Abnormal features of oceanographic characteristics in upwelling Vietnam waters under impact of El Niño events

Abstract

The summer upwelling that occurs in coastal waters of South Central Vietnam is one of the major hydrographic features in the East Sea. A weakening of the upwelling after major El Niño events was observed in the literature for previous El Niño events and was verified here from the analysis of new satellite image data sets of sea surface temperature (SST) and surface wind. The analysis of empirical orthogonal function (EOF) from of monthly SST as well as of temporal and spatial variations of SST and wind force allow us to identify abnormal characteristics in ocean surface water that happened after El Niño episode, in agreement with previous studies. Those abnormal characteristics in Vietnam upwelling waters appeared mainly during the summers of 1998, 2003, 2010 and 2016 years for the El Niño decline phase. The upwelling weakening during El Niño decline episodes is associated with the following signals: (1) Wind force and Ekman pump are very weak; (2) the cold and high chlorophyll-a tongue is shifted northward but not extended eastward; (3) for years when El Niño occurs, SST strongly increases and reaches a peak in May or early June of next year, during the declining phase of El Niño episode; (4) upwelling phenomenon typically occurs during August and not July. Using a reanalysis dataset derived from the HYCOM/NCODA system coupled with a local Finite Element Model (FEM) allow us to complete our knowledge about the abnormal oceanographic characteristics of deeper water layers after El Niño episodes. The analysis of spatial variations of oceanography fields derived from HYCOM/NCODA/FEM system along zonal and meridional sections and vertical profiles as well as the results obtained from water mass analysis allow us to identify in details the abnormal oceanic characteristics of deeper water layers during the declining El Niño phase. Those are; (5) Sea water in both surface and deeper water layers were transported dominantly northward but not eastward; (6) The thermo-halocline layer in South Vietnam upwelling center was deeper (about 90 -100m), compared with previous El Niño and normal years (50-60 m and 35-40 m, respectively); (7) Extreme global warming in recent years (2012)(2013)(2014)(2015)(2016) pressed the thermo-halocline layer in upwelling center deeper (90-100 m) during summer. Under the influence of the ocean global warming, this process should progress continuously, the depth of thermo-halocline layer should become therefore deeper and deeper in next years.

The monthly average distributions of sea-surface wind speeds and directions in the East Vietnam Sea

“…In contrast, the longshore current has a South-to-North direction during dominant Southwest monsoons, which is one of the causes of creating an anticyclonic gyre off the coast from Ninh Thuan to Binh Thuan, Vietnam (11 o N-13 o N). This anticyclonic gyre is the leading cause of the upwelling phenomenon in the South-Central Sea of Vietnam [28][29][30][31].…”

Monthly anomalies of sea surface chlorophyll-a concentration in the Khanh Hoa waters of Vietnam related to ENSO phenomenon

“…The studies on the variability of the upwelling system in the South-Central Vietnam waters under ENSO events and its impact on hydrographic conditions of the Ninh Thuan-Binh Thuan waters [18] have shown that the La Niña events can degrade and eliminate the upwelling phenomenon. The study of abnormal features of oceanographic characteristics in upwelling Vietnam waters under the influence of El Niño events [19] has partly clarified the impact of the El-Niño phenomenon on the occurrence time and position of the center region of the Vietnam upwelling waters. Based on 28 years of reanalysis data, the analyzed results showed that in the El Niño years, autumn rainfall in Central Vietnam is reduced by about 10 to 30% [20].…”