Evidence of Langmuir Mixing Effects in the Upper Ocean Layer During Tropical Cyclones Using Observations and a Coupled Wave-Ocean Model

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Mixing of the ocean beneath tropical cyclones (TC) cools the surface temperature thereby modifying the storm intensity. Modeling studies predict that surface wave forcing through Langmuir turbulence (LT) increases the mixing and cooling and decreases near-surface vertical velocity shear. However, there are very few quantitative observational validations of these model predictions, and the validation efforts are often limited by uncertainties in the drag coefficient (Cd). We combine EM-APEX and Lagrangian float measurements of temperature, salinity, velocity, and vertical turbulent kinetic energy (VKE) from five TCs with a coupled ocean-wave model (Modular Ocean Model 6—WAVEWATCH III) forced by the drag coefficient Cd directly constrained for these storms. On the right-hand of the storms in the northern hemisphere, where wind and waves are nearly aligned, the measured VKE is consistent with predictions of models including LT and 2–3 times higher than predictions without LT. Similarly, vertical shear in the upper 20 m is small, consistent with predictions of LT models and inconsistent with the large shears predicted by models without LT. On the left-hand of the storms, where wind and waves are misaligned, the observed VKE and cooling are reduced compared to those on the right-hand, consistent with the measured decrease in Cd. These results confirm the importance of surface waves for ocean cooling and thus TC intensity, through both Cd and LT effects. However, the model predictions, even with the LT parameterization, underestimate the upper ocean cooling and mixed layer deepening by 20%–30%, suggesting possible deficiency of the existing LT parameterization.

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Journal of Geophysical Research: Oceans