Meridional overturning circulation in the South China Sea envisioned from the high-resolution global reanalysis data GLBa0.08

Abstract

The pattern of meridional overturning circulation (MOC) in the South China Sea (SCS) is studied using a numerical Lagrangian tracing method with the HYCOM+NCODA Global 1/12 degrees Analysis (GLBa0.08) data. The SCS MOC has a sandwich structure, which consists of a layer of stronger clockwise circulation above 500 m depth, a counterclockwise layer in the mid layer between 500 and 1000 m depth, and a weaker clockwise layer below 1000 m. The deep (below 1000 m depth) clockwise layer is divided into three cells, namely, the deep southern MOC cell, DSMOC; the deep middle MOC cell, DMMOC; and the unclosed deep northern MOC cell, DNMOC. The inflow through the Luzon Strait is the main source for the SCS MOCs. The upper layer Luzon Strait inflow dominates the upper SCS MOC structure but has relatively less contribution to the DNMOC, whereas the deep layer Luzon Strait inflow mainly influences the DNMOC and it mostly rises near 18 degrees N. The inflow through the Taiwan Strait mainly contributes to the upper layer MOC. Moreover, inflows from the Mindoro and Karimata straits contribute negatively to the upper MOC but play a significant role on the DSMOC. The backward integration of Lagrangian trajectories further validates that the SCS deep water comes not only from the deep inflow but also from the entrainment of the middle and upper layer inflow through the Luzon Strait. In the SCS basin, there are three northwest-southeast tilted zones where tracers upwell, which correspond to the three deep MOC cells. One possible mechanism for these upwelling zones is the interaction between the continental slope-trapped waves and the westward planetary Rossby waves.