赤道MJO活动对南海夏季风爆发的影响

利用1979—2013年NCEP/DOE再分析资料的大气多要素日平均资料、美国NOAA日平均向外长波辐射资料和ERSST月平均海温资料,分析赤道大气季节内振荡(简称MJO)活动对南海夏季风爆发的影响及其与热带海温信号等的协同作用.结果表明,赤道MJO活动与南海夏季风爆发密切联系,MJO的湿位相(即对流活跃位相)处于西太平洋位相时,有利于南海夏季风爆发,而MJO湿位相处于印度洋位相时,则不利于南海夏季风爆发.赤道MJO活动影响南海夏季风爆发的物理过程主要是大气对热源响应的结果,当MJO湿位相处于西太平洋位相时,一方面热带西太平洋对流加强使潜热释放增加,导致处于热源西北侧的南海—西北太平洋地区对流层低层由于Rossby响应产生气旋性环流异常,气旋性环流异常则有利于西太平洋副热带高压的东退,另一方面菲律宾附近热源促进对流层高层南亚高压在中南半岛和南海北部的建立,使南海地区高层为偏东风,从而有利于南海夏季风建立;当湿位相MJO处于印度洋位相时,热带西太平洋对流减弱转为大气冷源,情况基本相反,不利于南海夏季风建立.MJO活动、孟加拉湾气旋性环流与年际尺度海温变化协同作用,共同对南海夏季风爆发迟早产生影响,近35年南海夏季风爆发时间与海温信号不一致的年份,基本上是由于季节转换期间的MJO活动特征及孟加拉湾气旋性环流是否形成而造成,因此三者综合考虑对于提高季风爆发时间预测水平具有重要意义.

Impact of equatorial MJO activity on summer monsoon onset in the South China Sea

Previous studies show that the interannual and interdecadal variability of sea surface temperature(SST) and the variation of the atmospheric intraseasonal variations are the factors that affect summer monsoon onset in the South China Sea(SCS). However there is little analysis about physical relationship between the equatorial Madden-Julian Oscillation(MJO) activity and the SCS summer monsoon onset. This paper attempts to study the impact of MJO activity on SCS summer monsoon onset and its synergy with tropical SST signal.The 35a(1979-2013) daily mean NCEP-DOE Reanalysis data, daily outgoing longwave radiation(OLR) and monthly extended reconstructed SST(ERSST.v2) from NOAA are used to analyze the impact of MJO activity on SCS summer monsoon onset and its synergy with tropical SST signal.Results show that MJO in the West Pacific phase is favorable to the onset of SCS summer monsoon, and MJO in Indian Ocean phase is unfavorable to SCS summer monsoon onset. When MJO is in western Pacific phase, more latent heat releases from enhancing convection, resulting in an anomalous cyclonic circulation at tropospheric low level in SCS to Northwest Pacific as Rossby wave respond. The anomalous cyclonic circulation is favorable to the retreating eastward of subtropical high. On the other hand, the heat source over Philippines vicinity promotes the establishment of South Asia high over Indochina Peninsula and the northern SCS, and wind changes to easterly over SCS at tropospheric high level. Therefore the SCS summer monsoon begins. When MJO is in Indian Ocean phase, tropical western Pacific changes to cold source because of the weakening of convection, the situation opposite with above appears, which is not conducive to the SCS summer monsoon onset. The years which are inconsistent between SCS summer monsoon onset dates and SST signal are basically due to the characteristics of MJO and the cyclone circulation over Bay of Bengal during the seasonal transition period.The equatorial MJO activity is related closely with SCS summer monsoon onset. The physical process in which MJO activity poses impact on SCS summer monsoon onset is primarily the response of atmosphere to the heat source. MJO activity and cyclonic circulation over Bay of Bengal cooperated with interannual SST signal influence the SCS summer monsoon onset. Comprehensive consideration of three factors, which are MJO, SST signal and circulation over Bay of Bengal, has great significance for improving monsoon onset prediction.