Abstract
The traditional view holds that a weakened upwelling has often been observed off the coast of southeast Vietnam during the post-El Niño summer. This study investigated a strong upwelling and concurrent phytoplankton bloom off the coast of southeast Vietnam in August 2016 by comparing it with another case in 1998. Analyses of the upwelling structure and formation mechanisms indicated that the abnormal strong upwelling in August 2016 was attributable to strong wind-driven offshore Ekman transport and Ekman pumping, which were caused by the accompanying southwesterly anomalies south of the anomalous cyclone (AC) over the western North Pacific (WNP), and vice versa in August 1998. This anomalous southwesterly wind associated with the AC over the WNP could not be explained by La Niña, the negative Indian ocean dipole, or the positive Pacific meridional mode events. Further analyses showed that the Madden–Julian oscillation (MJO)-induced westerly winds could have contributed more than 75% of the original zonal winds. Nine tropical cyclones generated over the WNP were favorable for excessive precipitation. The opposite configurations of precipitation patterns over the WNP and the Maritime Continent could have further strengthened the AC via a Gill response.
Highlights
Upwelling, one of the most important ocean circulation patterns [1], is characterized by a relatively low sea surface temperature (SST) and relatively high chlorophyll-a (Chl-a) concentration [2].The South China Sea (SCS), the largest semi-enclosed marginal sea in the western North Pacific (WNP), is dominated by the East Asian monsoon, with southwesterly and northeasterly monsoonal winds in summer and winter, respectively [3,4,5]
From the SSTa and the horizontal wind anomalies fields, we clearly found that the La Niña-like SST pattern and the associated anomalous easterly winds occurred in the central-western tropical Pacific during these two years
The off vertical thermohaline structures along the transect of the upwelling with system strong upwelling the coast of southeast
Summary
One of the most important ocean circulation patterns [1], is characterized by a relatively low sea surface temperature (SST) and relatively high chlorophyll-a (Chl-a) concentration [2].The South China Sea (SCS), the largest semi-enclosed marginal sea in the western North Pacific (WNP), is dominated by the East Asian monsoon, with southwesterly and northeasterly monsoonal winds in summer and winter, respectively [3,4,5]. All of the upwellings in the SCS and their formation mechanisms are primarily caused by monsoonal wind forcing, but can be greatly affected by topography, stratification, tide, and tropical cyclone (TC) [9,10,11,12]. The summer monsoonal wind forcing is the strongest in the western SCS due to the orographic induced southwesterly wind jet, which could lead to a strong coastal and open ocean upwelling off the coast of southeast Vietnam [13,14,15]. The Vietnam upwelling is one of the most important features of the ocean circulation pattern of the SCS in summer, and has been widely studied with respect to its structure, formation mechanism, and variabilities on intra-seasonal to decadal timescales.
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