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

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 degrees-35 degrees), and the interior-intensified type (Phillips type) occurs primarily between 5 degrees and 20 degrees 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 degrees-45 degrees), and westward in the low latitudes (30 degrees S-30 degrees N) of both hemispheres and in the high latitudes of the Northern Hemisphere (>50 degrees 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. Plain Language Summary Mesoscale eddies are ubiquitous in the ocean, accounting for similar to 90% of the ocean's kinetic energy. Eddies can be classified, according to their depths of maximum rotation velocity, into surface eddies, subsurface eddies, and bottom eddies. They are generally believed to be generated by baroclinic instability (BCI), a mechanism that extracts the potential energy stored in horizontal density gradient. Therefore, BCIs can be considered as the first stage of eddy generation. In order to shed light on the generation of different vertical structure of eddies, we identify four main types of BCIs according to their vertical structure: The Eady type (simultaneously surface- and bottomintensified, and weakened in the middle), the Charney surface type (surface-intensified), the Charney bottom type (bottom-intensified), and the Phillips type (subsurface-intensified). Each type of BCI may evolve into a different type of eddy. The global distribution of the four BCI types exhibits a latitudedependence. However, if the BCIs do not eventually evolve into eddies, they may manifest as propagating instability waves. We show that, the perturbative waves that determine the vertical structure of the instability waves, and the mean ocean currents that determine the waves' propagation speed, are all BCI type-dependent.