Dynamics of submesoscale processes and their influence on vertical heat transport in the southeastern tropical Indian Ocean

Submesoscale processes (SMPs) play profound roles in energy cascade, air-sea heat flux, and marine ecosystem, the variability of which significantly regulates regional and global climate. The southeastern tropical Indian Ocean (SETIO) has abundant and complex dynamic processes, yet the dynamics of SMPs in this region remain unclear. Based on the outputs of two high-resolution models, the seasonality and potential mechanisms of SMPs in the SETIO, as well as accompanying submesoscale vertical heat transport (SVHT) are investigated in this study. The SMPs and SVHT are much stronger during the southeast monsoon period (June-October). Mixed layer instability (MLI) dominates the generation of SMPs, while frontogenesis only plays a minor role. The enhanced horizontal density gradients partly resulting from strengthened large-mesoscale flow strain, coupled with a deeper mixed layer induced by surface cooling and strong southeast monsoon, account for the stronger MLI in the southeast monsoon period. Besides, symmetric instability (SI) also contributes to the generation of SMPs by extracting kinetic energy from the geostrophic flows. Upward SVHT in medium- and high resolution ROMS simulations surpasses that in low-resolution ROMS simulation by a factor of three during the austral winter and is significantly stronger than mesoscale vertical heat transport. These results confirm the importance of SMPs in upper-layer vertical heat transport, and SMPs resolving models can represent the vertical heat transport much better. Our findings could deepen our understanding on multiscale dynamic processes and vertical heat transport in the SETIO.