Weakest winter South China Sea western boundary current caused by the 2015–2016 El Niño event

Abstract

AbstractDuring the winter of 2015–2016, the strongest El Niño event of the twenty‐first century occurred. At the same time, volume transport (VT) time series of the South China Sea western boundary current (SCSWBC) exhibited a minimum value of 3.7 Sv (1 Sv = 1 × 106 m3 s−1) toward the southwest, indicating the weakest strength ever recorded in boreal winter (from November to February). The South China Sea (SCS) cyclonic gyre, inferred from the satellite‐derived surface absolute geostrophic current, was significantly reduced. It was considered that the weakened wind stress curl (negative anomaly) over the SCS resulting from an anticyclone over the Philippine Sea played an essential role. The anticyclone arose from a Rossby‐wave response to a negative sea surface temperature anomaly in the northwest Pacific. This idea is further supported by composite analysis, which shows that during El Niño (La Niña) winter, negative (positive) wind stress curl anomalies prevail in the Philippine Sea and the SCS; thus, the wind stress curl over the SCS is reduced (strengthened), leading to a weaker (stronger) SCS cyclonic gyre and SCSWBC. The mean VT of SCSWBC is 4.7 Sv (5.6 Sv), which is smaller (larger) than 5.2 Sv in normal years. This study provides robust observational evidence from long‐term in situ volume transport monitoring that El Niño can have a significant impact on the SCSWBC through an atmosphere‐bridged teleconnection.