Double-diffusive convection and baroclinic instability in a differentially heated and initially stratified rotating system: the barostrat instability

TL;DR

This study investigates baroclinic instability in a rotating, stratified fluid system with variable salinity profiles, revealing how stratification influences instability development and eddy formation, relevant to geophysical and astrophysical contexts.

Contribution

The paper introduces a novel laboratory setup to analyze baroclinic instability with stable stratification and develops statistical measures to quantify instability at different depths.

Findings

Stratification confines instability to shallow layers near top and bottom.

Stable stratification can promote multilayered baroclinic instability.

Stratification may enhance local eddy formation despite hindering full-depth overturning.

Abstract

A water-filled differentially heated rotating annulus with initially prepared stable vertical salinity profiles is studied in the laboratory. Based on two-dimensional horizontal particle image velocimetry (PIV) data, and infrared camera visualizations, we describe the appearance and the characteristics of the baroclinic instability in this original configuration. First, we show that when the salinity profile is linear and confined between two non stratified layers at top and bottom, only two separate shallow fluid layers can be destabilized. These unstable layers appear nearby the top and the bottom of the tank with a stratified motionless zone between them. This laboratory arrangement is thus particularly interesting to model geophysical or astrophysical situations where stratified regions are often juxtaposed to convective ones. Then, for more general but stable initial density profiles, statistical measures are introduced to quantify the extent of the baroclinic instability at given depths and to analyze the connections between this depth-dependence and the vertical salinity profiles. We find that, although the presence of stable stratification generally hinders full-depth overturning, double-diffusive convection can yield development of multicellular sideways convection in shallow layers and subsequently to a multilayered baroclinic instability. Therefore we conclude that by decreasing the characteristic vertical scale of the flow, stratification may even enhance the formation of cyclonic and anticyclonic eddies (and thus, mixing) in a local sense.