪数据中心(NSIDC)的北极海冰范围指数和NCEP/NCAR大气再分析数据,分析了2016~2017年秋冬季北极海冰的变化特点及其与大气环流的关系。2016~2017年秋冬季北极海冰远低于多年平均值,并且出现了若干次海冰减少或海冰发展停滞的现象。两个主要海冰异常区为巴伦支–喀拉海和东西伯利亚–楚科奇–波弗特海,其中巴伦支–喀拉海海冰异常最为明显。对北半球中高纬度大气环流的分析发现,2016年10月~11月,持续的、异常的大气环流是影响大气温湿度进而影响海冰的原因。异常高压对西风急流的引导作用使得大气从北大西洋和北太平洋向北冰洋运动,引起北极上空大气增温、水汽增加则导致长波辐射。2016年11月~2017年3月,没有明显的持续性的环流异常,但是在较短的时间尺度上,依然有强烈的平流作用使得大气升温,从而影响了海冰。

Fine-resolution simulation of surface current and sea ice in the Arctic Mediterranean Seas

A free-resolution model is developed for ocean circulation simulation in the National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics （LASG）, Chinese Academy of Sciences, and is applied to simulate surface current and sea ice variations in the Arctic Mediterranean Seas. A dynamic sea ice model in elastic-viscous-plastic rheology and a thermodynamic sea ice model are employed. A 200-year simulation is performed and a dimatological average of a 10-year period （14lst-150th） is presented with focus on sea ice concentration and surface current variations in the Arctic Mediterranean Seas. The model is able to simulate well the East Greenland Current, Beaufort Gyre and the Transpolar Drift, but the simulated West Spitsbergen Current is small and weak. In the March climatology, the sea ice coverage can be simulated well except for a bit more ice in east of Spitsbergen Island. The result is also good for the September scenario except for less ice concentration east of Greenland and greater ice concentration near the ice margin. The extra ice east of Spitsbergen Island is caused by sea ice current convergence forced by atmospheric wind stress.

Subpolar Gyre Index and the North Atlantic Meridional Overturning Circulation in a Coupled Climate Model

The subpolar gyre index (SPG), derived from the analysis of sea surface height (SSH), is proposed to be a potential indicator for the North Atlantic Meridional Overturning Circulation (AMOC) based on observation as well as the Ocean General Circulation Model (OGCM). We investigated the correspondence between the SPG and the AMOC in a coupled climate model. Our results confirm that the SPG can be used as an early indicator for the AMOC in the subtropical North Atlantic. Changes in the SPG are closely related to variations in the air-sea heat exchange in the Labrador Sea, and variations in deep water formation and southward dense water transport with the deep western boundary current (DWBC) in the North Atlantic.

Possible Impact of the Summer North Atlantic Oscillation on Extreme Hot Events in China

This paper reveals that the summer North Atlantic Oscillation (SNAO) is closely related to the extreme hot event (EHE) variability in China during the period of 1979 2009, with a positive-phase (negative-phase) SNAO corresponding to less (more) EHEs in northern China. The summer circulation anomalies associated with the SNAO give further confirmation of the above relationship. In a positive-phase (negative-phase) SNAO year, there is an anomalous cyclone (anticyclone) over central East Asia, which can increase (decrease) the total cloud cover over this region. Such changes of the total cloud cover can then decrease (increase) the solar radiation reaching the surface, which is consequently unfavorable (favorable) to the formation of EHEs over northern China.

Associations between the Autumn Arctic Sea Ice and North American Winter Precipitation

Associations between the autumn Arctic sea ice concentrations (SICs) and North American winter precipitation were examined using singular value decomposition. The results show that a reduced SIC in the majority of the Arctic is accompanied by dry conditions over the Great Plains, the southern United States, Mexico, eastern Alaska, and southeastern Greenland, and by wet conditions over the majority of Canada, the northeastern United States, and the majority of Greenland. Atmospheric circulation anomalies associated with the SIC variability show a wave train structure that is persistent from autumn to winter and is responsible for the covariability between the autumn Arctic SICs and North American winter precipitation. This relationship suggests a potential long-term outlook for the North American winter precipitation.

