初夏孟加拉湾东部降水异常对印度洋海温偶极子的触发作用

基于1982—2013年逐月NCEP资料及GODAS资料,采用回归分析、合成分析以及2.5层简化海洋模式数值模拟等方法,研究了热带东印度洋的大气和海洋过程对印度洋海温偶极子(IOD,Indian Ocean Dipole)东极(IODE,IOD East pole)海温异常的影响。结果表明,IODE海温异常的演变超前IOD西极(IODW,IOD West pole)海温异常的演变,并对IOD事件的生成和发展起到关键作用。初夏,来自阿拉伯海、中南半岛地区以及孟加拉湾西南部的水汽输送,导致孟加拉湾东部出现强降水。降水释放的潜热在热带东印度形成了一个跨越赤道的经向环流,有利于加强赤道东印度洋的过赤道气流,并在苏门答腊沿岸形成偏南风异常。该异常偏南风通过影响混合层垂向夹卷混合过程和纬向平流过程,导致IODE海温迅速下降。随后赤道东南印度洋异常东南风迅速增强以及赤道中印度洋东风异常的出现,增强了自东南印度洋向西印度洋的水汽输送,削弱了向孟加拉湾的水汽输送,使西南印度洋的降水增强,孟加拉湾东部的降水减弱。因此,IOD达到盛期前孟加拉湾东部的降水通过局地经向环流在苏门答腊沿岸形成偏南风异常,导致苏门答腊沿岸迅速的降温,并最终导致IOD事件的发生。

Possible triggering of the Indian Ocean Dipole by early summer rainfall anomalies over the eastern Bay of Bengal

Based on NCEP and GODAS monthly reanalysis data for the period 1982-2013,we explored the influence of atmospheric and oceanic processes on the SST anomaly over the equatorial Southeast Indian Ocean during Indian Ocean Dipole(IOD) events using regression analysis,composite analysis and a 2.5-layer dynamic upper-ocean model.Results show that the SST anomaly over the equatorial Southeast Indian Ocean,which plays a key role in the formation and maintenance of IOD events,has an advanced phase in comparison with that of the SST anomaly over the equatorial West Indian Ocean.The intensity of the SST anomaly over the equatorial Southeast Indian Ocean is stronger than that over the equatorial West Indian Ocean during the entire cycle of IOD events.The SST anomaly over the equatorial Southeast Indian Ocean matures in the fall,whereas over the equatorial West Indian Ocean it matures in winter.Strong moisture transport from the Arabian Sea,northeastern Indochina Peninsula,and southwestern Bay of Bengal,to the eastern Bay of Bengal,results in significantly increased rainfall over the eastern Bay of Bengal in early summer.Latent heat release because of the heavy rainfall anomaly over the eastern Bay of Bengal perhaps strengthens the local meridional circulation crossing the equatorial eastern Indian Ocean.The local meridional circulation crossing the equatorial eastern Indian Ocean may contribute to the southerly wind anomalies off the coast of Sumatra.The southerly wind anomalies off the coast of Sumatra may lead to a negative SST anomaly over the equatorial Southeast Indian Ocean through enhanced entrainment and zonal advection.Then,the rapid enhancement of southeast wind anomalies off the coast of Sumatra stimulated by the negative SST anomaly over the equatorial Southeast Indian Ocean and easterly wind anomalies in the central-eastern equatorial Indian Ocean,transport moisture from the equatorial Southeast Indian Ocean to the equatorial West Indian Ocean.This leads to less rainfall over the Bay of Bengal and equatorial Southeast Indian Ocean,and more rainfall over the West Indian Ocean.Westward-propagating warm Rossby waves south of the equator stimulated by anomalous anticyclones could lead to a warming SST anomaly over the Southwest Indian Ocean.Therefore,the southeast wind anomalies off the coast of Sumatra,because of the heavy rainfall anomaly over the eastern Bay of Bengal,through the local meridional circulation crossing the equatorial eastern Indian Ocean,leads to a negative SST anomaly over the equatorial Southeast Indian Ocean and contributes to the birth of an IOD event.