Post-El Nino Weakening of Coastal Upweling and Its Ecoliogical Impacts in the South-Central Viet Nam Region

Abstract

Upwelling that occurs in the summer offshore of South-Central Viet Nam in the South China Sea (SCS) also known as the East Viet Nam Sea (EVS), is one of the region’s key dynamic processes. The weakening of upwelling phenomena during post-El-Niño events and their ecological responses were studied based on a reanalysis dataset derived from the HYCOM/NCODA system, coupled with a local Finite Element Model (FEM) and observed data. Long-term warming, along with orographic and wind factors, play significant roles in the formation and weakening of upwelling phenomena during normal and post-El-Niño episodes, respectively. The weakening of upwelling during post-El-Niño periods is reflected by: Extreme weakening of wind forcing and Ekman pumping; a northward shift of the cold-saline tongue and current dipole, with a limited eastward extent; dominance of northward circulation in the surface layer and westward circulation in deeper layers; and the formation of a homogeneous surface thermohaline layer of about 50 meters thick, with a thermo-halocline layer at 50-60 meters depth. These abrupt changes strongly influenced the ecological characteristics of the upwelling area. Coral bleaching during the summers of 2010 and 2016, as well as anomalous distributions of chlorophyll-a (Chl-a) in the surface layer in 1998, 2003, 2010, and 2016, were concrete indicators of the ecological responses of Viet Nam's coastal upwelling waters in post-ElNiño years. Detailed intra-seasonal variations of temperature during the summer showed a warming of the waters, leading to coral bleaching and abnormal Chl-a levels during post-El-Niño years. Organisms in this region struggled to adapt to these rapid environmental changes, leading to a potential reduction in living resources.