Dynamics of the wind stress response to ocean mesoscale surface temperatures over the Agulhas Retroflection
International Pacific Research Center and Department of Oceanography
University of Hawai’i at Mānoa, Honolulu, Hawaii, USA
The surface and boundary layer dynamics of the response of the surface wind-stress curl and divergence to sea surface temperatures associated with the ocean mesoscale are deconstructed in the Agulhas retroflection region using output from a 19 year integration of an atmospheric general circulation model. The model reproduces observed regression (coupling coefficients) between wind stress divergence and curl and gradients of sea surface temperature. Linearization of the surface layer formulation of the wind stress around the large-scale winds reveals that 80% and 100% of coupling coefficients for wind stress divergence and curl, respectively, are due to stability dependence of the wind stress.
SST induced changes in surface wind speed account for 20% of the wind stress divergence coupling coefficients. Changes of wind direction do not affect wind-stress coupling coefficients. Boundary layer adjustments of sea surface temperature induced surface winds are poorly captured by coupling coefficients. Instead, spectral transfer functions describe the in-phase and phase-shifted surface wind response to ocean mesocale sea surface temperatures as a function of spatial scale and background wind direction and speed. A linear theory explains these spectral transfer functions by distinct physical regimes of the wind responses in the directions aligned and across background winds, and by scale-dependent magnitude of sea surface temperature induced vertical mixing and hydrostatic pressure gradients.
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