Impact Of The Madden-Julian Oscillation On The Precipitation And Onset Of The East Asian Summer Monsoon

The impact of the tropic Indian Madden-Julian Oscillation (MJO) on the precipitation of the East Asian summer monsoon, its onset and the possible mechanism are investigated using the observational rainfall data, the pentad MJO indices, NCEP/DOE AMIP-II Reanalysis. The results show that:(1)When the tropic Indian MJO is active(inactive), the rainfall of the middle and lower reaches of Yangtze River during the summer half year(especially in Apr-Jun) is enhanced(suppressed) in the same period and the following two pentads.(2)Such different rainfall regimes are associated with the corresponding changes in large scale back ground fields. When the tropic Indian MJO is active(inactive), an anomalous anticyclone(cyclone) develops in the WNP and an anomalous cyclone(anticyclone) arises in Bay of Bengal. The water vapor convergence and upward motion in the middle and lower reaches of Yangtze River are increased(decreased), such environment is in favor of rainfall enhancement(suppression).(3)It is suggested that the tropic Indian MJO may influence the precipitation of the middle and lower reaches of Yangtze River during Apr-Jun by forcing the classical Matsuno-Gill solution and a Rossby wave train. When it is active(inactive), anomalous low-level easterly over the tropical western Pacific, anomalous low-level westerly over the tropical Indian Ocean, anomalous local Hadley and Walker circulation all contribute to the anomalous cyclones and anticyclones in corresponding regions which benefit(against) the water vapor tranport. A negative anomaly of geopotential height to the west(east) of Southeast China and a positive anomaly to its east(west) contribute the anomalous south(north) wind which also benefit(against) the water vapor tranport.The anomalous water vapor tranport and geopotential height act on vertical motions which all affect the precipitation.(4) The SCSSM onset time shows prominent negative correlation with tropic Indian MJO activities during the period from 21 April to 5 May, in which the Bay of Bengal (BOB) vortex may act as a bridge. The intro-seasonal oscillation(ISO) could feed back to the SCS summer mean monsoon through nonlinear momentum transport.