Seasonality of Four Types of Baroclinic Instability in the Global Oceans

Abstract

Distinct geographical distributions of four types of mesoscale baroclinic instabilities (BCIs) exist in the global oceans, implying preferences of surface or subsurface mesoscale eddies to specific regions. This study explores seasonal variations of global BCI types and their dependence on varying background ocean states with a focus on three representative regions. First, the regions of the Kuroshio Extension and Gulf Stream are representative for distinctive seasonal transition of BCI types: the Charney surface BCIs prevail in winter, while the Phillips BCIs prevail in summer, which is controlled by weak (strong) upper‐layer stratification in winter (summer) but is less impacted by the seasonal changes in the mean flow‐induced shear. Second, the region covering both the tropics and subtropics of the North Pacific (10°N–35°N) is representative for seasonal meridional migration of the border between the domains of the Phillips and the Charney surface BCIs; their border migrates equatorward (poleward) in spring and summer (autumn and winter), which is primarily caused by the equatorward (poleward) shift of both the North Equatorial Current and the Subtropical Countercurrent in spring and summer (autumn and winter), and is secondarily modulated by the variation in the upper‐layer stratification. Third, the Antarctic Circumpolar Current region is chosen to represent a weak seasonal variation of BCIs. Those seasonal variations in BCI types are further demonstrated as being associated with seasonal variations of baroclinic conversion rate, implying possible generation of corresponding eddies.

Wave, Vortex and Wave‐Vortex Dipole (Instability Wave): Three Flavors of the Intra‐Seasonal Variability of the North Equatorial Undercurrent

“…(2021), Feng et al. (2022) and Hochet et al. (2015), who found that in this latitudinal band (8°–18°N) the fastest‐growing baroclinic instability is subsurface‐intensified and is generated by interaction of a pair of counter‐propagating second baroclinic Rossby waves.…”

“…Furthermore, according to the common vertical scale and structure of the sub-thermocline ISVs in this region (Figure S2 in Supporting Information S1), the WVD likely is the second baroclinic mode-like Rossby wave-initiated instability wave. This can be indicated by Feng et al (2021), Feng et al (2022 and Hochet et al (2015), who found that in this latitudinal band (8°-18°N) the fastest-growing baroclinic instability is subsurface-intensified and is generated by interaction of a pair of counter-propagating second baroclinic Rossby waves. Whereas, due to complicated top and bottom boundary conditions and mean flows, how well this baroclinic mode fits a normal second baroclinic mode needs further studies.…”

“…(2012) and Feng et al. (2022), $$BTR=\overline{{u}^{\prime }{u}^{\prime }}\frac{\partial \overline{U}}{\partial x}+\overline{{u}^{\prime }{v}^{\prime }}\left(\frac{\partial \overline{U}}{\partial y}\,+\frac{\partial \overline{V}}{\partial x}\right)+\overline{{v}^{\prime }{v}^{\prime }}\frac{\partial \overline{V}}{\partial y}$$ and $$BCR=-\frac{{g}^{2}}{{\rho }_{0}^{2}\overline{{N}^{2}}}\left(\overline{{u}^{\prime }{\rho }^{\prime }}\frac{\partial \overline{\rho }}{\partial x}+\overline{{v}^{\prime }{\rho }^{\prime }}\frac{\partial \overline{\rho }}{\partial y}\right)$$, where $$U$$, $$V$$ and $$\rho $$ are the monthly climatology of zonal and meridional velocities and density, respectively, obtained over 27 years (1993–2019); $${u}^{\prime }$$, $${v}^{\prime }$$ and $${\rho }^{\prime }$$ are the daily anomalies relative to …”

“…To identify the driving force of the ISVs, we calculate the barotropic and baroclinic energy conversion rate (BTR and BCR) associated with them. Here, following von Storch et al (2012) and Feng et al (2022),…”

From sheared annular centrifugal Rayleigh–Bénard convection to radially heated Taylor–Couette flow: exploring the impact of buoyancy and shear on heat transfer and flow structure

“…The coupling of buoyancy and shear is widespread in atmospheric motion and oceanic flow (Deardorff 1972;Khanna & Brasseur 1998). For example, the surface mesoscale eddies in the ocean are generated mainly by baroclinic instabilities, which are relevant to the pole-equator temperature gradient and a vertical shear (Hopfinger & van Heijst 1993;Pierrehumbert & Swanson 1995;Vincze et al 2014;Feng et al 2022). There are very many attempts to combine shear and buoyancy in one system and study their coupling effect.…”