海温强迫下的东亚夏季大气环流潜在可预报性特征

利用大气环流模式NCARCAM3,进行22a(1979—2000年)、每年8个初值的集合试验,并采用方差分析方法,研究了观测海温强迫下东亚夏季大气环流的潜在可预报性。结果表明,夏季东亚地区海平面气压场的潜在可预报性总体偏低,在中国区域呈东南高、西北低的分布特征;850hPa纬向风场、对流层500～200hPa平均温度场和500hPa位势高度场在低纬度地区的潜在可预报性明显高于中高纬度地区。500hPa位势高度场的潜在可预报性较高,东亚大部分地区大于0.5,尤其华南地区大于0.7。夏季东亚500hPa位势高度场的潜在可预报性具有明显的年际变化特征,并与夏季南海海温异常关系密切。与正常年份相比,在夏季南海海温偏暖或者偏冷年,东亚500hPa位势高度场的潜在可预报性较高。

欧亚大陆春季植被状况与东亚夏季大气环流的显著联系

地表植被覆盖的变化能通过改变陆面参数,以及生地化循环过程,对区域和全球气候产生重要影响。文中利用1982—2002年欧亚大陆春季归一化植被指数(NDVI)和欧洲中期数值天气预报中心再分析资料,采用奇异值分解分析方法,研究欧亚大陆春季植被状况与东亚夏季大气环流的关系。结果表明,贝加尔湖以西区域(55°—65°N,60°—100°E)春季植被状况与东亚夏季大气环流存在显著联系。当春季该区植被指数偏高时,在对流层高层从巴尔喀什湖、贝加尔湖至日本北部,以及中国华南和中南半岛上空存在显著的纬向风正异常,中国华北地区和江淮地区以北为显著负异常,异常中心自北向南依次为"正-负-正"分布,说明东亚夏季200hPa西风急流轴偏南;相应的在对流层中层15°—25°N地区西风偏强,伴随偏强上升气流,而在25°—42°N地区西风偏弱,并且在32°N附近存在显著下沉气流;在对流层低层,中国江淮流域以北、华北及贝加尔湖以东地区存在明显的反气旋型风场异常,而华南存在东北风异常。这种环流特征说明东亚夏季风偏弱,雨带偏南,并且使得中国华南降水偏多,华南以北大部分地区降水偏少,同时中国东南以及青藏高原东南部温度偏低,而中国北方以及江淮流域温度偏高。欧亚大陆春季植被状况与东亚夏季风的显著关系为东亚夏季风预测提供了有用的帮助。

太阳活动对夏季大气环流异常及云南降水的影响

利用1948～2005年太阳10．7cm射电流量资料，采用小波分析、相关合成分析方法对太阳活动与夏季大气环流异常的联系进行了初步的研究，分析了太阳活动对云南夏季降水的影响。研究表明：太阳活动有明显的年际和年代际变化特征，太阳流量特征时间尺度分别约为11年、22年；太阳活动与夏季中高层大气环流异常有十分明显的联系；太阳活动强弱年的大气环流存在显著差异：太阳活动峰值年，南亚高压异常强大，高压脊线偏南，副高表现出明显的偏强偏西；而太阳活动谷值年，南亚高压较弱，高压脊线偏北，副高表现出明显的偏弱偏东。进一步对太阳活动与异常风场的联系进行了分析，得出显著相关区主要位于中高纬，索马里附近为一显著正相关区。太阳活动峰值年，东亚地区西风急流异常偏南偏强，热带季风环流加强，梅雨锋区环流偏弱（夏季风环流加强）；太阳活动谷值年，东亚地区西风急流异常偏北偏弱，热带季风环流偏弱，梅雨锋区环流加强（夏季风环流偏弱）。云南夏季降水与太阳10．7cm流量在年代际尺度上有很好的对应关系，初夏、汛期降水分别有与太阳活动相关的11、20-24年明显周期。滇南、滇西北初夏5月的降水与太阳活动有一定的正相关关系，太阳活动强时，5月降水越多，太阳活动弱时，5月降水越少。

低阶谱模式的一种新方案及其应用

针对经典低阶谱模式存在的不足,提出了在最小二乘准则下通过计算不同函数组合与实测资料间的复相关系数,从实际观测资料中客观提取能反映具体物理现象的截断函数的新方案.克服了应用低阶谱模式研究物理现象时选取截断函数时的主观任意性.将此方法用于研究北半球夏季大气环流年际变化,利用1994年和1998年6月大气环流资料客观确定相应的函数组合,从描述大尺度大气运动的准地转两层斜压方程出发,揭示了造成这两年夏季大气环流演变存在差异的物理机制.

