Submesoscale processes and their seasonality generated by freshwater discharge at the Yangtze estuary

As the intersection between land and sea, the estuary is a hotpot for natural and human-induced processes that affect its physical and chemical characteristics, as well as biodiversity and ecosystems. Due to a combination of freshwater discharge and the unique topography of river estuaries, submesoscales with a spatial scale of O(1) km can be easily generated due to the presence of strong salinity fronts at river estuaries. As the biggest river in Asia, the basically dynamical characteristics and their seasonal mechanisms of submesoscales at the Yangtze estuary remain unclear. Using a nested high-resolution (300 m) simulation established by the Coastal and Regional Ocean Community model (CROCO), the seasonal variability of submesoscales at the Yangtze estuary is investigated and the underlying generation mechanisms are analyzed based on the analysis of kinetic energy budget. The results of normalized relative vorticity, horizontal salinity gradient, and submesoscale kinetic energy (SMKE) show that submesoscale processes have a seasonal variation at the Yangtze estuary. This seasonality is tightly related to the freshwater input with a correlation coefficient of up to 0.66 (the confidence level >99%). To figure out the dominant energy sources that control the SMKE budget, five terms including the baroclinic term (BC), the barotropic term (BT), the advection of submesoscale kinetic energy term (ADK), the pressure work term (PW), and vertical mixing term (VM) are diagnosed. The results suggest that BC, BT, and PW are the source producing SMKE, while ADK and VM tend to dissipate SMKE. This work suggests the important role of submesoscale activities in the energy cascade and enhancing the transports of tracers at the estuary.