The Impact of Stratification on Surface-Intensified Eastward Jets in Turbulent Gyres

TL;DR

This paper investigates how stratification influences the formation and stability of eastward jets in turbulent gyres, revealing conditions under which surface-intensified eastward jets emerge without bottom friction, through modeling and stability analysis.

Contribution

It introduces a phase diagram linking stratification parameters to jet regimes, highlighting the role of surface stratification and boundary layer stability in jet emergence and persistence.

Findings

Eastward jets form when surface stratification is strong and boundary layers are stable.

Surface-intensified eastward jets emerge without bottom friction influence.

Asymmetry in baroclinic instability affects jet persistence and disintegration.

Abstract

This study examines the role of stratification in the formation and persistence of eastward jets (like the Gulf Stream and Kuroshio currents). 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 \( \xi \), which controls the emergence of baroclinic instability, and the ratio of layer depths \( \delta \), which describes the surface intensification of stratification. Eastward jets detaching from the western boundary are observed when \( \delta \ll 1 \) and \( \xi \sim 1 \), representing a regime transition from a vortex-dominated western boundary current to a zonostrophic regime characterized by multiple eastward jets. Remarkably, these surface-intensified patterns emerge without considering bottom friction. 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.