| The northwestern continental shelf of the South China Sea?SCS?is an important ocean area for fishery,port,shipping,and oil and gas resources exploitation.It is one of the main seasonal upwelling areas,where tropical cyclones and mesoscale eddies with strong vertical motion frequently occur.As a key factor in ocean dynamics and thermodynamics,the vertical circulation is also crucial for material transport and sedimentation.Therefore,determining the vertical circulation and its dominant mechanism is of great importance for bio-geo-physical processes,which help maintaining the high primary productivity and exploiting the marine resources in the northwestern SCS.Using mooring observation of temperature,salinity and horizontal velocities in July-August 2005 and April 2018,and satellite observations of surface wind and sea surface height,this study diagnoses the vertical velocities induced by the passage of tropical storm Washi?2005?and an anticyclonic warm eddy?AE?in the northwestern SCS.The temporal-vertical variation of the vertical velocities in the periods of pre-storm,during-storm and post-storm,and from the edge to the center of the AE are obtained.The dominant dynamic terms of the vertical circulation are analyzed.The vertical mass transports by vertical velocity and mixing are further compared.The results show that the total vertical velocity is of the order of O(1×10-4)m/s in the mixed layer above 25 m,and of O(1×10-5)m/s in the lower layer during the passage of tropical storm Washi?2005?.The diagnosed vertical velocity in the upper layer is one order greater than the averaged Ekman pumping velocity.It has oscillation as negative and positive velocities alternatively appeared with a near-inertial period.As tropical storm Washi?2005?passed by from July 29 to 31,2005,the vertical velocity was dominantly upward and significantly enhanced to the order of O(1×10-3)m/s.Dynamically,the geostrophic advection and unsteady behavior of density induced by near-inertial oscillation are dominant factors in the upper and lower layers,respectively.The vertical advection transport calculated from the diagnosed vertical velocity reaches O(1×10-5)kg/?s·m3?,one order greater than that induced by turbulent mixing.Time-averaged transports by vertical advection and mixing are both upward in the layer above the thermocline during the storm passage.For the vertical circulation during the AE passage in April 2018,the diagnosed results show that the upper layer above 100 m has larger vertical velocity with most values over 5×10-4m/s in the whole observation period.The AE moved westward and passed the mooring station on April 23,2018.The vertical velocity at the station changed from negative before April 23 to positive after April 23,implying that there is downwelling?upwelling?on the left?right?side of the eddy center.In the thermocline layer of 100-250 m,the maximum upwelling reached 6 10-4m/s at the left edge of the eddy.It significantly affected the velocity distribution during the whole observation period.The vertical velocity got weak near the eddy center and was dominated by negative downwelling,of which the magnitude was less than2×10-4m/s,only 1/3 of the vertical velocity at the edge of the eddy.The dominant mechanisms are different in different layers.In the mixed layer and the thermocline,the geostrophic component is significant.In the deep layer,the vertical velocity gets one order of magnitude smaller and is dominant by the unsteady behavior of the density.In this case study,the vertical circulation during passage of tropical storm and mesoscale eddy are diagnosed on the northwestern continental shelf of the SCS.Enhanced upward vertical velocities are found during such vortex processes,which are of great significance to understand the vertical transport and biochemical processes in the shelf seas. |
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