Flavours of Baroclinic Instability in the Global Ocean

The transfer of energy from the global ocean circulation to mesoscale eddies is primarily mediated by baroclinic instability, which releases the superabundant available potential energy stored in sloping isopycnals by basin-scale wind and buoyancy forcing. However, the details of the local shear and stratification can give rise to qualitatively distinct flavours of baroclinic instability. In particular, the presence of outcropping isopycnals (or, equivalent, a thermal wind shear at the upper surface) can have a strong impact on the necessary conditions for baroclinic instability and the resulting nonlinear cascade to submesoscales.

In this article, a simple framework is described for categorizing baroclinic instability in terms of two non-dimensional parameters, the Charney-Green number and the Phillips supercriticality. We analyze the influence of a non-zonal mean flow on growth rates and the baroclinic conversion of available potential energy to eddy kinetic energy. Finally, hydrographic profiles are used to form a global atlas of baroclinic instability in the ocean, and regional patterns are discussed.