1999年东亚夏季风异常活动的物理机制研究
A STUDY ON PHYSICAL MECHANISMS OF ANOMALOUS ACTIVITIES OF EAST ASIAN SUMMER MONSOON DURING 1999
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摘要: 文中从海-气相互作用的角度探讨了1999年东亚夏季风及与其相联系的雨带异常活动的物理机制.结果表明,由于1998年春季至1999年南海-热带西太平洋出现了近20a最强的异常暧海温,该地强异常海-气相互作用的维持使得这种局地的热力强迫成为1999年东亚夏季风和降水异常的最主要外强迫机制,并使得1999年的季风活动和降水分布有别于一般的统计情形.从1998年秋到1999年,由于热带大气对南海-西太平洋暧海温所诱发的局地强加热的响应,热带西太平洋地区所出现的Gill模态的异常环流分布从冬季一直发展到夏季,并因此在海洋和大气之间形成了局地的强烈正反馈,不仅使得异常环流得以持续发展,而且也使得暖海温得以维持,成为影响1999年环流异常的最强前期信号.随着从冬到夏的季节演变,大气基本态对上述持续性异常环流的影响导致了冬、夏异常环流呈现出不同的纬向非对称,诱发了盛夏期间东亚到北美沿岸的遥相关波列.在东亚沿岸异常气旋性环流的影响下,大尺度异常东风在东亚沿岸的维持形成了极不利于季风西风在南海北部转向的条件,导致了季风在中国东部北进的异常偏弱和低纬西风转向位置的异常偏东.Abstract: The physical mechanisms causing the anomalous activities of East Asian summer monsoon and related rainfall-belts in 1999 have been investigated from the viewpoint of air-sea interaction.The results have shown that during the period from 1998 spring to 1999,because of the appearance of the strongest warm sea surface temperature (SST) anomalies in the region from South China Sea to the tropical western Pacific in past 20 years,the local thermal forcing induced by the persistent anomalous strong air-sea interaction in this region became the most important external forcing mechanism.This made the monsoon activities and precipitation pattern this year not consistent with the normal statistical relationship.During the period from 1998 fall to 1999,because of the response of tropical atmosphere to local strong heating induced by the anomalous warm SST from the South China Sea to the western Pacific,the anomalous circulation pattern similar to Gill mode was observed in the tropical western Pacific.The easterlies and westerlies were respectively found on eastern and western flanks of heat sources owing to the response of Kelvin and Rossby waves and a cyclone twin appeared in heat source region.This anomalous circulation pattern persisted from the previous winter to the summer and thus caused a strong Local positive feedback between the atmosphere and ocean.This allowed the anomalous circulation pattern and the strong warm SST to maintain in this region and thus evolved into the strongest signal that year.With the seasonal progress from the winter to summer,the impact of atmospheric basic state on the above persistent anomalous circulation resulted in the different tonal asymmetry of anomalous circulation in winter and summer,thus inducing the teleconnection wavetrain from the East Asia to North America in mid-summer.Under the influence of anomalous cyclonic circulation over the East Asian coast,the persistence of anomalous easterly winds over the East Asian coast was unfavorable to the turning of monsoonal westerlies in the South China Sea,which led to the anomalous weak northward advance of summer monsoon in East China and eastward shift of turning position of monsoonal westerlies in low latitudes.The sensitivity experiments of numerical model NCAR/CCM3 further supported that the persistent anomalous SST in the tropical Pacific from 1998 spring to 1999 can lead to the anomalous circulation and rainfall in East Asia in 1999.For that year,the local air-sea interaction in the western Pacific was the most important affecting factor and the remote forcing in the eastern Pacific took the second place.
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