Events of the Cross-Equatorial Flows over the Maritime Continent during 1979–2020

Abstract

Abstract This study proposed a definition of the cross-equatorial flow (CEF) events over the Maritime Continent (MC) based on the daily mean data derived from ERA5 from 1979 to 2020. Following this definition, a total of 138 CEF events were identified. The statistical analysis suggests that CEF events are concentrated from May to October, with the peak frequency appearing in July. The occurrence frequency generally decreases along with the increase in duration, with a total of 66% of events persisting no longer than the average (7.1 days), while approximately 16% persist exceeding 10 days. The intensity of events is generally the strongest in August, consistent with the climatological annual cycle of CEF. The CEF events are characterized by a seesaw pattern of equatorial meridional wind anomalies between the upper and lower troposphere, with the strongest southerly (northerly) anomalies located at 925 (150) hPa. Both the upper- and lower-level wind anomalies propagate westward along with the evolution of events. The CEF events are highly connected with the tropical dipole convection, featured by the enhanced convection over the western North Pacific (WNP) and suppressed convection over MC. The convection enhancement over the WNP can be traced back to the equatorial central Pacific, which explains the westward movement of the events-related equatorial meridional wind anomalies. By contrast, the suppressed convection over the Indian Ocean and MC keeps stagnant. The events-related convection anomalies act as a branch of boreal summer intraseasonal oscillations (BSISOs), but CEF events are not fully dependent on BSISOs.