Statistical quantification of potential impacts of upper-ocean stratification on tropical cyclone-induced sea surface cooling in the western North Pacific

Abstract

Using high-resolution reanalysis products and observed tropical cyclones (TCs) in the western North Pacific during July-October from 1994 to 2015, this study investigates the relative importance of TC intensity and upper-ocean stratification in controlling sea surface cooling within 24 h before and after TC passages, with a focus on vertical mixing, a dominant process for the cooling. To minimize the influence of differences in TC translation speeds, only TCs with typical translation speeds are retained. The upper-ocean stratification is quantified by density-based virtual potential energy change ( $$\:\varDelta\:$$ VPE) that incorporates the effects of both mixed layer depth (MLD) and the density gradient beneath it. A statistical classification method reveals that a 43% difference in $$\:\varDelta\:$$ VPE, associated with a shallower MLD and a steeper density gradient, leads to an additional surface cooling of 0.27℃ for stronger TCs. In addition, correlation and regression analyses, including both strong and weak TCs and simply assuming that cooling amplitude is explained solely by TC intensity and $$\:\varDelta\:$$ VPE, indicate that each predictor explains approximately 77% and 23% of the variance in cooling, respectively. Furthermore, $$\:\varDelta\:$$ VPE outperforms four other metrics in estimating the impact of upper-ocean stratification on TC-induced cooling. These results provide a basis for improving the representation of ocean feedbacks in real-time TC intensity forecasts.