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

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. Strong wind under tropical cyclones increases wind forcing at the sea surface, enhances upper-ocean currents and turbulence, and brings deep cold water to the sea surface. The resulting cooler sea surface temperature decreases the heat supply to the storm and weakens the storm. Therefore, it is important to understand how the upper ocean responds to tropical cyclone wind in order to improve the storm intensity forecast. In this study, we investigate how ocean surface waves affect such ocean responses by combining field observations and numerical simulations under five tropical cyclones. We find that upper ocean turbulence becomes more intense because of surface waves. Its intensity is strongly dependent on sea states as well as on wind speed. We also find that ocean responses are weaker on the left of the storm track, possibly because wind forcing at the sea surface is reduced when ocean swell direction and wind direction are misaligned. Field observations and numerical modeling are combined to study upper ocean responses under five tropical cyclonesWe find two evidences of the Langmuir turbulence, enhanced vertical velocity variance and reduced mean current shearLangmuir turbulence is weaker on the left of storm track possibly because drag coefficient is lower due to swell misaligned from wind