孟加拉湾东部夏季风爆发的定义及其年际变化

Definition and Interannual Variability of the Eastern Bay of Bengal Summer Monsoon Onset

利用合成分析、相关分析等方法,对孟加拉湾东部夏季风(Eastern Bay of Bengal Summer Monsoon,简称EBOBSM)爆发日期进行定义,并在此基础上研究其年际变化特征。规定3天滑动平均的850hPa纬向风在90°E^100°E,7.5°N^12.5°N的范围平均下,从平均值大于2m/s的当天开始,连续15天均值都大于0,且15天纬向风平均值大于2m/s,则该日期为EBOBSM夏季风爆发日期,1979—2008年气候平均爆发日期为5月5日。EBOBSM爆发时间的年际变化特征明显,EBOBSM爆发偏早(晚)年的前期冬季,赤道中东太平洋海温异常偏低(高),对应厄尔尼诺/南方涛动(ENSO)冷(暖)事件和南极涛动正(负)位相,这种位相分布自北半球冬季至春季由强变弱。澳大利亚及西南印度洋受异常气旋(反气旋)式环流控制,并一直持续至春季3、4月份,使得早年与晚年位势高度异常的差异自南向北表现为低-高-低的类波列分布,并伴随气旋-反气旋-气旋式波动环流。南极涛动异常信号通过经向异常波动,自南印度洋中高纬度传播至孟加拉湾。偏早年热带地区的气旋式环流使得索马里越赤道气流加强,更多的水汽及热量随赤道西风被携带至孟加拉湾,使得当地降水充沛,对流活跃,较易激发季风爆发,偏晚年情况相反。

Based on composite and correlation analyses, the interannual variability characteristics of the Eastern Bay of Bengal summer monsoon （EBOBSM） are examined according to a new definition of EBOB- SM onset date introduced in this paper. The date of each individual year is defined as the day when the following criteria are satisfied; （1） the 3-day running mean averaged （90~~100~E, 7.5~~12. 5~N） zonal wind exceeds 2m/s~ （2） the averaged zonal wind remains positive for more than 15 days （including the on- set day） and the mean value exceeds 2m/s. The long-term mean onset date between 1979 and 2008 deter-mined by the definition is 5th May. Besides the obvious interannual variability, the onset of EBOBSM also demonstrates an interdecadal change with a period of quai-20a. During the boreal winter before an early （late） onset EBOBSM, the sea surface temperature over tropical central and eastern Pacific is lower （high-er） than meteorology which corresponds with a La Nina （El Nino）event and a positive （negative） phase of Antarctic Oscillation. The phase distribution in the Southern Hemisphere strengths contemporaneously and gets weaker gradually from January to April. An anomalous cyclone （anticyclone） has controlled Aus-tralia and southwest Indian Ocean until boreal spring. Because of the tendency, the difference of anoma-lous geopotential height between early and late onset years shows a low-high-low wave pattern from south to north accompanied by a cyclonic-anticyclonic-cyclonic circulation. This distribution allows abnormal sig- nals of Antarctic Oscillation spreading from middle and high latitude of southern Indian Ocean to the BOB, The cyclonic circulation over the tropical region in the early onset years strengthens the Somali cross-equa-tor flow and carries more moisture and heat to the BOB which induces plenty of rainfalls and active convec-tion. Thus the environment is favorable for EBOBSM onset and vise versa.