Role of Coherent Eddies in Potential Vorticity Transport in Two-layer Quasigeostrophic Turbulence

TL;DR

This study investigates how coherent eddies in a two-layer quasigeostrophic model influence potential vorticity transport, revealing their propagation characteristics and their role in upgradient PV transport, which differs from standard diffusion estimates.

Contribution

It introduces a method to identify coherent eddies via Lagrangian vorticity contours and analyzes their transport properties across different flow regimes with varying bottom friction.

Findings

Coherent eddies become more prevalent with increased bottom friction.

Eddies propagate zonally near the deformation radius, with cyclones moving poleward and anticyclones equatorward.

Upgradient PV transport occurs due to coherent eddy cores, opposing background PV transport.

Abstract

The transport by materially coherent eddies is studied in a two-layer quasigeostrophic model of geophysical turbulence. The coherent eddies are identified by closed contours of the Lagrangian-averaged vorticity deviation obtained from Lagrangian particles advected by the flow. A series of flow regimes with different bottom friction strengths are considered---it is found that coherent eddies become more prevalent and longer-lasting as the strength of the bottom drag increases. These coherent eddies, with average core radius close to the deformation radius, propagate zonally with speeds close to the long baroclinic Rossby wave speed and meridionally with a preference for cyclones to propagate poleward and anticyclones to propagate equatorward. The meridional propagation preference of the coherent eddies gives rise to a systematic upgradient potential vorticity (PV) transport, which is in the opposite direction as the background PV transport and not captured by standard Lagrangian diffusivity estimates. The upgradient PV transport by coherent eddy cores is less than 10% of the total PV transport, but the PV transport by the periphery flow induced by the PV inside coherent eddies is significant and downgradient. This clarifies the distinct roles of the trapping and stirring effect of coherent eddies in PV transport in geophysical turbulence.