Stratification governs the Existence of Surface-Intensified Eastward Jets in Turbulent Ocean Gyres

We investigate the impact of stratification on the formation and persistence of turbulent eastward jets in the ocean (like the Gulf Stream and Kuroshio extensions) [1]. Using a wind-driven, two-layer quasi-geostrophic model in a double-gyre configuration, we construct a phase diagram to classify flow regimes. The parameter space is defined by a criticality parameter ξ, which controls the emergence of baroclinic instability, and the ratio of layer depths δ, which describes the surface intensification of stratification. Eastward jets detaching from the western boundary are observed when δ < 1 and ξ ~ 1, representing a regime transition from a vortex-dominated western boundary current [2] to a zonostrophic regime characterized by multiple eastward jets. The emergence of the coherent eastward jet is further addressed with complementary 1.5-layer simulations and explained through both linear stability analysis and turbulence phenomenology. In particular, we show that coherent eastward jets emerge when the western boundary layer is stable, and find that the asymmetry in the baroclinic instability of eastward and westward flows plays a central role in the persistence of eastward jets,while contributing to the disintegration of westward jets.

[1] Miller, L., Deremble, B., & Venaille, A. (2024). Stratification governs the Existence of Surface-Intensified Eastward Jets in Turbulent Gyres without Bottom Friction. ( https://arxiv.org/abs/2411.05660 )

[2] Miller, L., Deremble, B., & Venaille, A. (2024). Gyre turbulence: Anomalous dissipation in a two-dimensional ocean model. Physical Review Fluids, 9(5), L051801.