Flavors of Baroclinic Instability in the Global Ocean
Baroclinic instability is the fundamental mechanism responsible for large-scale and mesoscale motions in rotating, stratified fluids. In Earth's atmosphere, it produces weather systems in midlatitudes; in the ocean, it drives the formation of the mesoscale eddy field, releasing the superabundant available potential energy stored in sloping density surface (isopycnals). The details of the vertical shear and stratification can give rise to distinct “flavors" of baroclinic instability, however, with qualitatively different linear and nonlinear dynamics.Â
In this talk, I will describe the classical models of baroclinic instability — Phillips, Eady, and Charney — in the context of the ocean. I will show that a net change in isopycnal slope over the water column supports a depth-independent potential vorticity gradient due to vortex stretching. This tilting-induced gradient can reinforce or oppose the planetary vorticity gradient, modifying the necessary conditions for instability. I will then describe a simple framework for categorizing flavors of baroclinic instability in terms of a small set of non-dimensional parameters: the Phillips supercriticality, the Charney-Green numbers associated with the top and bottom surface buoyancy gradients, and the orientation of the baroclinic mean shear. Different flavors of baroclinic instability are represented by different regimes in parameter space. Within this framework, hydrographic temperature and salinity profiles will be used to form a global atlas of baroclinic instability types in the ocean, and patterns of regional and seasonal variability will be discussed.Â