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.

High-resolution simulation of upper-ocean submesoscale variability in the South China Sea:Spatial and seasonal dynamical regimes

Submesoscale processes in marginal seas usually have complex generating mechanisms,highly dependent on the local background flow and forcing.This numerical study investigates the spatial and seasonal differences of submesoscale activities in the upper ocean of the South China Sea(SCS)and the different dynamical regimes for sub-regions.The spatial and seasonal variations of vertical vorticity,horizontal convergence,lateral buoyancy gradient,and strain rate are analyzed to compare the submesoscale phenomenon within four sub-regions,the northern region near the Luzon Strait(R1),the middle ocean basin(R2),the western SCS(R3),and the southern SCS(R4).The results suggest that the SCS submesoscale processes are highly heterogeneous in space,with different seasonalities in each sub-region.The submesoscale activities in the northern sub-regions(R1,R2)are active in winter but weak in summer,while there appears an almost seasonal anti-phase in the western region(R3)compared to R1 and R2.Interestingly,no clear seasonality of submesoscale features is shown in the southern region(R4).Further analysis of Ertel potential vorticity reveals different generating mechanisms of submesoscale processes in different sub-regions.Correlation analyses also show the vertical extent of vertical velocity and the role of monsoon in generating submesoscale activities in the upper ocean of sub-regions.All these results suggest that the sub-regions have different regimes for submesoscale processes,e.g.,Kuroshio intrusion(R1),monsoon modulation(R2),frontal effects(R3),topography wakes(R4).

Submesoscale motions and their seasonality in the northern Bay of Bengal

The unbalanced submesoscale motions and their seasonality in the northern Bay of Bengal(BoB)are investigated using outputs of the high resolution regional oceanic modeling system.Submesoscale motions in the forms of filaments and eddies are present in the upper mixed layer during the whole annual cycle.Submesoscale motions show an obvious seasonality,in which they are active during the winter and spring but weak during the summer and fall.Their seasonality is associated with the mixed layer instability that depends on the mixed layer depth(MLD).During the winter,the MLD provides a much greater reservoir of the available potential energy,which promotes mixed layer instability to develop active submesoscale motions.The variations of MLD are likely modulated by the larger scale motions and the influxes of freshwater.Further investigations imply that the MLD and the stratified barrier layer are combined to determine the vertical structure of the submesoscale motions.The shallow MLD and strong stratification below during the summer and fall seem to prevent the downward extension of submesoscale motions.But in spring when the weak stratification exists,the penetration depth exceeds the base of the barrier layer.