Comparisons between Short- and Long-Lived Cross-Equatorial Flow Events over the Maritime Continent: Perspective from the Intraseasonal Oscillation

Abstract

Abstract The duration of cross-equatorial flow (CEF) events over the Maritime Continent (MC) varies greatly, from 3 days to more than 1 month. In this study, we classify CEF events into short- and long-lived ones using a threshold of 7 days, and conduct separate investigations on their statistical and evolution characteristics, as well as the associated physical processes from the perspective of two dominant modes of the intraseasonal oscillation. Results indicate that short-lived events, characterized by westward-propagation southerly anomalies, are largely dependent on the 10–25-day oscillation, while the contribution of the 30–60-day component is negligible. The associated enhanced convection is primarily confined to the western North Pacific (WNP) and moves northwestward, accompanied by an anomalous cyclone. In contrast, long-lived events show consistent changes among different CEF branches, characterized by southerly anomalies dominating the MC, which benefit from the favorable background of the 30–60-day oscillation. Associated convection anomalies show a dipole pattern, with enhanced convection over WNP and suppressed convection over the Indian Ocean. The enhanced WNP convection gradually migrates northward, inducing CEF anomalies via a continuous anomalous cyclone, while the suppressed convection can be found to slightly expand to the MC. Meanwhile, the 10–25-day oscillation shows similar magnitude and evolution with that in short-lived events, but is no longer crucial to the establishment of long-lived events.