The Emergence of Near-Permanent Marine Heatwave State in the Tropical Indian Ocean During 2023–2024

In 2023/24, global mean surface temperatures exceeded 1.5°C above pre-industrial levels, and the Tropical Indian Ocean (TIO) experienced a record-breaking basin mean anomalous warming of 0.88°C. This unprecedented warming in the TIO is linked to the severe and long-lasting marine heatwave (MHW) events ever recorded in the region, reconciling with the ongoing long-term warming and internal climate variabilities. The MHW events of 2023/24 were primarily centred in the Arabian Sea (AS) and southwestern TIO regions, with mean intensities of 0.73°C and 0.89°C, respectively. The prolonged MHW event developed in the AS region in August 2023 and in the southwestern TIO in July 2023 due to increased oceanic heat gain associated with increased insolation and reduced latent heat loss. The MHW event in the AS was sustained throughout the remainder of the year and the following year by subsurface warming due to the convergence of surface winds in the AS and suppressed cooling by vertical processes. In the southwestern TIO, the event intensified and lasted till December 2024, driven by thermocline warming induced by downwelling Rossby waves, triggered by intensified easterlies associated with an anomalously westward extended cold tongue in the equatorial Indian Ocean, linked to the co-occurrence of strong El Niño and extreme equatorial positive Indian Ocean Dipole (PIOD) events. The extreme warming of the TIO during 2023/24 underscores the ongoing impact of climate change on global ocean surface warming.