Evaluation and progression of marine heatwaves over the northwestern Indian Ocean: A comparative study using satellite and numerical model data

This study analyzes the spatio-temporal characteristics and drivers of marine heatwaves (MHWs) across the northwestern Indian Ocean using satellite-based observations (SAT) and high-resolution ocean model outputs from NEMO and HYCOM over the period 1994–2023. MHW events were estimated by adopting seasonally varying 90th percentile thresholds computed from three climatological baselines: 1982–2005, 1982–2023, and 1994–2023. The frequency and intensity of MHWs increased in recent years, particularly when evaluated against the colder historical baseline (1982–2005). SAT MHWs showed a strong spatial variability, with peak intensities and cumulative exposure in the northern Red Sea, western Arabian Gulf, and northern Arabian Sea (NAS). NEMO model highlighted strong warm bias over NAS (0.28) and Red Sea (∼0.26) compared to Arabian Gulf (∼0.1). However, HYCOM exhibited a weak-to-moderate warm bias over all the regions (0.03–0.13)except NAS, which displayed moderate cold bias (0.15), leading to overestimation of MHW events, particularly in the NEMO dataset. Detailed analysis of major MHW years highlighted local dynamics such as anomalous net shortwave heat fluxes, reduced wind-driven mixing and increased ocean heat content contribute to MHWs persistence. The correlation matrix of MHWs frequency and climate modes showed a strong relation with Atlantic multidecadal oscillation. The negative long-term trends for chlorophyll-a and net primary production, coinciding with increasing MHW trends. Overall, this study noted that HYCOM and NEMO were able to capture the MHWs with slight uncertainty, especially over the NAS and the southern Red Sea, which could be due to the difference in surface forcing. This also highlights the importance of developing and testing regional ocean-biological models to better understand the MHWs and their impacts.