冬季东亚-西太平洋西风急流基本结构及其异常的诊断分析
STUDY ON THE CHARACTERISTICS AND VARIABILITIES OF THE EAST ASIAN-WESTERN PACIFIC WESTERLY JET STREAM IN WINTER SEASONS
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摘要: 利用ECMWF再分析资料ERA40中的逐月风场和温度场资料,对冬季位于东亚-西太平洋上空的西风急流(WJS)基本结构和异常特征进行了分析。结果表明,一年当中最为强盛的冬季200 hPa WJS主体稳定地分布于东亚至西太平洋上空,向上向下均迅速减小。在对流层中高层,从东亚沿海到太平洋上空的西风系统较为深厚,低层对应着显著的大气斜压区。通过对东亚-西太平洋地区冬季200 hPa纬向风异常的EOF分解得到三种异常空间分布型,第一模态的中纬度西风异常主要发生于日界线以东的中东太平洋上空,第二、三模态则分别对应WJS主体发生增强/减弱和位置的北/南移动。WJS的这三种空间异常形态与中纬度低层大气斜压性的三种异常分布型相对应。通过对冬季WJS长期趋势的初步分析发现,近二十年来,洋面上空西风急流有增强趋势,这是由于急流轴南侧洋面上空大气存在显著增暖趋势、而急流轴北侧洋面上空却有变冷的趋势所造成的。Abstract: In this study,the monthly wind and temperature fields from ECMWF(European Center for Medium-Range Weather Forecasts) reanalysis dataset ERA40 are used to investigate the characteristics and variabilities of the East Asian-western Pacific westerly jet stream(WJS) in winter seasons.The results indicate that the strong 200 hPa WJS band spans over the East Asian and western Pacific region steadily during the whole winter season.It decreases dramatically upward and downward.The winter WJS extending from the East Asian coast region to the Pacific is a deep westerly wind accompanied with robust baroclinicity at the low level.Three leading anomalous patterns,representing deviations of the 200 hPa zonal flow from climatology over the East Asian and western Pacific regions,are obtained by using EOF statistical technique.The first leading anomalous pattern occurs over mid-eastern Pacific,while the second and the third anomaly patterns have been interpreted as representing the fluctuation in jet strength and width,and north/south jet movement,respectively.The above different anomalies of the westerly wind are corresponding to diversity in anomalies of the baroclinicity at the low level.The trend of 200 hPa WJS is also examined and the results show that the WJS over the Pacific region is enhanced during the recent 20 years,due to a positive trend of the air temperature south of the jet axis and a negative trend north of the axis.
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Key words:
- westerly jet stream /
- EOF /
- atmospheric baroclinicity /
- trend
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