Modulation of the Vietnam coastal upwelling in the 2016 Summer by an abnormal anticyclonic eddy

The summer upwelling along the southeast coast of Vietnam (SUEV) is a prominent seasonal feature of the South China Sea (SCS), characterized by cold sea surface temperatures (SSTs). The SUEV is induced by the southwest monsoon during boreal summer (June–August, JJA) and is probably modulated by the El Niño–Southern Oscillation (ENSO). Generally, when an El Niño event occurs in the previous winter followed by a La Niña event in the following winter, easterly anomalies in the summer weaken the southwest monsoon, leading to a significant reduction in the SUEV. This type of summer has occurred in the past 30 years in 1998, 2010, and 2016, and is hereafter referred to as the Niño-Niña SUEV event. However, the Niño-Niña SUEV in August 2016 exhibited an anomalous intensification caused by positive westerly anomalies induced by the Madden–Julian Oscillation (MJO). This raises a question of how oceanic processes contributed to this unexpected intensification. To address this issue, this study uses reanalysis and satellite datasets, including ERA5 winds and SSTs, Copernicus Marine Environment Monitoring Service (CMEMS) “GLOBCURRENT” surface current fields (Ekman+geostrophic), and altimeter data (sea level anomaly). The results show that the late-summer intensification of the 2016 Niño-Niña SUEV was strongly modulated by a mesoscale anticyclonic eddy propagating from northeast to southwest along the eastern flank of the SUEV. The persistent northeastward flow on the western flank of this eddy enhanced the 2016 Niño-Niña SUEV by +19.8 cm/s. In contrast, the Niño-Niña SUEV weakened by −43.4 cm/s and −48.4 cm/s in 1998 and 2010, respectively. These findings highlight the role of oceanic internal variability in modulating the 2016 SUEV intensity. This study provides new insights into ocean climate variability, with implications for the United Nations 2030 Sustainable Development Goal 13 (Climate Action).