Mass transport of a mesoscale eddy in the South China Sea identified by a simulated passive tracer

To quantitatively investigate the water mass transport of mesoscale eddies, the mass transport induced by a simulated anticyclonic eddy in the South China Sea was evaluated by using the Regional Ocean Modelling System (ROMS) and a built-in passive tracer module. The results indicate that the eddy can trap and transport 51% of the initial water in the eddy core to 689 km from its origin during its lifetime of 100 days, with a stable loss rate of 6‰ per day. During propagation- there is drastic horizontal water exchange between the inside and outside of the eddy. Meanwhile- the vertical mass transport is significant- and 65% of the water initially in the mixed layer of the eddy is eventually detrained into the subsurface. A tracer budget analysis of eddy shows that advection is the dominant dynamic process of transport- while the effect of mixing is weak- and horizontal process plays a controlling role. Horizontal and vertical advection exhibit opposite patterns and strongly offset each other. Particularly- a distinct dipole pattern is found in the local velocity field of the eddy- with significant convergence (downwelling) and divergence (upwelling) zones in the anterior and posterior of the eddy- respectively- which is likely related to the driving mechanism of the westward propagation of the eddy. The dipole further induces a vertical overturning cell- through which the surface water in the anterior of the eddy detrains into the subsurface by downwelling and resurface from the posterior of the eddy by upwelling and gradually spreads out of the eddy. The temporal variability in the tracer budget is significant- in which horizontal advection is dominant. The propagation acceleration and temporal derivative of the deformation rate are highly correlated with tracer transport- suggesting the potential effect of the temporal instability of eddies on the eddy mass transport.