Toward parameterization of the eddy field near the Gulf Stream

Abstract

Analysis of measurements from two long-term moored arrays in and near the Gulf Stream suggests a simple parameterization of eddy spatial covariance statistics: a parameterization that can be referred to as “quasi-homogeneous and isotropic”. Taking the normalized covariance function (i.e. the correlation function) for streamfunction to be homogeneous and isotropic and assuming motions to be horizontally nondivergent and hydrostatic permit the velocity and temperature covariances to be derived from the streamfunction covariance. Statistical tests indicate that deviations from these assumptions are indistinguishable from Gaussian random noise. The spatial correlation function used in Gaussian with a decay scale of about 140 km, which is only weakly depth dependent. A simple form is also suggested for the vertical lag dependence. This parameterization permits calculation of derived quantities such as the eddy vorticity flux divergence which is discussed in the context of the mean potential vorticity balances for the depth integrated circulation and for the subthermocline layer. The divergence of the relative vorticity flux is found capable of driving two counter-rotating gyres of strength 30–40 Sv on either side of the Stream, as are observed. The “thickness flux” dominates the lower layer eddy potential vorticity flux and is of the correct sign to make the recirculation more barotropic. The lower layer eddy forcing is weak and the gyres exist in a region of nearly uniform mean potential vorticity.