Langmuir turbulence under Hurricane Gustav

Rabe, Tyler
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University of Delaware
Extreme winds and complex wave field, which vary in space and time, drive upper ocean turbulence in tropical cyclone conditions. Motivated by Lagrangian float observations of mixed layer averaged (i.e. bulk) vertical kinetic energy (VKE) under Hurricane Gustav, upper ocean turbulence is investigated based on large eddy simulation (LES) of the wave-averaged Navier-Stokes equations. The wave-driven residual current (Stokes drift) interacts with the sheared Eulerian currents to create Langmuir circulations, whose wide range of temporal and spatial scales characterizes them as a type of turbulence. To realistically capture wind and wave-driven Langmuir turbulence (LT), the LES model imposes the Stokes drift vector from spectral wave simulations; both, the LES and the wave model are forced by the NOAA HRD surface wind analysis product (H*WIND). Results strongly suggest that without LT effects, simulated VKE underestimates the observed VKE. LT increases the VKE indicating that it plays a significant role in upper ocean turbulence dynamics. Consistent with observations, the LES predicts a suppression of VKE near the hurricane eye due to wind-wave misalignment. However, this decrease is weaker and of shorter duration than that observed, potentially due to large scale horizontal advection not captured in our LES. LES results agree better with observations for smaller wind stresses, suggesting that the air-sea drag coefficient is lower that previously estimated in high wind tropical cyclone conditions. Both observations and simulations are consistent with a highly variable upper ocean turbulence field beneath tropical cyclone cores. Bulk VKE, a TKE budget analysis, and anisotropy coefficient (ratio of horizontal to vertical velocity variances) profiles all indicate that LT can suppress turbulence to levels closer to that of shear turbulence (ST) due to misaligned wind and wave fields. VKE approximately scales with the directional surface layer Langmuir number that incorporates the wind stress and Stokes drift vectors and the upper ocean boundary layer depth. Such a scaling provides guidance for the development of an upper ocean boundary layer parametrization that explicitly depends on sea state. Enhanced mixing from LT leads to greater sea surface temperature (SST) changes under the hurricane core, which can provide a direct negative feedback on tropical cyclone strength.