Forced and Unforced Changes of Indian Ocean Temperature and Land-Sea Temperature Gradient

Sea surface temperature (SST) over the Indian Ocean is directly connected with circulation, winds, precipitation, humidity, etc. over India. Increased SSTs are a major consequence of climate change driven largely by anthropogenic factors. Recent literature points to weakening of the Indian Summer Monsoon possibly because of decreased land-sea temperature gradient due to faster rate of warming of the oceans compared to land regions. We examine changes in the SST over the Indian Ocean using two observational datasets; HadISST (v1.1) and ERSST (v3b). Based on trend differences between two time periods (1979-2009 and 1948-1978) we identify four regions in the Indian Ocean with different signatures of change - Bay of Bengal (BOB), Arabian Sea (AS), Southwest Indian Ocean (SWIO), and Southeast Indian Ocean (SEIO). We first quantify the extent to which the SST trends over multiple time-scales (20, 30, 50 and 100-years) are outside of the range expected from internal variability of the climate system. We make use of output data from long control run simulations from the Coupled Model Intercomparison Project Phase-5 (CMIP5) database in order to estimate the contribution of external forcings to the observed trends. Using optimal fingerprint Detection and Attribution methods we quantify the contributions of various natural and anthropogenic forcings by making use of the suite of experiments (piControl, historical, historicalNat, historicalAnt, historicalGHG, and historicalAA) from CMIP5 are used in this study. We will also address the question of what drives the observed weakening of land-ocean temperature gradients.