Impact of European Black Carbon on East Asian Summer Climate

The remote response of the East Asian summer monsoon （EASM） to European black carbon （EUBC） aerosols was studied by using an ensemble of sensitivity experiments with the Geophysical Fluid Dynamics Laboratory （GFDL） atmospheric general circulation model （AGCM） Atmospheric Model version 2.1 （AM2.1）.The results show that EUBC causes an enhanced EASM.The resulted enhanced southwesterly brings more moisture supply from the Bay of Bengal,which causes an increase in precipitation over the Yangtze River valley,northeastem China,the eastern part of the Yellow River valley,and the Tibetan Plateau.Diagnostic examination suggests that EUBC induces enhanced tropospheric heating over most of the Eurasian Continent through a propagating wave train and horizontal air temperature advection.This phenomenon results in intensified thermal contrast between land and ocean,which accounts for the enhanced EASM.Moreover,reductions in EUBC emission in 1992 may have contributed to decadal weakening of the EASM in the early 1990s.

Improving the Seasonal Forecast of Summer Precipitation in China Using a Dynamical-Statistical Approach

A dynamical-statistical post-processing approach is applied to seasonal precipitation forecasts in China during the summer.The data are ensemble-mean seasonal forecasts in summer (June August) from four atmospheric general circulation models (GCMs) in the second phase of the Canadian Historical Forecasting Project (HFP2) from 1969 to 2001.This dynamical-statistical approach is designed based on the relationship between the 500 geopotential height (Z500) forecast and the observed sea surface temperature (SST) to calibrate the precipitation forecasts.The results show that the post-processing can improve summer precipitation forecasts for many areas in China.Further examination shows that this post-processing approach is very effective in reducing the model-dependent part of the errors,which are associated with GCMs.The possible mechanisms behind the forecast's improvements are investigated.

An Effective Approach for Improving the Real-Time Prediction of Summer Rainfall over China

This paper has two purposes. One is to evaluate the ability of an atmospheric general circulation model (IAP9L-AGCM) to predict summer rainfall over China one season in advance. The other is to propose a new approach to improve the predictions made by the model. First, a set of hindcast experiments for summer climate over China during 1982-2010 are performed from the perspective of real-time prediction with the IAP9L-AGCM model and the IAP ENSO prediction system. Then a new approach that effectively combines the hind-cast with its correction is proposed to further improve the model's predictive ability. A systematic evaluation reveals that the model's real-time predictions for 41 stations across China show significant improvement using this new approach, especially in the lower reaches between the Yellow River and Yangtze River valleys.

DECADAL CLIMATE VARIABILITY OF RAINFALL AROUND THE MIDDLE AND LOWER REACHES OF THE YANGTZE RIVER AND ATMOSPHERIC CIRCULATION

This study examined the rainfall around the middle and lower reaches of the Yangtze River and related atmospheric circulation by using NCEP reanalysis data. The purpose of this study is to analyze their decadal variation and the relationship among rainfall, atmospheric circulation around East Asia and the ENSO episodes. Current results are presented as follows: (1) Very clear increasing trend of the rainfall around the middle and low reaches of the Yangtze River during the Meiyu period and June to July is found in the recent 15 years. Meanwhile, the geopotential height at 500 hPa around the Okhotsk Sea also holds similar increasing trend. It is noticeable that ENSO episodes tend to occur more frequently in the recent 15 years. (2) An index describing East Asian summer monsoon is well correlated with the SST in the Nino-3 region in preceding autumn in the recent 20 years but is not prior to the period. This means that the El Nino phenomenon exerts more impacts on East Asian summer monsoon recently. (3) The warm phase of PDO in the recent 20 years basically coincides with the increasing trend of the atmospheric circulation in East Asia.

