Direct Atmospheric Forcing of Geostrophic Eddies. Part II: Coherence Maps

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  • 1 Institut für Meereskunde Hamburg, Universität Hamburg, Hamburg, Germany
  • | 2 Department of Oceanography, University of Hawaii, Honolulu, Hawaii
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Abstract

The theory of stochastic atmospheric forcing of quasigeostrophic eddies is applied to calculate coherence maps, that is, the coherence between the oceanic response at one location and the atmospheric forcing at another location as a function of separation for different frequencies. The theory determines the linear quasigeostrophic response of an infinite or periodic, β-plane ocean to stochastic wind stress forcing transmitted to the ocean by Ekman pumping. All transfer and dissipation processes in the ocean are parameterized by a linear friction law with a scale-independent damping rate (Rayleigh damping). In Part I, it was shown that the theory reproduces observed energy levels in midocean regions far removed from strong mean currents. Here it is shown that the theory also reproduces basic features of observed coherence maps between the barotropic flow component and the wind stress curl. The theory provides a simple explanation for the existence and location of nonlocal or secondary maxima in coherence maps. The theory assumes statistical homogeneity. The existence and location of nonlocal or secondary maxima is related to the wavenumber structure of the forcing spectrum and response function and does not reflect nonlocal forcing and energy propagation along wave group trajectories. Overall, the results support the notion that much of the subinertial barotropic variability in the ocean is directly forced by fluctuations in the atmospheric wind stress and that a simple linear stochastic forcing model can provide the basis for explaining this variability.

Abstract

The theory of stochastic atmospheric forcing of quasigeostrophic eddies is applied to calculate coherence maps, that is, the coherence between the oceanic response at one location and the atmospheric forcing at another location as a function of separation for different frequencies. The theory determines the linear quasigeostrophic response of an infinite or periodic, β-plane ocean to stochastic wind stress forcing transmitted to the ocean by Ekman pumping. All transfer and dissipation processes in the ocean are parameterized by a linear friction law with a scale-independent damping rate (Rayleigh damping). In Part I, it was shown that the theory reproduces observed energy levels in midocean regions far removed from strong mean currents. Here it is shown that the theory also reproduces basic features of observed coherence maps between the barotropic flow component and the wind stress curl. The theory provides a simple explanation for the existence and location of nonlocal or secondary maxima in coherence maps. The theory assumes statistical homogeneity. The existence and location of nonlocal or secondary maxima is related to the wavenumber structure of the forcing spectrum and response function and does not reflect nonlocal forcing and energy propagation along wave group trajectories. Overall, the results support the notion that much of the subinertial barotropic variability in the ocean is directly forced by fluctuations in the atmospheric wind stress and that a simple linear stochastic forcing model can provide the basis for explaining this variability.

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