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2005年初夏亚洲季风异常及对华南强降雨影响
Anomalous Features of Asian Summer Monsoon and Their Influence on the Continual Torrential Rain in South China in the Early Summer of 2005
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摘要: 利用国家气象中心1957-2006年基本观测站点逐日降水量资料，NCEP/NCAR所提供的2005年5—7月逐日OLR资料、日平均再分析资料及其相应的多年平均资料，采用诊断分析方法讨论了2005年初夏华南持续强降雨雨情、亚洲季风异常特征及其对华南持续强降雨的影响。结果表明：2005年南海季风在5月第6候爆发，较正常偏晚2候左右，导致华南前汛期比常年同期明显推迟；之后，南亚西南季风倾向于向东传播，西北太平洋副热带高压偏东偏弱，越赤道气流偏弱，使西南季风偏北风分量偏小，而东亚槽偏深，诸多因素使得西南季风在华南一带长时间维持，造成华南持续强降雨。同时还分析了澳大利亚冷高压、青藏高原南部到孟加拉湾一带热源的异常特征及其对亚洲季风和华南降雨的影响。Abstract: Anomalous features of Asian summer monsoon and their influence on the continual torrential rain in South China in early summer of 2005 are analyzed. The results show that Southwest monsoon surges play a very important role in the rain band movement in Eastern China from the end of May to June. The South China Sea (SCS) summer monsoon doesn't start until the end of May (about three pentads later than normal) and leads to the obvious delay of the flood season in South China. From then on, Southwest monsoon over the Bay of Bengal moves mainly eastwards with smaller components towards the southeast part of Tibet Plateau than normal. At the same time, the western Pacific subtropical anticyclone (WPSA) is weaker and remains farther away from China mainland, and the cross-equatorial flow between 105°-115°E is weaker so that the northern component of the Southwest monsoon is also smaller. Meanwhile, East Asia trough is deeper and leads stronger cold air to China. All of these result in the Southwest monsoon main taining in south China and cause continual torrential rain there. Weaker Australia High is an important factor for the weaker cross-equatorial flow, and the weaker southward ITCZ and WPSA. Due to the stronger cold air over the large area from southern part of the Tibet to the northern India, the Indian monsoon breaks out late and marches slowly northwards so that the convection and the latent heat and the heat sources over southern part of Tibet Plateau and Bay of Bengal are obviously weaker than normal. The negative thermal feedback mechanism is conversely unfavorable for the northward marching of southwest monsoon. This is another important cause for the southwest monsoon and torrential rain's long-time staying in South China.
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图 1 1957—2005年6月江南南部和华南地区 (21°~27°N, 105°~120°E) 47个测站平均降水量逐年演变图 (a) 和2005年5-7月105°~120°E平均降水量经向-时间剖面图 (b)
Fig. 1 Time series of the mean precipitation of 47 stations over the area from southern part of the Yangtze Valley to South China (21°-27°N, 105°—120°E) from 1957 to 2005 (a) and the daily mean precipitation over 105°-120°E from May to July in 2005 (b)
图 2 2005年5—7月850 hPa 105°~120°E平均风 (阴影部分表示风速大于5 m·s-1) (a) 和 OLR (阴影部分表示OLR值小于240 W·m-2) (b) 经向-时间剖面图
Fig. 2 Time series of longitude (105°—120°E) mean winds at 850 hPa (the shaded means wind speed larger than 5 m·s-1) (a) and OLR (unit：W·m-2; the shaded means less than 210 W ·m-2) (b) from May to July in 2005
图 3 2005年5—6月10°~20°N平均OLR时间-纬向剖面图 (单位：W ·m-2; 阴彩部分表示数值小于240W·m-2)
Fig. 3 Time series of zonal mean (10°—20°N) OLR from May to June in 2005 (unit：W·m-2; the shaded means less than 240W·m-2)
图 4 2005年5月第5候 (a), 第6候 (h) 850 hPa风场及200 hPa平均高度场
(粗虚线表示2500 m地形商度等值线，下同)
Fig. 4 Mean wind at 850 hPa and potential height at 200 hPa of the fifth pentad (a) and the sixth pentad (b) in May 2005
(the thicker dotted line means geographical height of 2500 m hereinafter)
图 5 2005年6月1-24日850 hPa风场 (单位: m·s-1) 和500 hPa高度场 (单位:gpm) (a) 及距平场 (b) 、6月25—28日 850 hPa风场 (单位:m·s-1) 和500 hPa高度场 (单位: gpm) (c)
(阴影表示西风风速大于5 m· s-1)
Fig. 5 Mean winds at 850 hPa (unit: m · s-1) and heights at 500 hPa (unit： gpm) during June 1—24, 2005 (a) with their anomalies (b), and mean winds at 850 hPa (unit: m·s-1) and heights at 500 hPa (unit: gpm) during June 25—28, 2005 (c)
(shaded areas mean western wind speed larger than 5 m ·s-1)
图 6 2005年5—7月1000 hPa 110°~150°E平均高度 (a) 以及 6月1—22日平均1000 hPa位势高度和风场距平 (b)
Fig. 6 Time series of heights of longitude 110°—150°E at 1000 hPa of May to July in 2005 (a) and mean anomalies of winds and potential heights at 1000 hPa from 1 to 22 in June 2005 (b)
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