Attributing the External Forcings Responsible for Sea Surface Temperature Changes over Indian Ocean

Sea surface temperature (SST) is a major physical attribute of the world's oceans and is a strong signal of productivity, pollution, and global climate change. Thus changes in SST can alter marine ecosystems in several ways. Recent studies pointed out that the decreased land-sea temperature gradient due to ocean warming weakened the Indian Summer monsoon and could increase droughts in South Asia. Attribution of the causes of SST change is vital for making informed adaptation and mitigation policies.

We use multiple observational SST dataset (HadISST (v1.1) and ERSST (v3b)) to account for observational uncertainty to investigate the changes in SST over the Indian Ocean. We carried out our detection and attribution analysis on the four selected regions in the Indian Ocean - Bay of Bengal (BOB), Arabian Sea (AS), Southwest Indian Ocean (SWIO), and Southeast Indian Ocean (SEIO). We identified these regions based on the trend differences between two time periods (1979-2009 and 1948-1978) in the Indian ocean. A regression-based Regularised Optimal Fingerprinting method is used for studying the changes in SST pattern over the four regions. We make use of outputs from 7 climate models in the Coupled Model Intercomparison Project Phase-5 (CMIP5) and Phase-6 (CMIP6) databases. We are able to detect changes and attribute them to GHG forcing over BoB only for 1906-1955, but the results are sensitive to the choice of the observed dataset (ERSST or HadSST). However, for the full 100-year 1906-2005 period and the 50-year 1956-2005 period show a clearer attribution to GHG forcings and are partially offset by other anthropogenic forcings (primarily anthropogenic aerosols) over the AS and SEIO. We find better agreement between observations and CMIP6 models - a possible consequence of improved forcing datasets and/or model improvements. We extend this detection & attribution study till the recent historical period (upto 2020 by utilising RCP/SSP scenarios simulation beyond 2005, 2015 in CMIP5, CMIP6 respectively). We will present the differences in the warming patterns and influences over the 4 subregions of the Indian Ocean.