Ctop Year4: Coupled Typhoon-Ocean Prediction System: Understanding & Forecasting the Coupled Response of Ocean and Typhoon in a Changing Climate

In 2019, MOST-funded research has resulted in a total of 5 papers; please visit http://mpipom.ihs.ncu.edu.tw/RecentPublications.php. This proposal will continue the work of two of these papers: Zhang & Oey 2019a: An observational analysis of ocean surface waves in tropical cyclones in the western North Pacific Ocean. Journal of Geophysical Research Oceans, 124, 184-195, https://doi.org/10.1029/2018JC014517; and Zhang & Oey, 2019b: Young ocean waves favor the rapid intensification of tropical cyclones - a global observational analysis. Monthly Weather Review, 147, 311-328, https://doi.org/10.1175/MWR-D-18-0214.1; they can be downloaded from http://mpipom.ihs.ncu.edu.tw/RecentPublications.php. Identifying the condition(s) of how tropical cyclones intensify, in particular rapid intensification, is challenging, due to the complexity of the problem involving internal dynamics, environments and mutual interactions; yet the benefit to improved forecasts may be rewarding. Ocean surface waves and sprays modulate surface heat and moisture fluxes in tropical cyclones, which in turn can alter storm’s intensification. In paper 2019a, we studied surface waves in western North Pacific tropical cyclones. In paper 2019b, we extend the analysis to global tropical cyclones and demonstrate, for the very first time in the literature, how surface wave-induced surface exchanges are coupled to atmospheric low-level moisture convergence to induce intensification. To make the analysis more tractable, we focus near the sea surface, by examining 23-year global observations comprising over 16,000 cases of tropical cyclone intensity change, together with upper-ocean features, surface waves and low-level atmospheric moisture convergence. Contrary to the popular misconception, we found no statistically significant evidence that thicker upper-ocean layers and/or warmer temperatures are conducive to rapid intensification. Instead, we found in storms undergoing rapid intensification significantly higher coincidence of low-level moisture convergence and a dimensionless air-sea exchange coefficient closely related to the youth of the surface waves under the storm. This finding is consistent with the previous modeling results, verified here using ensemble experiments, that higher coincidence of moisture and surface fluxes tends to correlate with intensification, through greater precipitation and heat release. The young waves grow to saturation in the right-front quadrant due to trapped-wave resonance for a group of Goldilocks cyclones that translate neither too slowly nor too quickly, which 70% of rapidly-intensifying storms belong. Young waves in rapidly-intensifying storms also produce relatively less (compared to the wind input) Stokes-induced mixing and cooling in the cyclone core. A reinforcing coupling between tropical cyclone wind and waves leading to rapid intensification is also proposed in the paper. This proposal seeks further funding to continue our coupled ocean-typhoon modeling and data analysis.