Arctic Oscillation Responses to Black Carbon Aerosols Emitted from Major Regions

The responses of the Arctic Oscillation(AO) to global black carbon(BC) and BC emitted from major regions were compared using the atmospheric general circulation model Geophysical Fluid Dynamics Laboratory(GFDL) atmospheric general circulation model(AGCM) Atmospheric Model version 2.1(AM2.1). The results indicated that global BC could induce positive-phase AO responses, characterized by negative responses over the polar cap on 500 h Pa height fields, and zonal mean sea level pressure(SLP) decreasing while zonal wind increasing at 60°, with the opposite responses over midlatitudes. The AO indices distribution also shifted towards positive values. East Asian BC had similar impacts to that of global BC, while the responses to European BC were of opposite sign. South Asian BC and North American BC did not affect the AO significantly. Based on a simple linear assumption, we roughly estimated that the global BC emission increase could explain approximately 5% of the observed positive AO trend of +0.32 per decade during 1960 to 2000.

Will typhoon over the western North Pacific be more frequent in the Blue Arctic conditions?

How would typhoon activity over the western North Pacific change for various scenarios of future global warming?Using the model projections of the Coupled Model Intercomparison Project phase 3(CMIP 3)under the SRES A1B scenario,we generated summer(September)ice-free Arctic conditions,also referred to as Blue Arctic conditions,and then used the corresponding monthly sea surface temperature(SST)and a set of CO2concentrations to drive an AGCM model to simulate the resulting changes in background conditions affecting typhoon activity over the western North Pacific.Our results show that,during typhoon season(June to October),atmospheric and ocean circulations over the western North Pacific would be significantly different from the present circulations.Changes in the vertical shear of zonal wind and outgoing longwave radiation(OLR)in the western North Pacific are favorable for westward and northward shift,respectively,of the location of typhoon genesis.Moreover,changes in the above fields over the key area may be conducive to less frequent typhoons.In addition,the tropical cyclone genesis potential index(GPI)over the western North Pacific would decrease(increase)east(west)of 150°E(140°E).

North Indian Ocean tropical cyclone activities influenced by the Indian Ocean Dipole mode

Using Joint Typhoon Warning Center tropical cyclone(TC)track data over the North Indian Ocean(NIO),National Centers for Environmental Prediction monthly reanalysis wind and outgoing long-wave radiation data,and National Oceanic and Atmospheric Administration sea surface temperature data from 1981 to 2010,spatiotemporal distributions of NIO TC activity and relationships with local sea surface temperature(SST)were studied with statistical diagnosis methods.Results of empirical orthogonal function(EOF)analysis of NIO TC occurrence frequency show that the EOF1 mode,which accounts for 16%of total variance,consistently represents variations of TC occurrence frequency over the whole NIO basin.However,spatial dis- tributions of EOF1 mode are not uniform,mainly indicating variations of westward-moving TCs in the Bay of Bengal.The prevailing TC activity variation mode oscillates significantly on a quasi-5 year interannual time scale.NIO TC activity is notably influenced by the Indian Ocean dipole(IOD)mode.When the Indian Ocean is in a positive(negative)phase of the IOD, NIO SST anomalies are warm in the west(east)and cold in the east(west),which can weaken(strengthen)convection over the Bay of Bengal and eastern Arabian Sea,and cause anticyclonic(cyclonic)atmospheric circulation anomalies at low levels. This results in less(more)TC genesis and reduced(increased)opportunities for TC occurrence in the NIO.In addition,positive(negative)IOD events may strengthen(weaken)westerly steering flow over the Bay of Bengal,which further leads to fewer(more)westward-moving TCs which appear in regions west of 90°E in that bay.