Intensification of Pre-Monsoon Cyclone: Insights from spring snowmelt discharge

Abstract The intensification of pre-monsoon cyclones in the northern Bay of Bengal, particularly Cyclones Yaas (2021), is increasingly attributed to the rising influx of snowmelt water from the Ganges and Brahmaputra rivers. Cyclone Yaas intensified over high sea surface temperatures (31.5 to 32°C) and significant freshwater discharge, leading to notable stratification and increased ocean heat content in the upper layers. As Yaas moved through a warm core eddy, the sinking of water caused the isotherms to deepen by 25–50 meters, a process that was further intensified by freshwater discharge due to spring snowmelt. Key ocean heat content in the top 30-meter layer was critical during Yaas's formation, with total incoming shortwave radiation of 420 W/m² and net radiation of 390 W/m² closely aligning with the heat content of 400 W/m² in the central and western Bay, providing the necessary energy for intensification. This study employs integrated datasets, including in situ observations, ERA5-based freshwater flux analysis, and high-resolution numerical simulations, to reveal cyclone-ocean interactions. Findings indicate that similar snowmelt dynamics contributed to the intensification of recent cyclones. With the rising temperatures due to global warming, even minor changes in increasing snowmelt and surface runoff can significantly affect upper ocean dynamics. This highlights the critical role of pre-monsoon freshwater discharge in strengthening cyclones and emphasises the need for better prediction models to understand future cyclone behavior, which can help reduce risks in this vulnerable region. Additionally, in terms of biological processes, the increased fresh water discharge during spring in the northern Bay of Bengal caused high stratification and intense downwelling, which suppressed nutrient-rich subsurface waters and resulted in limited chlorophyll concentrations (1 mg/m³) along Yaas's track, despite phytoplankton blooms in regions of high wind stress.