The baroclinic instability of a mid-ocean circulation

The instability of a flow modelling a linear oceanic thermocline is investigated with a view towards understanding the intermediate-scale fluctuations observed by Crease (1962) and others. Robinson’s (1962) analysis of the stability of a thermal ocean circulation is extended to include amplifying quasi-geostrophic perturbations. North-south wavelengths of several hundred kilometers are predicted and agree with previous measurements.

For the special case of perturbations independent of longitude, a uniformly valid solution is obtained for oceans of large latitudinal extent. Growth rates are of the order of one year, and the north-south wavelength is a function of latitude.

Although the basic state chosen is different from its atmospheric counterpart examined by Eady (1949) and Green (I960), both are baroclinically unstable and have perturbations with similar properties. At maximum instability, both have perturbations which are constant in the direction parallel to the basic lateral density gradient, and this result is true both on an f-plane and β-plane.