Tropical Anomalies Associated with the Interannual Variability of the Cross-Equatorial Flows over the Maritime Continent in Boreal Summer

In this study, we investigate circulation, convection, and sea surface temperature (SST) anomalies associated with interannual variability of the cross-equatorial flow (CEF) intensity over the Maritime Continent (MC) in boreal summer. Observational diagnostics show that strengthened CEF is associated with large-scale circulation anomalies featuring weakened Walker circulation, upper-level northeasterly anomalies across the MC, and lower-level cyclonic anomalies over the tropical western North Pacific (WNP). Further analyses indicate that strengthened CEF is associated with both La Nina-like SST anomalies in the preceding winter and El Nino-like SST anomalies in the simultaneous summer. These relationships between CEF and ENSO are established by two key convection regions: enhanced convection over WNP and depressed convection over the MC. A linear baroclinic model is applied here to further discuss the causality between circulation and convection. Results suggest that both the WNP heating and MC cooling can induce the strengthened CEF. Moreover, the stability of the relationship between CEF and El Nino-Southern Oscillation (ENSO) is also discussed. Results show that the relationship between CEF and SST anomalies in the simultaneous summer is stable and remains significant, whereas that between CEF and SST anomalies in the preceding winter experienced a decadal strengthening around 1997/98 from insignificant to significant. After 1998, the preceding winter ENSO is followed by strong summer SST anomalies in the MC that significantly affect CEF via modulating local convection. However, this ENSO-summer MC SST relationship is weak before 1997, failing to establish the relationship between the preceding ENSO and CEF.