Submesoscale baroclinic instability is believed to be a primary source of submesoscale eddy kinetic energy. While the widely recognized mixed layer instability theory relies on a rigid boundary condition at the base of the mixed layer, that is an unrealistic condition in the global ocean. In contrast, the Charney-type submesoscale baroclinic instability (C-SBCI) theory assumes a bottom boundary far away from the surface, making it more applicable in the global ocean. In this study we carefully isolate the C-SBCI, characterized by an intensified surface amplitude, and attribute it to the opposite-sign quasi-geostrophic potential vorticity (QGPV) gradient, restricted within a certain depth range. We define the lower boundary of the specific depth range as the Charney depth, representing the vertical scale or the effective depth of the C-SBCI. At the Charney depth, amplitude, phase change, and lateral eddy buoyancy flux derived from vertical structure of the C-SBCI exhibit regular patterns, indicating that the Charney depth serves as a reliable indicator for the baroclinic energy conversion and lateral eddy buoyancy flux induced by the C-SBCI. Comparative analysis of the Charney depth and the mixed layer depth reveals that 81%–89% of the former are shallower than the latter, while 11–19% of the former are deeper. While the former tends to arise from weak stratification, the latter is attributed to strong stratification along with strong vertical shear of velocity.

submesoscale baroclinic instability