Outer-core size asymmetry and intensification of North Atlantic tropical cyclones

Abstract

Challenges persist in accurately predicting tropical cyclone (TC) intensification and intensity change, owing to restricted understanding of the mechanisms involved. Here the relationship between TC asymmetry (TCA) of outer-core wind field and intensification over the North Atlantic is investigated using best-track and satellite-based wind analysis data from 2000 to 2021. The results suggest a negative correlation between TCA and the upper limit of intensification rate (IR), except in nearly symmetric TCs. TC records are categorized into groups of low, medium, and high asymmetry based on the distribution of TCA. The 24-h evolution of TCA and its variation preceding intensification onset reveal significant differences between TCs with low-to-medium and high asymmetry. TCs with low-to-medium asymmetry tend to exhibit axisymmetrization of the wind field under smaller vertical wind shear (VWS). In contrast, highly asymmetric TCs display sharp increases in wind speed on the downshear-left side of greater-magnitude VWS vector, which can reduce vortex tilt and fortifies TC's resilience against VWS. Furthermore, TCs with higher TCA encounter greater challenges during their intensification stage, especially those with lower lifetime maximum intensity (LMI). TCA reflects the worsening disorganization of convection or the expansion of the inner-core wind field, rendering weaker TCs more susceptible to the effects of VWS. These findings underscore the nuanced relationship between TCA and TC intensification, demonstrating that TCA can serve as an additional indicator to improve the accuracy of TC intensity predictions, while also providing novel insights into the mechanisms driving TC intensification.