Stability of the Sargasso Sea Subtropical Frontal Zone

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Abstract
Recent studies suggest that eddy Properties are significantly influenced by the mean current shear associated with the western Sargasso Sea subtropical frontal zone (SFZ). Between 19° and 34°N, the mean density structure is characterized by three layers separated by a climatologically permanent upper (“seasonal”) thermocline that shoals, and a lower (main) thermocline that deepens, toward the north; the thermoclines are separated by the wedge-shaped southern part of the subtropical mode water pool. The SFZ is evident as a zonal band between about 26° and 32°N where subtropical frontogenesis between the westerlies and trades enhances the slope of the mean seasonal thermocline. Classical linear as well as more recent nonlinear stability theories predict that the mean SFZ flow should be unstable. The linear eigenvalue problem suggests that the most unstable perturbations have wavelengths between 150 and 200 km. Analysis of a channel version of the Miami isopycnic-coordinate primitive equation numerical model verified these predictions and also characterized the further nonlinear evolution of the eddy field. As nonlinear effects become increasingly important, the eddies with wavelengths of 150-200 km predicted by linear theory that initially dominate the model fields continue to grow as the wavenumber spectrum becomes saturated. Following this, the eddies stop growing and energy shifts to longer wavelengths as predicted by geostrophic turbulence theory and observed in Geosat altimeter data. Zonal bands of mean flow also appear after the onset of the nonlinear energy cascade to larger scales as predicted by theory. Model results suggest baroclinic energy conversion and atmospheric forcing contribute roughly equally to eddy variability within the SFZ. Over three-fourths of the available potential energy released by the instability is extracted from the model seasonal thermocline. This agrees with the strong dependence of the strength of the instability on seasonal thermocline slope predicted by linear theory, and also agrees with the concentration of eddy potential energy within the seasonal thermocline revealed by analysis of historical XBT data. This may be one reason why clear evidence of baroclinic instability in the Sargasso Sea SFZ was not obtained from earlier moored measurements in this region (e.g., MODE); measurements in the main thermocline were emphasized.

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