Weakening Coherence of Tropical Cyclogenesis Between the Northwestern and Southeastern Western North Pacific After the Early‐2000s

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ABSTRACT The western North Pacific (WNP) stands out as the world's foremost region for tropical cyclone (TC) genesis. El Niño typically leads to increased TC genesis frequency in the southeastern WNP (SEW) and decreased TC genesis in the northwestern WNP (NWW), forming a robust out‐of‐phase pattern in the TC genesis variation. The present study investigates the stability of this inverse relationship during active TC seasons from 1979 to 2024 and finds a pronounced interdecadal weakening of the inverse relationship after the early‐2000s. This weakened relationship is primarily attributed to changes in the large‐scale environmental fields over the WNP. In the early period, El Niño induces a distinct dipole pattern in mid‐level vertical motion and specific humidity over the WNP, favouring and suppressing TC genesis in SEW and NWW, respectively. In the recent period, however, these opposing anomalies become indistinct, particularly over the NWW. Further analysis indicates that this change is linked to the interdecadal enhancement of the coupling between the central‐eastern Pacific and the northeastern Atlantic Ocean (EAO) sea surface temperature (SST) variation after the early‐2000s. The EAO warming during recent El Niño events counteracts the El Niño‐induced dipole pattern, thereby weakening the inverse relationship. Our findings reveal that the non‐stationary behaviour of El Niño‐Southern Oscillation's (ENSO) impact on WNP TC genesis distribution is primarily driven by the evolving inter‐basin SST cooperation, especially the intensified ENSO–North Atlantic connection, which should be considered in seasonal predictions of subregional TC activity.

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