Contributions of Wind, Ocean Intrinsic Variability, and ENSO to the Interannual Variability of the South Vietnam Upwelling: A Modeling Study

The summer South Vietnam Upwelling (SVU) is a major component of the South China Sea circulation where it also influences ecosystems. Here we revisit the existing knowledge of the SVU interannual variability. Based on a set of 15-year eddy-resolving sensitivity simulations, we quantify the respective contributions from different factors (atmospheric, river, and oceanic forcings; ocean intrinsic variability; and El Nino Southern Oscillation) to this interannual variability and explore the underlying mechanisms. Our sea surface temperature upwelling indices allow us to quantify the strong SVU interannual variability in terms of strength and spatial distribution. Strong SVU years are offshore dominant with SVU centers located within 11-12 degrees N and 110-112 degrees E, whereas weak SVU years are coastal dominant with SVU centers located near the coast and spanning a larger 10-14 degrees N latitude range. Our study confirms the leading influence of the summer wind and further reveals that coastal SVU variability is related to the variability of the eastward jet that develops from the coast, whereas offshore SVU variability is strongly driven by the spatiotemporal collocation of wind stress curl with cyclonic eddies. Ocean intrinsic variability, and to a lesser extent perturbations induced by river discharge and lateral oceanic conditions, strongly triggers interannual variability of background eddy circulation, thus of the SVU. El Nino Southern Oscillation influences the SVU mainly through its direct influence on summer winds.