Role of Anomalous Ocean Warming on the Intensification of Pre-Monsoon Tropical Cyclones Over the Northern Bay of Bengal

The northern Indian Ocean, typically the Bay of Bengal (BoB), has experienced several stronger tropical cyclones during the pre-monsoon season in the last few years. Similarly, in 2021, cyclone Yaas reached the Very Severe Cyclonic Storm (VSCS) stage despite forming above 14 degrees N. In comparison to the other three cyclones that formed in 2007 (Akash), 2009 (Aila), and 2017 (Mora) with a similar cyclogenesis location and track, the pre-cyclonic condition in 2021 is more favorable. It was found that from 2007 to 2021, the area associated with Sea Surface Temperature greater than 31 degrees C increased from 0.1 percent to 29 percent. The Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction buoy at 15 degrees N also recorded an increasing trend of 0.1 degrees C per year in both average and maximum temperatures during May. The warming in 2021 is primarily linked with Marine Heat Wave events, which extensively cover the north BoB for a longer duration starting from the first week of May. Such warming extends even to the subsurface, providing the heat required to convert the depression into VSCS within 2 days. The weaker western boundary current during 2021 made the northern BoB fresher and favorable for cyclone, intensification. The associated anticyclonic eddy along the track extended the abnormal distribution of low saline water even up to the subsurface (similar to 100m), which is vital in increasing the ocean heat content. These simultaneous pre-cyclonic oceanic conditions can be incorporated into the weather model to understand and predict the rapid intensification within a shorter duration. In recent years, the northern part of the Indian Ocean, especially the Bay of Bengal (BoB), has seen a larger number of intense tropical cyclones during the summer. Only few of them form over the northern basin above 14 degrees N. Among four such similar cyclones during 2007, 2009, 2017, and 2021, the latest cyclone Yaas was intensified unusually because of a much suitable prevailing oceanic condition. It was found that from 2007 to 2021, the area associated with Sea Surface Temperature greater than 31 degrees C increased from 0.1 percent to 29 percent. The northern area experienced a gradual warming with a rate of 0.1 degrees C per year. Eventually, this unusual warming corresponded to a widespread long lasting Marine heat Wave event during the entire month of May. This typical phenomenon of ocean warming is extended deeper and confined longer because of the abnormal spread of freshwater over northern BoB. The embedded clockwise ocean circulation on the track of the cyclone Yaas also ensured to keep the area warmer, which further fueled the cyclone. The unusual oceanic condition and its response can be considered to improve the forecasting model for a better prediction of cyclones, which intensify in no time. The area over the Bay of Bengal with Sea Surface Temperature greater than 31 degrees C has increased from 0.1 percent to 29 percent from 2007 to 2021 A Marine Heat Wave event with more than 20 days during May 2021 played a key role in the rapid intensification of the cyclone Yaas The warming is also extended to the subsurface and confined because of widespread freshwater plumes during the early summer of 2021