Seminar, Suraj Singh, post-doc at MISU

Seminar

Date: Tuesday 18 June 2024

Time: 11.15 – 12.15

Location: C609 Rossbysalen, MISU, Svante Arrhenius väg 16C, 6th floor

Title: On Baroclinic instability of curved fronts

 

Abstract

Baroclinic instability has traditionally been examined using a model of a front in approximate geostrophic and hydrostatic balance–i.e., a straight front in thermal wind balance. However, mesoscale eddies and curved fronts are ubiquitous in the oceans and it is critical that we understand the effect of curvature. In this study, we present modifications of the classical Eady and Charney problems, introducing a small amount of curvature in the small-Rossby, large-Richardson number limit. Employing quasi-geostrophic scalings for a predominantly zonal flow in cylindrical polar coordinates, we derive the governing equation of perturbation pressure in the presence of small curvature, treating this quantity as a deviation from a straight front. We find that tthe importance of curvature principally arises through the potential vorticity (PV) gradient. Consequently, although curvature enters the Eady model via an introduction of so-called Green modes, the introduction of curvature does not modify the most unstable mode. In Charney’s model, however, the curvature of the flow introduces a depth scale that governs the vertical extent of the unstable modes and whose importance often presides over planetary beta. We also find that introducing cyclonic curvature in Charney’s model increases the horizontal wavelength of the most unstable mode. We also report that curvature modifies the vertical buoyancy flux, making the unstable modes deeper. The consequences of these results are discussed. Since our present-day understanding of baroclinic instability assumes centrifugal forces in the mean state to be zero and since this undergirds existing submesoscale eddy parameterizations, this study proposes a different interpretation of at least some of the observed vortices in the ocean and suggests the weakly-curved Charney model might inform sub-grid-scale parameterizations of baroclinic instability of curved fronts in the oceans.
 

 

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