The impact of tropic Indian M,JO activities on the Intra-Seasonal Oscillation of the Southwest Summer Monsoon over Bay of Bengal

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