Evaluation and future projection of marine heatwaves in CMIP6 models in the Indian Ocean

Marine heatwaves (MHWs) are prolonged periods of anomalously high sea surface temperature (SST) that threaten global marine ecosystems, yet their future projections in the Indian Ocean remains poorly understood. Hence, this study systematically evaluates and projects MHWs using 14 Coupled Model Project Intercomparison Phase 6 (CMIP6) models under three shared socioeconomic pathways (SSP126, SSP245, SSP585). Results show that raw CMIP6 models tend to overestimate MHW duration and underestimation of intensity due to regional biases, which are substantially reduced through quantile delta mapping bias correction, improving projection accuracy. Bias-corrected projections reveal a sharp increase in MHW intensity and duration by 2100, specifically under SSP585, with SST anomalies exceeding 3 degrees C and events lasting over 350 days annually compared to 1985-2014. While SSP126 maintains relatively moderate MHW conditions, both SSP245 and SSP585 drive substantial intensification, with SSP585 producing the most severe outcomes. Projected MHWs reveal pronounced seasonality, with lower area coverage during summer monsoon and peaks exposure in late winter and spring. MHW categories undergo a robust shift by 2080, indicating critical ecological and socioeconomic risks. Crucially, the fixed baseline method uncovers that the Arabian Sea and northern Indian Ocean emerge as future MHWs hotspots, transitioning towards near permanent MHWs state as early as 2040-2050, revealing an irreversible ecological threat that is otherwise masked by moving-baseline methods. However, the CMIP6 model skill is found to depend more on physical parameterizations than on spatial resolution. Biases in lower-performing models can substantially alter projected MHW occurrence and severity, often projecting the onset of permanent MHW conditions by several decades earlier and challenging the development of policy and mitigation strategies.