Global Distribution and Seasonal Variations of Charney-Type Submesoscale Baroclinic Instabilities (C-SBCIs)

Abstract

Previous studies primarily focused on the upper-ocean submesoscale baroclinic instabilities (SBCIs) influenced by mesoscale eddies. However, both idealized simulations and observations suggest that the mean ocean state may also play an important role in the generation of SBCIs. In this study, we investigate the Charney-type SBCIs (C-SBCIs) using seasonal climatological data, excluding mesoscale eddies, to offer a new perspective on the upper-ocean SBCIs. The C-SBCIs, characterized by surface-intensified and depth-decaying amplitudes, are generated by the opposite-sign quasi-geostrophic potential vorticity gradient at the surface and in the interior. The growth rates of C-SBCIs exhibit a geographic distribution, with substantial enhancement in western boundary currents and a remarkable seasonal variation, being larger in winter than in summer. The horizontal wavelengths of C-SBCIs decrease with increasing latitude, ranging from 2 km poleward of 60$°$N/S to 30 km equatorward of 10$°$N/S, and display a pronounced seasonal variation, being longer in winter than in summer. The vertical scale of C-SBCIs, termed the Charney depth, is first identified as the depth range of the potential vorticity gradient necessary for the C-SBCIs. Although the C-SBCIs resemble mixed layer (ML) instabilities in terms of growth rates and horizontal wavelengths, they differ in vertical scales with approximately a 0.2 probability of C-SBCIs extending below the ML depth. These deep-reaching C-SBCIs are caused by strong stratification and strong vertical velocity shear and may account for observed enhanced vertical buoyancy fluxes below the ML.