The shallow meridional overturning circulation in the South China Sea and the related internal water movement

The structure of the annual-mean shallow meridional overturning circulation (SMOC) in the South China Sea (SCS) and the related water movement are investigated, using simple ocean data assimilation (SODA) outputs. The distinct clockwise SMOC is present above 400 m in the SCS on the climatologically annual-mean scale, which consists of downwelling in the northern SCS, a southward subsurface branch supplying upwelling at around 10A degrees N and a northward surface flow, with a strength of about 1x10(6) m(3)/s. The formation mechanisms of its branches are studied separately. The zonal component of the annual-mean wind stress is predominantly westward and causes northward Ekman transport above 50 m. The annual-mean Ekman transport across 18A degrees N is about 1.2x10(6) m(3)/s. An annual-mean subduction rate is calculated by estimating the net volume flux entering the thermocline from the mixed layer in a Lagrangian framework. An annual subduction rate of about 0.66x10(6) m(3)/s is obtained between 17A degrees and 20A degrees N, of which 87% is due to vertical pumping and 13% is due to lateral induction. The subduction rate implies that the subdution contributes significantly to the downwelling branch. The pathways of traced parcels released at the base of the February mixed layer show that after subduction water moves southward to as far as 11A degrees N within the western boundary current before returning northward. The velocity field at the base of mixed layer and a meridional velocity section in winter also confirm that the southward flow in the subsurface layer is mainly by strong western boundary currents. Significant upwelling mainly occurs off the Vietnam coast in the southern SCS. An upper bound for the annual-mean net upwelling rate between 10A degrees and 15A degrees N is 0.7x10(6) m(3)/s, of which a large portion is contributed by summer upwelling, with both the alongshore component of the southwest wind and its offshore increase causing great upwelling.