| There are thousands of mesoscale eddies in the 360 million square kilometers of ocean.These strong nonlinear mesoscale eddies rotating clockwise or anticlockwise will transport a large amount of water in the process of eddies movement,which plays an important role in the oceanic transports of heat,salt,dissolved oxygen,chlorophyll,and other tracers.Therefore,the study of mass transport in mesoscale eddies is crucial for us to deeply understand global climate change.At present,there are still many problems in the eddy-induced mass transport.For example,(1)There is no clear quantitative research on how strong the ability of eddyinduced mass transport is;(2)What are the contributions of advection and mixing to the mesoscale eddy transport?(3)How does the vertical structure of mesoscale eddies change and how does the vertical dynamic process affect the transport?(4)The spatial structure of eddies is highly time-varying,and which structural variables have significant effects on the mass transport? In order to explore the above problems,the Regional Ocean Simulation System(ROMS)and the built-in passive tracer tracking module are used to quantitatively evaluate the mass transport caused by an anticyclone mesoscale eddy(warm eddy)in the South China Sea by combining Lagrangian coordinate transformation and tracer budget analysis.The tracer tracing results show that during the 85 days movement of the eddy,the initial trapping water diffuses to the outside at an average rate of 6 ‰ per day,and finally 51% of the initial trapping water is transported to a distance of 689 km away from its source.It can be seen that the mesoscale eddy has a strong ability of longdistance mass transport,but the transport process is not conservative,but there is obvious loss phenomenon.The results remind us that it is necessary to consider the loss of water mass and make quantitative evaluation when analyzing the contribution of eddy-induced mass transport to large-scale circulation.On the other hand,the vertical mass transport of eddy is also very significant.The statistical results show that during the whole movement,the water masses in the eddy mixing layer continuously subduct into the mixing layer.Up to the last time of its extinction,about 65% of the water mass in the mixing layer at the initial time descend below the mixing layer.At the same time,in the process of eddy motion,there is intense water exchange between the eddy inside and outside in the horizontal and vertical directions,resulting in the subsurface water mass in the eddy becoming warmer,fresher and lighter.The dynamic mechanism of the mass transport process of the eddy is further analyzed through the calculation of the eddy tracer budget analysis.The results show that the advection process is the most important dynamic process(120%),while the mixing effect is relatively weak(-20%)and has a positive contribution to the conservation of the water mass in the vortex;The effect of horizontal process is about twice that of vertical process,which indicates that horizontal process plays a leading role.Especially in the process of budget analysis,we found and explained an important feature of the warm eddy: there is an obvious "dipole" structure in the vertical velocity field along the forward direction of the mesoscale eddy.That is,there is a significant convergence and downwelling region in the front,and a significant divergence and upwelling region in the rear of the eddy.This phenomenon is different from the classical theory that the warm eddy is a unified convergence sink.The results show that in the eddy moving coordinate system,the variation trend of the tracer concentration in the eddy is nearly symmetric with the eddy center,but in the earth coordinate system,the local variation trend presents an obvious "dipole" structure.Further analysis shows that the structure exactly corresponds to the vertical velocity and divergence field: there are obvious convergence(downwelling)and divergence(upwelling)regions in the front and rear of the eddy,respectively.The time variation of the total amount of water wrapped by the eddy and its correlation with the characteristic variables of the vortex show that there is no direct correlation between the time variation trend of the eddy mass transport and the characteristics such as radius,intensity,moving speed and deformation,but it is significantly related to the acceleration of the eddy motion and the time variation rate of deformation.The results show that when the eddy accelerates,the tracer flow loss rate increases,that is,more water flow out of the eddy;On the contrary,when the eddy decelerates,the tracer flow loss rate decreases,that is,the water in the eddy will be more conservative.At the same time,when the eddy shape changes from circular to irregular,the water in the eddy is more likely to be lost from the eddy core;On the contrary,if the shape of the eddy develops from irregular to circular,the water be better bound by the eddy core.The results show that the time variation of eddy has an important influence on the eddy mass transport.In this study,the mass transport in a warm eddy over the South China Sea was quantitatively studied by using ROMS model and passive tracer tracking method.It is helpful for us to understand quantitatively the mechanism of mass transport and the dynamic process of mass transport in mesoscale eddies. |
