Upper Ocean Dynamics During Binary Interaction of Tropical Cyclones: A Case Study in the Southeastern Indian Ocean

Tropical cyclones (TCs) are among the most powerful and destructive atmospheric phenomena, significantly influencing the ocean's surface and subsurface dynamics. While individual TC interactions with the upper ocean are well-studied, the effects of binary or multiple TC interactions on the physical setting of the upper ocean remain relatively underexplored. In this study, we have investigated the coupled dynamics of the two co-occurring TCs Seroja and Odette in the southwestern Indian Ocean, focusing on their binary interactions and the impacts on the upper ocean layer (0-200 m). For the first time, we examined the impact on the upper ocean during stalling and complete merging of TCs by using a combination of observational data and numerical simulations.

During the interaction of the weak TCs Seroja and Odette, we observed cooling of up to 3.0°C within 72 hours, typical known only for strong TCs. This cooling persisted for at least 8 days and was associated with significant upwelling processes in the upper 200 m of the ocean. Our analysis revealed drastic changes in vertical ocean velocities, with sudden reversals from downward to upward velocities of up to 30 m d^-1, observed down to depths of at least 750 m. These changes were particularly pronounced during the merging of the two TCs, highlighting the extreme nature of such binary interactions.

This research contributes to our understanding of how even weaker TCs, when interacting, can cause extraordinary transport of deeper water masses to the ocean surface. With the potential increase in TC frequency and intensity due to climate change, our findings underscore the importance of studying these extreme events for better prediction and risk assessment in marine environments.