Pre-Storm Upper-Ocean Warm-Layer Conditions and Vertical Wind Shear Associated with Rapid Intensification in the Western North Pacific

Rapid intensification (RI) in the western North Pacific (WNP) occurs in broadly warm sea surface temperature (SST) conditions, making pre-storm ocean structure potentially informative. Using 1993–2024 best tracks, SST, the ERA5 atmospheric reanalysis, and the GLORYS12V1 global ocean reanalysis, we evaluate upper-ocean heat content above 26 °C (OHC26), 26 °C isotherm depth (D26), and atmospheric environmental conditions in relation to 24 h intensity increases of 15 and 20 m s−1 (RI15 and RI20). At fixed locations 24 h before arrival, RI15 environments had 33.91 kJ cm−2 higher OHC26 and 26.57 m deeper D26 than all non-RI records. Restricting controls to other intensifying records reduced, but did not remove, these contrasts: +18.28 kJ cm−2 and +15.37 m for RI15. Storm-grouped validation showed persistent gains from pre-storm OHC26 across pre- and post-2005 periods. RI frequencies were highest where weak vertical wind shear coincided with substantial warm-layer support; however, formal cluster-robust ocean–shear interaction terms were not statistically significant. The findings identify robust pre-existing warm-layer associations with RI while limiting inference about nonlinear environmental gating, ocean-response causality, or operational forecast skill.