1982—2009年冬夏两季热带季节内振荡的趋势特征

Trend of tropical intraseasonal oscillations in summer and winter during 1982—2009

采用1982—2009年美国国家海洋与大气管理局（National Oceanic and Atmospheric Administration,NOAA）逐日向外长波辐射（outgoing longwave radiation,OLR）资料,利用EOF方法,分析了20-70 d北半球夏季（6—9月）季节内振荡（boreal summer intraseasonal oscillation,BSISO）与冬季（12月—次年2月）季节内振荡（也称Madden-Julian Oscillation,MJO）不同的强度趋势。结果表明：BSISO指数有明显加强的趋势,而MJO指数的趋势则不明显。进一步利用频率—波数分析方法将季节内振荡（intraseasonal oscillation,ISO）分成西传和东传两部分。结果表明：东传的BSISO在其活动中心——热带印度洋地区有显著加强的趋势,而东传的MJO在其活动中心的趋势则不明显,仅在其活动中心西南部即热带印度洋西南部有减弱的趋势。为探究其原因,文章进一步分析了海表温度（sea surface temperature,SST）和纬向风垂直切变的趋势变化。结果表明：1982—2009年,西太平洋和印度洋SST无论冬夏均持续增暖,SST并不能解释冬夏两季ISO不同的趋势特征;而夏季热带印度洋地区对流层中低层东风垂直切变减弱,冬季海洋性大陆地区东风垂直切变增强。由此认为：热带印度洋东风垂直切变减弱有可能有利于东传的BSISO加强;而海洋性大陆地区东风垂直切变加强有可能削弱东传的MJO,但这种减弱效应被冬季海洋性大陆地区增强的上升运动产生的加强效应抵消,所以MJO的变化趋势并不显著。

Based on the daily outgoing long-wave radiation（ OLR） data of the National Oceanic and Atmospheric Administration（ NOAA） from 1982 to 2009,by using EOF method,this paper investigated the intensity changes of the 20—70 d boreal summer（ June-September; JJAS） intraseasonal oscillation（ BSISO） and winter（ December-next February; DJF） intraseasonal oscillation,also known as the Madden-Jul-ian Oscillation（ MJO）. The results show that BSISO index has a significant intensifying trend during1982—2009. On the other hand,little trend is found for boreal winter MJO index during this period. The wavenumber-frequency analysis method was applied to separate ISO（ intraseasonal oscillation） into westward-propagated and eastward-propagated parts. For the eastward-propagated BSISO,a significant intensified trend is observed over tropical Indian Ocean or its action center. For the eastward-propagated MJO,there is no evident trend observed over its action center,except that a weakened trend is observed over southwestern tropical Indian Ocean（ i. e. southwestern part of MJO action center）. To gain insight into the different ISO characteristics,the tendencies of sea surface temperature（ SST） and the vertical shear of zonal wind were analyzed. The results show that,in both seasons from 1982 to 2009,the global SST trends are similar,and thus they could not be used to explain the intensifying trend of BSISO. However,the lower-tropospheric easterly shear in boreal summer over tropical Indian Ocean has a decreasing trend,while the easterly vertical shear over the maritime continent is enhanced in winter. It is proposed that the reduced easterly vertical shear over tropical Indian Ocean favores the amplification of the eastward-propagated BSISO. The enhanced easterly vertical shear over the maritime continent might weakens the eastwardpropagated MJO,but its impact is offset by the enhanced upward motion over the maritime continent. As a result,there is little trend for MJO in boreal winter.