Physical Mechanism of the Impacts of the Tropical Atlantic Sea Surface Temperature on the Decadal Change of the Summer North Atlantic Oscillation

In this study,physical mechanism of the impacts of the tropical Atlantic sea surface temperature(SST)on decadal change of the summer North Atlantic Oscillation(SNAO)was explored using an atmospheric general circulation model(AGCM)developed at the International Centre for Theoretical Physics(ICTP).The simulation results indicate that the decadal warming of the SST over the tropical Atlantic after the late 1970s could have significantly enhanced the convection over the region.This enhanced convection would have strengthened the local meridional circulation over the Eastern Atlantic-North Africa-Western Europe region,exciting a meridional teleconnection.This teleconnection might have brought the signal of the tropical Atlantic SST to the Extratropics,consequently activating the variability of the eastern part of the SNAO southern center,which led to an eastward shift of the SNAO southern center around the late 1970s.Such physical processes are highly consistent with the previous observations.

Recognition of two dominant modes of EASM and its thermal driving factors based on 25 monsoon indexes

Based on three reanalysis datasets—ERA-Interim,NCAR–NCEP and JRA-55—the classification of25 commonly used indexes of the East Asian summer monsoon(EASM)was investigated.The physical nature of two categories of monsoon index,together with their circulation pattern,climate anomalies,and driving factors,were investigated.Results suggest that the selected 25 monsoon indexes can be classified into two typical categories(CategoryⅠandⅡ),which are dominated by interannual and decadal variabilities of the EASM,respectively.The anomalous circulation patterns and summer rainfall patterns related to the two categories of index also exhibit evident differences.CategoryⅠis closely linked to the low-latitude circulation system and the anomalous circulation pattern is a typical East Asia–Pacific teleconnection pattern.The summer rainfall anomaly exhibits a typical tripole pattern.However,CategoryⅡmainly reflects the impacts of the middle–high latitude circulation system on the summer monsoon and is closely linked to a typical Eurasian teleconnection pattern,which corresponds to a dipole of summer rainfall anomalies.Further analysis suggests that the underlying thermal driving factors of the two categories of monsoon are distinct.The main driving factors of CategoryⅠare the tropical sea surface temperature anomalies(SSTAs),especially ENSO-related SSTAs in the preceding winter and summer SSTAs in the tropical Indian Ocean.The winter signal of Category II summer monsoon anomalous activity mainly originates from the polar region and the middle and high latitudes of the Eurasian continent.CategoryⅡmonsoon activity is also associated with summer SSTAs in the equatorial central Pacific.

Interdecadal change of the connection between winter North Pacific Oscillation and summer precipitation in the Huaihe River valley

The relationship between the winter North Pacific Oscillation (NPO) and the following summer precipitation in the Huaihe River valley before and after the mid-1970s is investigated by using the Chinese 160-station precipitation data and NCEP/NCAR reanalysis data from 1951 to 2008.It is found that their linkage appears to have an apparent interdecadal variation.Before the mid-1970s, there was a prominent out-of-phase relationship between the winter NPO and the summer precipitation in the Huaihe River valley.However, such a relationship is significantly weakened afterwards.The change of atmospheric circulations related to the winter NPO before and after the mid-1970s is further addressed.Before the mid-1970s, a strong (weak) winter NPO was followed by the summer situations with anomalous low-level cyclonic (anticyclonic) circulation over the western Pacific and descending (ascending) over the Huaihe River valley.Meanwhile, the water vapor transporting to the Huaihe River valley was reduced (enhanced).These conditions are unfavorable (favorable) for the precipitation occurring in the Huaihe River valley, and thus the local precipitation was decreased (increased).After the mid-1970s, however, the impact of winter NPO on the summer atmospheric circulation system associated with the rainfall in the Huaihe River valley becomes diluted, thereby weakening its linkage to the summer precipitation in the Huaihe River valley.