Oceanic Response Observations due to Passing Tropical Cyclones: An Assessment of Drag and Sea Spray Parameterizations

Abstract

In 2015, Air-Launched Autonomous Micro Observer float 9077 was intercepted by Hurricane Ignacio and revealed a 40 m isotherm depression preceding the traditional, well-documented upwelling response. To replicate this unique and apparent downwelling ahead of Hurricane Ignacio and three othersﾗIrma, Florence and Michael, the Price-Weller-Pinkel model permitted comparisons of tropical cyclone intensity indices and vertical velocity. When modeling the stress, two other traditional ideas were challenged. First, many legacy drag coefficients linearly increase with wind speed, while the modern variety maximize near tropical cyclone strength. Second it is believed sea spray droplets are sheared off large wave crests, quickly accelerate, but upon reentry, dampen smaller waves while fluxing considerable momentum to the ocean. However, many models bulk parameterize air-sea interaction processes with a legacy drag coefficient and omit spray. Therefore, this study aimed to simulate the aforementioned downwelling using seven legacy/modern drag coefficients, with and without spray stress. These analyses illustrated downwelling ahead of every storm, legacy drag coefficients as statistical outliers, invalid for TC modeling, and although its addition reduced model accuracy, sea spray remains paramount through modern drag coefficient selection. The confirmation of downwelling is supported by Ekman dynamics and may be significant in storm surge enhancement.