Zonal jets in equilibrating baroclinic instability on the polar beta‐plane: Experiments with altimetry

Abstract

Results from the laboratory experiments on the evolution of baroclinically unstable flows generated in a rotating tank with topographic -effect are presented.We study zonal jets of alternating direction which occur in these flows. The primary system we model includes lighter fluid in the South and heavier fluid in the North with resulting slow meridional circulation and fast mean zonal motion. In a two-layer system the velocity shear between the layers results in baroclinic instability which equilibrates with time and, due to interaction with -effect generates zonal jets. This system is archetypal for various geophysical systems including the general circulation and jet streams in the Earth's atmosphere, the Antarctic Circumpolar Current or the areas in the vicinity of western boundary currents where baroclinic instability and multiple zonal jets are observed. The gradient of the surface elevation and the thickness of the upper layer are measured in the experiments using the Altimetric Imaging Velocimetry and the Optical Thickness Velocimetry techniques respectively. Barotropic and baroclinic velocity fields are then derived from the measured quantities. The results demonstrate that the zonal jets are driven by "eddy forcing" due to continuously created baroclinic perturbations. The flow is baroclinic to a significant degree and the jets are "surface intensified". The meridional wavelength of 3 the jets varies linearly with the baroclinic radius of deformation and is also in a good agreement with a modified Rhines scale. This suggests a linear dependence of the perturbation velocity in the equilibrated baroclinically unstable flow on the parameter.

Zonal jets at the laboratory scale: hysteresis and Rossby waves resonance

“…(2004, 2007) alternatively used a specific convective forcing by spraying dense (salt) water at the free surface of a fresh water layer, and in a similar fashion, Afanasyev et al. (2012), Slavin & Afanasyev (2012), Zhang & Afanasyev (2014), Matulka & Afanasyev (2015) performed a localized forcing involving sources of buoyancy. Another method, which has the advantage of allowing a close control of the location, scale and intensity of the forcing, consists in applying a mechanical forcing, provided that, again, it does not directly force the mean flow.…”

Complex environmental <i>β</i>-plane turbulence: laboratory experiments with altimetric imaging velocimetry

“…A classic model for a baroclinically unstable system is a rotating annulus where heating/cooling is provided at the outer wall/centre of the tank respectively. Different aspects of the dynamics of this system were studied in a number of experiments (Hide and Mason, 1975;Mason, 1975;Read, 1997, 1998;Wordsworth et al, 2008;Smith et al, 2014 Matulka and Afanasyev (2015). It was shown that the meridional scale of the jets is determined to a large extent by the radius of deformation and, at the same time, is in good agreement with the Rhines scale (Rhines, 1975).…”

“…The radial motion, in turn, causes zonal circulation. Measurements of barotropic and baroclinic components of velocity (for details see Matulka and Afanasyev, 2015) show that the upper layer rotates cyclonically while the lower layer rotates anticyclonically. The shear between the layers makes the system baroclinically unstable.…”

“…A baroclinic component which allows one to obtain the total velocity in the lower layer can be measured by a different technique (e.g. Afanasyev et al, 2009;Matulka and Afanasyev, 2015), but is not discussed here.…”

“…In this study we perform spectral analyses of the flows described in Matulka and Afanasyev (2015) and compare them with the results obtained for a somewhat different flow generated by thermal forcing (Zhang and Afanasyev, 2014). The latter experiment although forced baroclinically was more barotropic in its dynamics, while the former experiment was purely baroclinic.…”