Langmuir turbulence under Hurricane Gustav
Date
2013
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
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.