Mechanism Of The Impact Of MJO On The Onset Of The Indian Monsoon And Its Subseasonal Predictabilit

Based on the ERA-Interim reanalysis data,the National Oceanic and Atmospheric Administration(NOAA)outgoing longwave radiation(OLR)and Climate Prediction Center daily precipitation data and the reforecast data of three operational models [the European Centre for Medium-Range Weather Forecasts(ECMWF),the National Centers for Environmental Prediction(NCEP)and Institute of Atmospheric Physics,Chinese Academy of Sciences(IAPCAS)] that participated in the Subseasonal to Seasonal Prediction(S2S)project,the impacts of the tropical intra-seasonal oscillation(a.k.a Madden–Julian Oscillation,MJO)on the onset of the Indian subcontinent rainy season and its sub-seasonal predictability were investigated through moisture budget diagnosis,climatological composites and case studies.A number of findings are concluded as below:(1)The statistical analysis reveals the phase-locking relationship between MJO activity and rainy season onset over the Indian subcontinent.A significantly high probability of rainy season onset is observed when the MJO convection stays over the western-central Indian Ocean.On the other hand,the rainy season onset is infrequent when the MJO is over the Maritime Continent and western Pacific.(2)The MJO-associated convective instability with anomalous warm and moist air in the lower troposphere appears and grows during 10 days prior to the onset of rainy season,and drops substantially after the start of rainy season,suggesting its role as a trigger of rainy season onset.In contrast,the low-frequency background state(LFBS)with a period > 90 days favors a convectively unstable stratification even after the onset of rainy season,supporting the succeeding precipitation during the entire rainy season.(3)Based on the scale-decomposed moisture budget diagnosis,we further found that the key processes inducing the abrupt transition from a dry to wet condition come mainly from two processes: 1)convergence of LFBS moisture by the convergent disturbance which triggered by MJO-related circulation perturbations 2)the MJO-related circulation causes the warming of SST and the increase of disturbed moisture in the Arabian Sea region through the sea–air interaction,advection of MJO moisture anomalies by the background cross-equatorial flow contributes to the moisture convergence in the Indian subcontinent region.(4)The assessments of reforecast from three S2 S models show the diverse prediction skills in predicting the timing of rainy season onset over the Indian Subcontinent.The useful skill reaches 28-day(ECMWF),19-day(NCEP)and 17-day(IAP-CAS).The forecast errors are related closely to the MJO.The accurate predictions of the onset timing of rainy season are shown when the model-predicted MJO-circulation evolution is consistent with observations,while the predicted rainy season onset dates are later(earlier)than the observation when the predicted MJO circulation is shown later(earlier)or of weaker(stronger)amplitude in the Arabian Sea compared to observations.These results confirm that the MJO is an important source of subseasonal predictability of rainy season onset over the Indian subcontinent.