On the simulation of northeast monsoon rainfall over southern peninsular India in CMIP5 models

Abstract

The skill of 34 CMIP5 models to simulate the mean state and interannual variability of Northeast Monsoon Rainfall (NEMR) is studied here. The mean (1979–2005) NEMR over southern Peninsular India (SPIRF), Indian Ocean and Maritime continents (10°S-30°N,40°E-120°E) is simulated reasonably well by CMIP5 models with pattern correlation ranges from 0.6 to 0.93. A few individual models have been found to be outperformed the multi model ensemble (PCC-0.88). Diverse behaviour in the simulation of Indian and Pacific Ocean SST is observed in the CMIP5 models. A set of models (high skill models: HSM), which shows an NIOD like mean (1979–2005) SST bias in Indian Ocean and strong La Nina like mean SST bias in the Pacific Ocean, are able to simulate the mean NEMR more realistically. Another set of models (low skill models: LSM) which shows a Positive IOD (PIOD) like mean SST bias in the Indian Ocean and a weak La Nina like mean SST bias in the Pacific Ocean are not able to simulate the observed equatorial Indian Ocean westerlies. This leads to an abnormal ascending motion and unrealistic wet bias over the western Indian Ocean and dry bias over the southern Peninsular India, southeast Asia and southeast Indian Ocean. Observational analysis reveals that the ascending anomalies over warm pool in the climatological mean Walker circulation during NEM season is modified as ascending anomalies to the east and west of warm pool region and descending anomalies over warm pool region during El Nino and PIOD. This modulation is manifested as an interesting pattern of warm and wet western Indian Ocean, southern Peninsular India and central and eastern Pacific Ocean and cool and dry warm pool region including Maritime continents.The observation analysis also reveals that the establishment of South China Sea anticyclone and Bay of Bengal anticyclone during El Nino and PIOD are strongly related with the ascending motion over south peninsular India and hence enhances the south Peninsular Indian rainfall during NEM season. Around 70% of the CMIP5 models were not able to capture the observed positive correlation that exist between SPIRF and Nino3.4 SST as well as SPIRF and DMI. An unrealistic westward extension of warm anomalies over the equatorial Pacific cold tongue is observed in low skill models (LSM-IAV). Unrealistic westward extension of South China Sea anticyclone and Bay of Bengal anticyclone (up to 70°E) is also observed in the LSM-IAV model ensemble. This is manifested as the abnormal descending anomalies and unrealistic dry bias over the southern Peninsular India and hence the unrealistic negative CC between SPIRF and Nino 3.4 SST (SPIRF-DMI). The descending anomalies over South China Sea and ascending anomalies over the western Indian Ocean and southern Peninsular India (50°E-80°E) is well captured but with lower intensity in HSM-IAV and hence it shows the observed positive CC between SPIRF and Nino3.4 SST as well as SPIRF and DMI.