热带印度洋MJO活动对孟加拉湾西南夏季风季节内振荡的影响
The impact of tropic Indian M,JO activities on the Intra-Seasonal Oscillation of the Southwest Summer Monsoon over Bay of Bengal
-
摘要: 通过对1979-2008年热带太平洋30-60d振荡(Madden Julian Oscillation,MJO)指数、美国国家环境预报中心再分析资料和日本气象厅降水资料的分析,发现热带东印度洋MJO强度和传播状况影响孟加拉湾西南夏季风季节内振荡及相关低频环流、对流和降水分布.当热带东印度洋MJO在春末夏初较活跃时,孟加拉湾西南季风季节内振荡活动在4-8月比其不活跃时提前约20d(约1/2个周期),其对于孟加拉湾西南季风季节内振荡的影响可持续整个季风期,使西南季风的季节内振荡不仅酝酿期和活跃期提前发生,季风期有所延长,季节内振荡也更强.西南季风季节内振荡具有明显的北传和东传特征,北传沿孟加拉湾通道从赤道向副热带推进,而东传则沿10°-20°N从孟加拉湾向东传至南海地区.春末夏初时热带东印度洋MJO的异常状况,正是通过对西南季风季节内振荡东传和北传的影响,进而对孟加拉湾西南季风季节内振荡在季风期的酝酿、维持和活跃产生作用,这种作用同时体现在强度和时间上.孟加拉湾西南夏季风季节内振荡强度与热带东印度洋MJO在4月21日-5月5日的活动呈现显着负相关,当热带东印度洋MJO在春末夏初较活跃时,孟加拉湾西南夏季风季节内振荡的强度较大,在5-8月经历3次季节内振荡波动,低频对流场和环流场在1-3位相(孟加拉湾西南夏季风季节内振荡为正位相)和4-6位相(负位相)时呈反位相特征,这是由MJO低频对流的东传及在孟加拉湾和南海这两个通道上的北传引起的.从印度半岛到菲律宾群岛的降水在1-3位相和4-6位相上分别为正异常和负异常,其中,在第2位相(孟加拉湾西南季风季节内振荡波峰)和第5位相(孟加拉湾西南季风季节内振荡波谷)时分别为降水最大正异常和最大负异常.反之,在热带印度洋MJO在春末夏初不活跃年时,孟加拉湾西南夏季风季节内振荡活动较弱,强度偏弱且振荡也不规律.Abstract: Adopting the MJO Index, the NCEP rcanalysis data and the precipitation data from Japan during 1979-2008,the impact of the tropic eastern Indian MJO intensity and propagation on the Intra-Seasonal Oscillation (ISO) of the Southwest Summer Monsoon (SSM) over Bay of Bengal (BOB) and the related LF (Low Frequency) circulation and convection and precipitation distribution is analyzed. In years when tropic eastern Indian Madden and Julian Oscillation (MJO) is active during late spring to early summer, ISO activities of SSM over BOB during April-August is about 20 d (about half of a cycle) earlier than years when the tropic Indian MJO is inactive during late spring to early summer. Such impact on ISO of SSM over BOB can last for the whole monsoon period,so that the preparing period and activating period of ISO of SSM is advanced,the monsoon period is extended,and the ISO intensity is also stronger. Besides, ISO of SSM shows a pronounced feature of northward (from the tropics along the BOB path to the sub-tropic regions) and eastward (from BOB along 10-20°N to the South China Sca (SCS) propagation. By such an impact on northward and eastward propagation of ISO of SSM,the abnormal situation of tropic eastern Indian MJO during late spring to early summer influence the preparation,maintenance and activation of ISO of SSM over BOB, which arc shown in both intensity and time. (hhe ISO intensity of SSM over BOB shows prominent negative corrclation with tropic eastern Indian MJO activities during the period from 21 April to 5 May, when the tropic Indian MJO is more active during late spring to early summer, the ISO intensity of SSM over BOB is stronger with 3 times of ISO waves experienced during May-August. The LF convection and circulation fields at the phases of 1-3 (i. E. ISO of SSM over BOB is at positive phases) and that at the phases of 4-6 (negative phases) show anti-phase haracteristics,which arc caused by the eastward transportation of LF convection of MJO and its northward transportation along the BOB and SCS paths at the same time. The area from Indian Peninsula to Philippines is in positive Cor negative) anomalies of precipitation at the phases of 1-3 (or phases of 4-6),among which the phase 2
下载: