Phase-dependent modulation of the MJO during cross-equatorial northerly surges (CENS)

This study investigates phase-dependent associations between Cross-Equatorial Northerly Surges (CENS) and Madden-Julian Oscillation (MJO) precipitation and circulation patterns using 84 years (1940-2023). Approximately 79% of CENS days (76% of events) occurred during phases 4-7 of the active MJO. Composite analysis reveals distinct phase-dependent co-variations between CENS occurrence and MJO structure: during phases 4-5 (MJO over the Maritime Continent), CENS occurrence is associated with enhanced precipitation around Java and along the northern coast of Australia through enhanced meridional convergence. In contrast, during phases 6-7 (MJO over the Western Pacific), CENS-associated patterns are meridionally broader and vertically deeper, with enhanced precipitation over the western flank (rear) of the MJO. Notably, phases 6-7 exhibit significant positive sea surface temperature anomalies in the Western Pacific, a pattern that persists even when controlling for ENSO phase and MJO amplitude variations, consistent with a southward shift of the MJO propagation path. These results indicate that CENS are not merely passively affected by the MJO but are systematically associated with distinct phase-dependent differences in MJO precipitation patterns, vertical structure, and propagation characteristics.