The cause of extreme precipitation events, which deadly flooded Tamil Nadu state of southern India during the northeast monsoon season of 2015 was investigated, and the results were presented in this paper. Though a strong El Nino prevailed during the events, the effect of El Nino is suppressed by the tropical variabilities in the Indian Ocean. A power spectrum analysis was performed to find out the kind of tropical variabilities in NCEP variables like wind fields, Omega, precipitation rate, and soil moisture at 0-10 cm. The spectrum analysis resulted in significant periodicities of 30-40 days and 7-20 days during the extreme events over southern India. Those frequencies were linked with the convectively coupled equatorial waves (CCEWs) like Madden-Julian Oscillations (MJO), and, it was found that the cause of El Nino’s suppression is a manifestation of the CCEWs. The dynamical mechanism behind those interactions was investigated to know the specific connections of two major tropical variabilities El Nino and MJO. Further exploration was done by performing composite analysis of extreme precipitation events during historical El Nino (moderate to very strong) and MJO (active phases over the Indian Ocean) events from 1997-2014 to know the possible interaction between El Nino and MJO. The composite analysis contributed an insight into the interactions of El Nino and MJO. This analysis concludes a hypothesis, which states that if a prevailing, moderate to very strong El Nino as a background low-frequency wave superimposed with high-frequency wave like active MJO in the equatorial Indian Ocean during October-December season, then blended El Nino & MJO wave suppresses the effect of background prevalent El Nino. Such a clampdown of El Nino by blended El Nino & MJO wave roots the cause of extreme precipitation over the southeastern India. This study reveals a new dimension to the El Nino and MJO interactions in intraseasonal time scale, which could be exploited in the prediction of extreme precipitation events during northeast monsoon season.

Dynamical downscaling of Indian summer monsoon rainfall (ISMR) by using regional climate models (RCMs) portrays the inability of the RCMs in simulating the ISMR, and certain systematic biases appear in the seasonal monsoon rainfall climatology. The inconsistency in RCMs simulation of ISMR can be due to the improper representation of convection by convective and/or microphysical parameterization schemes in different RCMs. In this study, we conducted convection permitting simulations in WRFv3.8.1 and compared with parameterized simulations, to understand the difference of reproducibilities of time-space patterns in the ISMR. Our experimental set-up consists of two sets of simulations with parameterized and explicit convection on a grid resolution of 25 km. The simulations are conducted for three different monsoon seasons: flood, drought, and normal years, to ascertain robustness in the analysis of the model output. These simulations are forced by using ERA-Interim reanalysis as the lateral boundary and large-scale forcing input. The mean large-scale circulation, the spatial distribution of rainfall, seasonal northward propagation of rain bands, and magnitude-phase of the Indian summer monsoon rainfall are verified against the JRA55 reanalysis and India Meteorological Department gridded rainfall datasets. The results show that regional simulations with explicit convection have benefited in the simulation of ISMR features. Simulated seasonal mean rainfall in parameterized convection shows positive bias over Gangetic plains and the Western Ghats. The same bias reduced in explicit simulations and seasonal mean ISMR behaves realistically concerning IMD observations. The added value in the simulation of ISMR in explicit experiments is found to be consistent during the flood, drought, and normal monsoon seasons. Further evaluation of the results reveals that over Indian region, explicit convection simulations of Indian summer monsoon are more realistic than parameterized convection simulations. Therefore, the current study tried to show up the uncertainties in ISMR simulation associated with parameterizations, and explicit convection experiments highlight the reduction of these uncertainties.