MONSOON INTRASEASONAL VARIABILITY DURING EXTREME YEARS: OBSERVATION & COUPLED MODEL SIMULATION

The present study examines the space-time characteristics of the Indian summer monsoon intraseasonal variability (ISV) during strong monsoon (SM) and weak monsoon (WM) years, and further provides insight on the seminal role of the seasonal mean states on modulating the ISV statistics. The observational evidences indicate prominent asymmetry in the duration and intensity of active/break spells associated with the monsoon intraseasonal oscillations (MISO) during SM and WM years. Substantial differences in the northward propagation of MISOs during wet/dry spells are also evident. It is found that the MISOs have reasonably faster (slower) propagation during active phase of WM (SM), while it has slower (faster) propagation during break episodes of WM (SM) over Indian monsoon region. It is speculated that considerable differences in the seasonal mean states during extreme years could be responsible for these asymmetries. In support of this hypothesis, it is found that during WM (SM) years, the seasonal mean zonal winds associated with the convective region are anomalously westerly with barotropic (baroclinic) vertical structure, while a baroclinic (barotropic) structure dominates over region of suppressed convection. The evolution of meridional-vertical structure of the MISOs during SM and WM also highlights the role of barotropic zonal wind anomalies in the northward propagation of MISOs. From the barotropic zonal momentum budget analysis, it is demonstrated that the meridional and vertical advection of the seasonal mean zonal wind anomaly may be largely responsible for such asymmetry in intensity, life span and propagation characteristics of the MISOs during SM and WM years. Further, the ability of a state-of-the-art coupled climate model (Climate Forecast System Model, CFSv2) in simulating such observed asymmetry in the ISV during extreme years has been evaluated. It is found that the model has a reasonable intraseasonal-to-interannual monsoon variability and it could capture the observed asymmetry in the MISO characteristics during extreme years realistically.