Four Types of Baroclinic Instability Waves in the Global Oceans and the Implications for the Vertical Structure of Mesoscale Eddies

Abstract

Linear stability analysis is re-conducted to fully understand the geostrophic distribution of the different types of baroclinic instability (BCI) in the global oceans, their correspondence to the different vertical structures of the observed mesoscale eddies, and the properties and formation mechanisms of the instability waves. Four principal vertical types of BCI are identified, which are found to exhibit large-scale patterns in the global ocean. The surface- and bottom-intensified type (called the Eady type hereafter) is mainly located in the Antarctic Circumpolar Current (ACC) region, locations of the bottom-intensified type (Charney_b type) are scattered around the Eady type, the surface-intensified type (Charney_s type) primarily occurs in the subtropics (10°–35°), and the interior-intensified type (Phillips type) occurs primarily between 5° and 20° in both hemispheres. More specifically, both geostrophic locations and the depths of the maximum perturbation velocities of the Phillips type BCIs match those of observed subsurface eddies. Moreover, the BCI waves show regions of uniform propagation properties: eastward in the ACC and the mid-latitudes (25°–45°), and westward in the low latitudes (30°S–30°N) of both hemispheres and in the high latitudes of the Northern Hemisphere (>50°N). These waves resemble normal mode Rossby waves in structure (i.e., first baroclinic, second baroclinic, and topographic Rossby waves), but their propagation speeds are found to be Doppler shifted by the mean flows relevant for the corresponding BCI type. Propagating signals with the same dispersion relationships as the BCI waves are captured with numerical ocean general circulation models.