The Influence of Remote Wind Forcing and Kelvin Waves on the Java Upwelling in Positive IOD Years

Abstract:

The seasonal upwelling region just south of Java, Indonesia plays a critical role in the development and evolution of Indian Ocean Dipole (IOD) events. In this study the effects of local and remote surface wind anomalies on the interannual variability of SST south of Java are investigated using remote sensing datasets. Though upwelling-favorable winds along the Java coast are responsible for the seasonal SST cooling (June-September) in most years, the anomalously enhanced upwelling that precedes positive IOD (IOD+) events is almost entirely associated with remote wind forcing. Surface wind-SST anomaly correlations for May-July and composites of the early phases of IOD+ events both suggest that the anomalous Java upwelling is closely linked to strengthened southeasterly winds in the eastern equatorial Ocean and along the coast of Sumatra. In contrast, only minor wind anomalies are observed along coastal Java during IOD+ years.

To explain the influence of remote winds on the interannual variability of Java upwelling, we also quantify oceanic Kelvin wave activity along the equator-Sumatra-Java waveguide. A sea level projection method is applied to altimetric observations, removing sea level anomaly (SLA) variations due to steric height changes and Rossby waves. The method produces a Kelvin wave amplitude that is more robustly correlated with SST cooling south of Java than gridded SLA. Extreme negative values of the Kelvin wave amplitude in May-July, corresponding to high-amplitude upwelling waves, precede all of the strong IOD+ events in the period of record. These results suggest an outsize impact from anomalous winds in the equatorial-coastal waveguide during austral late fall/early winter, and emphasize the important role of oceanic Kelvin waves in conveying remote wind forcing to the upwelling region.