Four Types of Baroclinic Instability Waves in the Global Oceans and the Implications for the Vertical Structure of Mesoscale Eddies
June 07,2021

Ling Feng, Chuanyu Liu, Armin Köhl, Detlef Stammer, Fan Wang

Published in Journal of Geophysical Research: Oceans, February 2021

Surface mesoscale eddies are ubiquitous in the world’s oceans and are believed to account for ∼90% of ocean kinetic energy (Wunsch & Ferrari, 2004) and to play an important role in meridional heat and mass transport (Sun et al., 2019; Zhang et al., 2014). Recent years, a large amount of subsurface mesoscale eddies are detected in global oceans (Xu et al., 2019; Zhang et al., 2017). However, Dilmahamod et al. (2018) pointed out that ~27% surface eddies detected by satellite altimetry are indeed subsurface eddies, which indicates the distribution of surface eddies is remain vague; meanwhile, lack of subsurface observational data leads to poor knowledge of subsurface eddies. Baroclinic instability (BCI) is one of the most important mechanism for generation of eddies, therefore, researches about BCIs will be instructive for eddies.

A recent study conducted jointly by IOCAS and University of Hamburg showed the BCIs can be mainly classified into four types, which are surface- and bottom- intensified Eady-like type, surface-intensified Charney-like type, bottom-intensified Charney-like type and subsurface-intensified Phillips-like type. For convenience, hereafter we refer to those four types of BCIs as Eady, Charney_s, Charney_b, and Phillips, respectively.

Fig. All profiles of ψ grouped into four BCI types and sorted by water depth (ψ is standardized to have an amplitude of 1).

The global distribution of the four types of BCIs is pretty regular. The Eady type BCIs, accounting for 13% of all the 31678 BCIs, mainly occurs in high latitudes, such as the main axis of Antarctic Circumpolar Current (ACC); the Charney_b type BCIs, accounting for only 7%, usually scatter around the Eady type BCIs; the Charney_s type BCIs, accounting for 47%, widely distribute in global ocean and concentrate at subtropical oceans; the Phillips type BCIs, accounting for 33%, mainly occurs in the tropics, the Kuroshio Extension, in the eastern return flow of the subtropical gyres, and in the high latitude of North Atlantic.

Fig. Global distribution of the BCI types. The locations are left blank where no fastest growing BCI that meets the criterion K < 5Kdef is detected. (Kdef is the wavenumber of the local first baroclinic radius of deformation.)

Different type of BCIs will generate different types of eddies. Firstly, the vertical structures of the Charney_s type and the Phillips type BCIs are consistent with those of surface and subsurface eddies, respectively. Secondly, the Charney_s type and the Phillips type BCIs dominate regions where surface and subsurface eddies are active, respectively. Consequently, the global distribution of four types of BCIs in this study may be instructive for the global distribution of various eddies.

Ling, Feng, Chuanyu, Liu, Armin Köhl, Detlef Stammer, & Fan Wang (2021). Four types of baroclinic instability waves in the global oceans and the implications for the vertical structure of mesoscale eddies. Journal of Geophysical Research: Oceans, 126 (3), 1-24, e2020JC016966.

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