Vertical structure and occurrence patterns of the cross-equatorial northerly surge under different ENSO and MJO phases

This study investigated the vertical structure of 6 cross-equatorial northerly surge (CENS) events during the Year of Maritime Continent-Cold Surge Observation in 2021 (YMC-CSO2021) campaign. These events, named CENS1 (Jan. 18-20), CENS2 (Jan. 29-30), CENS3 (Feb. 2-5), CENS4 (Feb. 5-9), CENS5 (Feb. 18-20), and CENS6 (Feb. 25-26), occurred under different environmental conditions associated with cold surges (CSs) and the Madden-Julian Oscillation (MJO). Using radiosonde observations, we identified distinct characteristics in the northerly wind layer thickness, westerly wind bursts, and potential temperature anomalies among the events. Notably, CENS6 featured a deep northerly wind layer reaching 400 hPa, influenced by a southward pressure gradient linked to a cyclone in the Southern Hemisphere. Statistical analysis of past CENS events using reanalysis dataset showed that 80% occurred in January and February, with higher frequencies during La Niña years and active MJO phases over the western Pacific, aligning with YMC-CSO2021 observations. Composite analysis showed that CENS events induced significant ascending motion and localized potential temperature gradients, leading to positive precipitation anomalies around the Maritime Continent. These findings enhance our understanding of CENS dynamics and their impact on regional climate variability.