Marine heatwaves becoming more intense, frequent in Indian Ocean: Incois study

Kochi: A worrying climate change signal is emerging from the Indian Ocean, particularly over Arabian Sea, where marine heatwaves (MHWs) are becoming increasingly frequent and intense, according to observations from Indian National Centre for Ocean Information Services (Incois). Based on INCOIS observations between 2016 and July 2024, Arabian Sea was identified to remain in the ‘Watch' state of Incois Marine Heatwave Advisory Services for close to 40 days every year. During these days, sea surface temperature was noticed to increase by about 0.5° above normal, which was much more than what was witnessed in other areas of the Indian Ocean. This warming anomaly occurred along the shelf regions of the western coastline of India. Further analysis revealed that marine heatwave conditions became intense for about 20 days every year, wherein sea surface temperatures increased by about 1° above normal levels, putting the area in the ‘Alert' stage. Furthermore, extremely high marine heatwaves with temperatures above 1° above normal were noticed for approximately five days per year in Arabian Sea, classified in the ‘Warning' category. The increased warming in the Arabian Sea is mainly due to increased heat absorption, decreased winter cooling and also the warming of global oceans caused by climate change. In addition to this, the semi-closed nature of the Arabian Sea makes it easier for heat to get trapped there. Experts in Incois said recent studies indicate that weaker summer monsoonal circulation in parts of Arabian Sea may be one factor causing escalation of the marine heatwaves. The threats posed by the rising marine heat include mass coral bleaching, deaths of marine life, biodiversity loss, impacts on fisheries and rising sea levels. "Arabian Sea has experienced relatively increasing sea surface warming trend due to enhanced heat absorption, reduced winter cooling, and increasing background global warming. Reduced wind stress leads to weaker vertical mixing, shallow mixed layer ventilation, stronger upper-ocean stratification, and enhanced surface heat trapping. This creates favourable conditions for long-duration MHWs," said Incois director Balakrishnan Nair T M. "Rising ocean temperatures are one of the major consequences of climate change. When sea surface temperatures increase, evaporation from the oceans also rises, adding more moisture to the atmosphere. This enhanced moisture contributes to the formation of low-pressure systems over the seas, which can eventually intensify thunderstorm activity and trigger extreme rainfall events over the land areas. Higher marine activity also influences atmospheric circulation patterns, leading to increase in land surface temperatures and changes in precipitation patterns," said Sivananda Pai, head of the regional meteorological centre of IMD in Chennai.