Extended Abstract
Introduction
Tropical cyclones are among the most destructive natural hazards in the North Indian Ocean, particularly in the Bay of Bengal. The intensification of these systems is critically influenced by air-sea interactions, specifically the exchange of surface latent heat flux (SLHF). SLHF represents the heat transfer associated with the phase change of water, exchanged between the Earth's surface and the atmosphere. An increase in SLHF correlates with a rise in the number and intensity of cyclone systems, as these systems derive their energy from the ocean. Sea surface temperature (SST) is a crucial indicator that influences climate patterns and significantly affects the development and intensity of tropical cyclones. Given that the Bay of Bengal is prone to the formation of many tropical cyclones, the aim of this study is to investigate the advection of surface latent heat flux, sea surface temperature and wind speed in tropical cyclone Amphan (May 2020) and tropical cyclone Nivar (November 2020). Tropical cyclone Amphan and tropical cyclone Nivar occurred in the pre-monsoon and post-monsoon periods, respectively. The use of reanalysis data to investigate surface latent heat flux (SLHF), sea surface temperature (SST), and wind speed, which play a crucial role in the formation and evolution of tropical cyclones, can be significant.
Materials and Methods
The Bay of Bengal is recognized as one of the world's most active basins for tropical cyclone formation, with two primary seasons for cyclone development: pre-monsoon (March, April, and May) and post-monsoon (October, November, and December). This research focuses on two significant events in 2020: Tropical Cyclone Amphan, which occurred from May 16 to 21 and was classified as a super cyclonic storm, and Tropical Cyclone Nivar, which developed from November 22 to 27 and was categorized as a very severe cyclonic storm by the Indian Meteorological Department (IMD). To investigate the interaction between SLHF, SST and Wind Speed during these two tropical cyclones, reanalysis data were utilized. SLHF data were obtained from the ERA5 reanalysis, which provides hourly estimates with a spatial resolution of 0.25° × 0.25°. For SST and wind speed, MERRA-2 reanalysis data were employed, offering a temporal resolution of 1 hour and a spatial resolution of 0.625° × 0.5°. All data were processed and analyzed using Python. The main track positions of the Amphan and Nivar tropical cyclones were extracted from IMD reports and IBTrACS data, and their tracks were mapped using ArcMap.
Results and Discussion
The analysis of the two tropical cyclones revealed distinct temporal and spatial variations in sea surface temperature (SST), surface latent heat flux (SLHF), and wind speed during their development and intensification stages. Tropical Cyclone Amphan formed on 16 May 2020 under SST values exceeding 30°C, which provided a favorable environment for rapid intensification. SLHF increased sharply from approximately 250 W/m² to more than 300 W/m² and reached values above 400 W/m² on 19 May, coinciding with Amphan’s peak intensity. Wind speeds on 19 May, 15 m/s. Subsequently, as the cyclone approached land, SST decreased, leading to weakening. In contrast, Tropical Cyclone Nivar developed on 22 November 2020 with lower SST values (around 29°C). SLHF exhibited a gradual increase, reaching its maximum (300–400 W/m²) on 25 November. Wind speeds intensified to about 12.5 m/s. Overall, the results demonstrate that pre-monsoon conditions favor stronger heat flux exchange and more intense cyclone development compared to the post-monsoon period.
Conclusion
This study investigated the interaction between surface latent heat flux (SLHF), sea surface temperature (SST), and wind speed during the evolution of Tropical Cyclone Amphan (pre-monsoon) and Tropical Cyclone Nivar (post-monsoon) in the Bay of Bengal. The results show that pre-monsoon conditions create a more favorable environment for cyclone intensification due to higher SST and stronger heat exchange. During Amphan’s peak on 19 May, SLHF 400 W/m², SST reached about 31°C, and wind speed increased to nearly 15 m/s. In contrast, during Nivar’s peak on 25 November, SLHF ranged between 300 and 400 W/m², SST decreased to 26–29°C, and wind speed reached approximately 12.5 m/s. Overall, the findings of 2020 indicate that the interaction between latent heat flux, sea surface temperature, and wind speed has a time-dependent nature, and the seasonal differences play a significant role in the intensity and evolution of tropical cyclones. This study practically demonstrates that seasonal variations in surface latent heat flux (SLHF), sea surface temperature (SST), and wind speed can enhance the accuracy of tropical cyclone forecasts in the Bay of Bengal.