Cross-Equatorial Northerly Surges Associated with Extratropical Cold Surges and Tropical Variability over the Maritime Continent

Abstract Cross-equatorial northerly surge (CENS) is characterized by a strengthening of northerly moist monsoon winds over the south of the South China Sea in the western Maritime Continent. The CENS typically lasts a few days in boreal winter and is frequently reported as a crucial synoptic forcing of heavy rainfall and flood events over northern Java Island. The occurrence of CENS has been generally understood as an extension of a northerly cold surge (CS) coming from East Asia. However, out of 117 CENS events identified over the last 42 years, only 59% of the events were induced by cold surges (CENS-CS). We further found that CENS events occurred with no association to cold surges (CENS-noCS) are mostly attributed to several tropical variabilities (Madden–Julian oscillation phases 4–7, mixed Rossby gravity waves, equatorial Rossby waves, and tropical cyclones). These phenomena induce strengthening of northerly winds mainly by generating meridional southward pressure gradient force over the CENS region. In terms of potential impacts, precipitation anomalies over northern Java Island associated with CENS-CS are slightly larger than those of CENS-noCS, but significant flood events had occurred following any CENS. These imply that both types of CENS are important for controlling hydrometeorological events over the region of interest. Significance Statement Day-to-day variation during the rainy season in the south of Indonesia is modulated by incoming northerly moist flow called cross-equatorial northerly surge (CENS) events. These events often cause heavy rain and flooding in the most densely populated island of Java, especially in Jakarta. Understanding CENS dynamics is crucial for reliable weather monitoring and forecasting systems. However, the current knowledge of CENS initiation, which is controlled by extratropical cold surges, only explains 59% of CENS events. To improve the predictability of CENS, this study identifies other potential drivers that manifest as several tropical phenomena, which provide favorable conditions for CENS events. Furthermore, we confirm that floods can occur during any CENS event, suggesting that forecasters should pay attention to all CENS events regardless of their origin.