Future seasonal changes of marine heatwaves in the Arabian Sea and Bay of Bengal

This study investigated the marine heatwaves (MHWs) in the Arabian Sea (AS) and Bay of Bengal (BoB) during pre-monsoon (March-April-May, MAM), monsoon (June-July-August-September, JJAS), and post-monsoon (October-November-December, OND) seasons using observations from NOAA and the simulations from 17 CMIP6 Global Climate Models (GCMs) under four Shared Socioeconomic Pathways (SSPs). MHW metrics of frequency, maximum intensity, and duration were analyzed for the historical period (1985–2014) and for the near (2031–2060, NF) and far (2061–2090, FF) future periods. Two approaches, fixed-baseline and shifted-baseline, were applied for the future projection of MHWs. Observed NOAA values indicated higher trends in AS compared to BoB for all MHW metrics during all seasons. Future projections under the fixed-baseline approach indicated model agreement on positive shifts ranging from 43 % to 136 % for frequency, 89 % to 173 % for maximum intensity, and 545 % to 1225 % for duration in the NF compared to the historical, with BoB showing higher positive changes than AS during JJAS and OND under all SSPs. The fixed-baseline approach suggested MHWs attaining a permanent state towards the FF. Model agreement on future trends under the shifted-baseline indicated higher trend values in the NF compared to historical from SSP2-4.5 to SSP5-8.5 across all MHW metrics during all seasons, while the FF indicated higher trends from SSP3-7.0 to SSP5-8.5. Additionally, under the shifted-baseline, FF exhibited higher trend values than NF. Model agreement under the fixed-baseline approach indicated more vulnerability in the northwestern regions of AS to an increase of MHW frequency during MAM and JJAS in the NF.