Dynamical analysis of submesoscale fronts associated with wind-forced offshore jet in the western South China Sea

This study investigates the submesoscale fronts and their dynamic effects on the mean flow due to frontal instabilities in the wind-driven summer offshore jet of the western South China Sea(WSCS),using satellite observations,a 500 m-resolution numerical simulation,and diagnostic analysis.Both satellite measurements and simulation results show that the submesoscale fronts occupying a typical lateral scale of O(~10)km are characterized with one order of Rossby(Ro)and Richardson(Ri)numbers in the WSCS.This result implies that both geostrophic and ageostrophic motions feature in these submesoscale fronts.The diagnostic results indicate that a net cross-frontal Ekman transport driven by down-front wind forcing effectively advects cold water over warm water.By this way,the weakened local stratification and strong lateral buoyancy gradients are conducive to a negative Ertel potential vorticity(PV)and triggering frontal symmetric instability(SI)at the submesoscale density front.The cross-front ageostrophic secondary circulation caused by frontal instabilities is found to drive an enhanced vertical velocity reaching O(100)m/d.Additionally,the estimate of the down-front wind forcing the Ekman buoyancy flux(EBF)is found to be scaled with the geostrophic shear production(GSP)and buoyancy flux(BFLUX),which are the two primary energy sources for submesoscale turbulence.The large values of GSP and BFLUX at the fronts suggest an efficient downscale energy transfer from larger-scale geostrophic flows to the submesoscale turbulence owing to down-front wind forcing and frontal instabilities.In this content,submesoscale fronts and their instabilities substantially enhance the local vertical exchanges and geostrophic energy cascade towards smaller-scale.These active submesoscale processes associated density fronts and filaments likely provide new physical interpretations for the filamentary high chlorophyll concentration and frontal downscale energy transfer in the WSCS.

Submesoscale-enhanced filaments and frontogenetic mechanism within mesoscale eddies of the South China Sea

Submesoscale activity in the upper ocean has received intense studies through simulations and observations in the last decade,but in the eddy-active South China Sea(SCS)the fine-scale dynamical processes of submesoscale behaviors and their potential impacts have not been well understood.This study focuses on the elongated filaments of an eddy field in the northern SCS and investigates submesoscale-enhanced vertical motions and the underlying mechanism using satellite-derived observations and a high-resolution(∼500 m)simulation.The satellite images show that the elongated highly productive stripes with a typical lateral scale of∼25 km and associated filaments are frequently observed at the periphery of mesoscale eddies.The diagnostic results based on the 500 m-resolution realistic simulation indicate that these submesoscale filaments are characterized by cross-filament vertical secondary circulations with an increased vertical velocity reaching O(100 m/d)due to submesoscale instabilities.The vertical advections of secondary circulations drive a restratified vertical buoyancy flux along filament zones and induce a vertical heat flux up to 110 W/m^(2).This result implies a significant submesoscale-enhanced vertical exchange between the ocean surface and interior in the filaments.Frontogenesis that acts to sharpen the lateral buoyancy gradients is detected to be conducive to driving submesoscale instabilities and enhancing secondary circulations through increasing the filament baroclinicity.The further analysis indicates that the filament frontogenesis detected in this study is not only derived from mesoscale straining of the eddy,but also effectively induced by the subsequent submesoscale straining due to ageostrophic convergence.In this context,these submesoscale filaments and associated frontogenetic processes can provide a potential interpretation for the vertical nutrient supply for phytoplankton growth in the high-productive stripes within the mesoscale eddy,as well as enhanced vertical heat transport.