Abstract
Upwelling off Vietnam in the South China Sea (SCS) and the Sumatra–Java upwelling in the Indian Ocean significantly modulate regional variation in climate. Although located in different hemispheres, these upwellings nearly concur during the boreal summer; both are the result of wind-induced Ekman divergence. Beyond seasonal time scales, the two upwellings were not synchronous in 1998. In the summer of 1998, upwelling off Vietnam was almost absent, generating the warmest summer on record in the SCS. We demonstrated that the El Niño–Southern Oscillation, which was highly correlated with the upwelling in previous studies, was not solely responsible for this variability. Wind trajectory analyses revealed that cross-equatorial winds, which had passed over the Sumatra–Java upwelling site about 2 weeks earlier, were a rapid force acting on SCS summer upwelling. In the summer of 1998, SCS winds were greatly perturbed due to an anomalous wind path that blew toward the SCS through the Sulu Sea. Our findings suggest that not only the resulting weakening but also the perturbation of the SCS winds prevented the formation of summer upwelling off central Vietnam in that year.
Highlights
Observational SST data used in this study are monthly 1° × 1° Hadley Centre Sea Ice and Sea
The Dipole Mode Index (DMI) is defined as the SST anomaly difference between the western equatorial Indian Ocean (50°E–70°E, 10°S–10°N) and the southeastern equatorial Indian Ocean (90°E–110°E, 10°S–Equator)[17]
Summary
As a result of the close relationship between the two upwelling sites due to their connection via cross-equatorial winds, upwelling variation at the IOU site led SCSU fluctuation by about 2 weeks during boreal summer. The correlations indicate that, during boreal summer, variability in the IOU index is a proxy for the DMI in the Indian Ocean, and can be served as a precursor to the SCSU index in the SCS. In early boreal summer (July/August), cold SST anomalies first occurred around the Lombok Strait (~9°S, 116°E), accompanied by southeasterly winds in the southeastern tropical Indian Ocean[17]. Cold SST anomalies appeared in the Lombok Strait, and the accompanying southeasterly wind anomalies increased the easterly anomalies and slightly weakened the local thermocline This cooling enhanced and further strengthened the easterly anomalies, triggering westward-propagating downwelling Rossby waves[18].
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