孟加拉湾夏季风爆发的判断指标及其年际特征

Studies on onset criterion and interannual characteristics of summer monsoon over Bay of Bengal

利用高低层大气环流、OLR(向外长波辐射)、CMAP降水、SST(海表温度)等资料分析了孟加拉湾地区3—5月多年气候平均大气环流及不同要素的演变特征,定义了一个新的孟加拉湾夏季风(BOBSM,下同)爆发指标为孟加拉湾地区(5°N—15°N,90°E—97.5°E)850 hPa和200 hPa纬向风区域平均的变化同时满足U850>3 m·s^(-1)和U200<-5 m·s^(-1),并持续5天的第一天即作为BOBSM爆发日期.该季风指数有明确的天气学意义,可以反映孟加拉湾低层西南风持续稳定和南亚高压在青藏高原建立早晚的特征.文章进一步分析了BOBSM爆发的年际特征及其前兆海洋信号特征,结果表明:1981—2010年BOBSM爆发的平均日期为5月10日,季风爆发有显著的年际波动,爆发最早在1999年(4月11日)和最晚在1968年(6月1日),年代际尺度上表现为由爆发偏晚至偏早的变化趋势;BOBSM爆发早(晚)与热带印度洋地区850 hPa的越赤道气流和西风异常加强(减弱),以及200 hPa青藏高原南亚高压的季节性建立偏早(晚)等密切联系;前期冬季赤道西太平洋的海温冷(暖)变化对BOBSM爆发早(晚)有很好的指示意义,前期冬季海温偏高(低)有利于季风偏早(晚),其影响的主要途径是通过热源变化激发纬向垂直环流及其热带印度洋和太平洋低层环流异常,进而影响季风爆发早晚.

Based on atmospheric circulation,outgoing longwave radiation(OLR),CMAP precipitation and sea surface temperature(SST)data,the characteristics of circulation evolution and different variable at upper and lower level over Bay of Bengal(BOB)from March to May are investigated in detail.Based on the analysis,a new onset criterion of BOB summer monsoon(BOBSM)is defined by using regionally averaged zonal wind at 850 hPa(U850)and 200 hPa(U200)over Eastern BOB(5°N-15°N,90°E-97.5°E).When U850 and U200 get to the criterion of U850>3 m·s-1 and U200<-5 m·s-1 and persist for five days,we define the first day as the BOBSM onset date.The criterion definition has clear synoptic significance which can reflect the stably persist variation of low\|level southwest wind and the establishment of high\|level southern Asian high(SAH)over the Tibetan plateau.On the basis of BOBSM onset date obstained using above criteria,the interannual characteristics of BOBSM onset and precusory singal of tropical ocean are further analyzed.The results showed that the averaged date of BOBSM onset for 1981-2010 is 10 th May.BOBSM onset indicate significant interannual fluctuations with onset as early as 1999(11 th April)and the latest in 1968(1 st June).The decadal change of onset date display the trend from the late outbreak to early onset.The BOBSM onset is close related to upper and lower atmospheric circulation.In BOBSM onset early(late)years,850 hPa cross equatorial flow and westerly anomalies over tropical Indian Ocean strengthen(weaken),and 200 hPa SAH over Tibaten plateau establish early(late).The SST in the preceding winter over the Western Pacific has a good indicative sense to BOBSM onset.The monsoon onset is early(late)while SST is cold(warm).The possible impact way is to change the zonal vertical circulation and its anomalous circulation in the tropical India Ocean and the Pacific Ocean by the change of heat source,and then affect the monsoon onset early or later.