Pre-requisite conditions for the intensification of pre-monsoon cyclones over the Bay of Bengal

The intensification of pre-monsoon cyclones in the northern Bay of Bengal, particularly Cyclone Yaas (2021), is increasingly attributed to the rising influx of freshwater 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 m, a process further influenced by the freshening of the surface layers during the pre-monsoon period. 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. Present findings indicate that freshwater dynamics and resulting stratification significantly contributed to the intensification of recent cyclones. With rising temperatures due to global warming, even minor changes in freshwater input and surface runoff can affect upper ocean structure and cyclone behaviour. This highlights the critical role of pre-monsoon freshwater discharge in strengthening cyclones and emphasises the need for improved prediction models to understand future cyclone behaviour. Additionally, from a biological perspective, the increased freshwater discharge during spring in the northern Bay of Bengal caused strong 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 adjacent regions of high wind stress.