Changes in the upper and deep cells of the South China Sea overturning circulation under global warming

Abstract The South China Sea Meridional Overturning Circulation (SCSMOC), a key prominent component of the Pacific meridional overturning circulation, features two profound circulation cells (upper and deep cells) in the vertical direction. Variations in the SCSMOC play a significant role in regulating meridional heat transport within the SCS and influencing the broader Pacific Ocean water cycle. For instance, a weakened deep cell inhibits the upwelling of North Pacific Deep Water and hinders water cycling in the Pacific, whereas a strengthened upper cell intensifies northward oceanic heat transport, potentially affecting the climate of Eastern Asia. Using a high-resolution coupled climate model, we identify a strengthening trend of 0.04 Sv·century⁻¹ in the upper cell, equivalent to 3% of its mean transports during 1900–2100. In contrast, the deep cell weakens at a rate of 0.05 Sv·century⁻¹, representing a 20% reduction over the same period. The increased sea surface height difference between the northern and southern Luzon Strait—linked to weaker subtropical wind in a warming climate—leads to a stronger upper-layer inflow into the SCS, thereby intensifying the upper cell of the SCSMOC. In contrast, the more rapid freshening of the Pacific Ocean than the SCS diminishes the deep-layer meridional pressure difference in the Luzon Strait, resulting in a reduced deep inflow and a weakened deep cell of the SCSMOC. Our findings highlight the importance of long-term monitoring of the SCSMOC and comprehensive understanding of associated impacts.