Statistical Characteristics Of Mesoscale Eddies On The Continental Slope In The Northern South China Sea:Spatiotemporal Distribution,Propagation And Influence On Cross-Shelf Movement

Mesoscale eddies are very active on the continental slope in the northern South China Sea(SCS),but their statistical features remain unclear.In this study,we investigate the properties of eddy propagation over 24 years(1993-2016)by using the altimeter data and Eulerian eddy detection method.147 eddies are found in the continental slope region(CSR),including 70 cyclonic eddies(CEs)and 77 anticyclonic eddies(ACEs).For the eddies that appear in the CSR,the surrounding areas of Dongsha Islands(DS)and southwest of Taiwan(SWT)are considered as the main sources,which contribute more than 60% of the total.According to the spatial distribution of eddy relative vorticity,the eddies tend to weaken when propagating westward,and ACEs are relatively stronger than CEs.The vorticity of vortexes of different polarities response differently to the terrain,which might be explained by the potential vorticity conservation or eddy-terrain interaction.Furthermore,although both types of eddies roughly propagate along the slope isobaths,there are discrepancies between the CEs and the ACEs.The ACEs move slightly faster in the zonal direction,but the CEs tend to cross the isobaths than ACEs.However,after leaving the CSR,the ACEs generally move further to the basin areas.The eddy propagation is likely to be impacted by mean flow to a certain degree because the eddy trajectories have notable seasonal signals due to the seasonal cycle of geostrophic current on the continental slope.A further investigation indicates that the eddy propagation velocity is consistent with geostrophic velocity in both magnitude and direction.The majority of mesoscale eddies in the CSR move westward as nonlinear eddies rather than linear Rossby waves which was commonly accepted before.More ACEs are nonlinear compared with the CEs,and nonlinearity dissipates when eddies propagate westward.Besides,eddies could also contribute to the cross-shelf movement in the surface layer.We find that the eddy-induced transport has various spatiotemporal variabilities,which peaks at about 115°E in space and reaches its maximum in June.