Abstract

A two-dimensional regime diagram for geostrophic turbulence in the ocean is constructed by plotting observation-based estimates of the nondimensional eddy radius and unsuppressed mixing length against a nonlinearity parameter equal to the ratio of the root-mean square eddy velocity and baroclinic Rossby phase speed. For weak nonlinearity, as found in the tropics, the mixing length mostly corresponds to the marginal stability threshold for baroclinic instability whereas the eddy radius corresponds to the Rhines scale; it is suggested that this mismatch is indicative of the inverse energy cascade that occurs at low latitudes in the ocean and the zonal elongation of eddies. At larger values of nonlinearity, as found at mid- and high-latitudes, the eddy length scales are much shorter than the stability threshold, within a factor of 2.5 of the Rossby deformation radius. However, better agreement is found with a turbulent $\beta$ length scale across much of the ocean.