Intraseasonal variability in sea surface height over the South China Sea

Intraseasonal sea surface height (SSH) variability and associated eddy energy in the South China Sea is studied using satellite observations and an eddy-resolving, global ocean general circulation model. In both the model hindcast and satellite observations, a conspicuous minimum of intraseasonal SSH variance is found along the continental break between the shallow shelf and deep basin. Specifically strong intraseasonal variability (ISV) exists in the following regions: on the northern continental shelf, in the Gulf of Thailand, and along two bands in the deep basin with the northern band located west of Luzon Strait and the southern one southeast of Vietnam. SSH ISV exhibits clear seasonality. During active seasons, ISV in the deep water, high-variance bands displays robust propagations in the direction of mean flow. Low correlation between observations and model hindcast suggests the importance dynamical instabilities for ISV in the deep basin, in agreement with an energetics analysis. An exception is along the Vietnam offshore jet during summer, where ISV is forced by wind curls created by Annam Cordillera. In shallow waters, especially in the Gulf of Thailand, SSH ISV is dominated by barotropic response to intraseasonal wind stress forcing. The agreement between altimetry and the model simulation in the Gulf of Thailand demonstrates the ability of satellite altimeters to observe SSH variability in shallow shelves of weak tides.