Understanding the Cross-Equatorial Pathway: How Tropical Variability Modulates East Asian Cold Surges Propagation into the Southern Hemisphere

The transition of an East Asian cold surge (CS) into a cross-equatorial northerly surge (CENS) is a significant meteorological process affecting heavy rainfall across the Maritime Continent during boreal winter. However, a strong CS does not guarantee an occurrence of CENS and pre-existing tropical conditions are suspected to modulate their connection, indicating a nonlinear and complex relationship between them. Here, we analyzed 40 winters of reanalysis data to identify the differences between CS that crossed the equator and developed into CENS (CS-CENS), and those that did not (CS-noCENS). We found that, out of 201 cold surges, only 36.8% evolved into CS-CENS. This CS-CENS exhibits a low-pressure anomaly center south of the Maritime Continent, facilitating a southward pressure gradient toward the Southern Hemisphere. In contrast, CS-noCENS events are associated with high-pressure anomalies that inhibit the southward flow. This dynamic is potentially supported by preceding sea surface temperatures over the South China Sea and vicinity, where cold anomalies appear to favor CS-CENS and warm anomalies favor CS-noCENS. In addition to the effect of seasonality, the probability of a CS driving a CENS is also strongly modulated by the Madden-Julian oscillation and equatorial Rossby waves. The location of the Borneo vortex (BV) also affects the CS-to-CENS transition: a BV over Borneo Island promotes it, whereas one over the Karimata Strait inhibits it. These findings demonstrate that the CS-to-CENS transition is not determined solely by the surge’s intrinsic strength but is critically controlled by the large-scale tropical environment.