A laboratory study of global-scale wave interactions in baroclinic flow with topography II: vacillations and low-frequency variability

Abstract

A laboratory investigation is presented with the aim of studying systematically the occurrence and characteristics of low-frequency variability of flows resulting from the interaction of a baroclinic flow with periodic bottom topography. Low-frequency variability within the baroclinic wave regime occurred in two distinct forms in separate regions of parameter space. One corresponded to the transition region between the baroclinic travelling and stationary wave regimes. It involved primarily an interaction between the drifting baroclinic waves and stationary components of the topographically forced wave. The resulting flow had characteristics similar to amplitude vacillation and had a time-scale of 30-60 annulus revolutions (days), which also corresponded to the wave drift period. A new regime of low-frequency amplitude vacillation was discovered in the transition region with the axisymmetric flow regime. As the complexity of the flow increased the period of the vacillation cycles grew to ∼100-180 "days". This slower vacillation seemed to involve a cyclic enabling and disabling of nonlinear interactions between the forced stationary wave and the growing and azimuthally drifting wave, which in turn was linked to a decrease in mean flow shear. Subsequent chains of wave-wave interactions characterised the complex but robust oscillation phenomenon. The resulting behaviour has several features in common with some recent models of intraseasonal oscillations in the mid-latitude troposphere and with sudden stratospheric warmings.

An experimental investigation of blocking by partial barriers in a rotating baroclinic annulus

“…Taylor number, this component was never found to dominate the flow structure, and thus, in contrast to earlier work (e.g. Read and Risch 2011;Risch and Read 2015;Marshall and Read 2015), no clear evidence of topographic resonance was discovered.…”

“…Instead the flow always exhibits a drifting wave structure, transitioning from a dominant wavenumber-2 to wavenumber-3, and with occasional transitions to wavenumber-4, as the rotation rate increases. Compared to experiments with pure wavenumber-3 bottom topography, such as investigated by Read and Risch (2011), Marshall and Read (2015) and Risch and Read (2015), amplitude and/or structural vacillations were encountered in similar areas of parameter space, but some points, denoted in Figure 3 as the dashed (green online) Transition region, exhibit a greatly increased number of apparent transitions between wavenumbers over time. In these flows, which lie near the 'irregular' region, a given dominant wavenumber is only foremost for part of the time, with other wavenumbers irregularly dominating the structure.…”

“…In this way, the sensitivity of the problem under investigation to truncation effects can be studied, complementing the results from finite resolution numerical models, regardless of complexity. As such, it appears that a suitable approach would be both to employ a baroclinic annulus flow, thus testing the sensitivity of Tian et al's (2001) barotropic results to the presence of additional vertical degrees of freedom, and to use quantitative analyses, systematically exploring parameter space and thus verifying and extending the qualitative findings of Risch (1999), Read and Risch (2011) and Risch and Read (2015). Combining this with the fully nonlinear interactions of a physical annulus, an investigation into the existence of multiple equilibria could be launched, stringently putting to the test the assertions of Tung and Rosenthal (1985) and Cehelsky and Tung (1987).…”

Rossby Waves in Astrophysics

“…For example, the reader is referred to the works by the groups at Florida State University (Pfeffer et al 1980;Buzyna et al 1984), at Japanese universities (Ukaji and Tamaki 1989;Tajima and Kawahira 1993;Sugata and Yoden 1994;Tajima and Nakamura 2000;Tamaki and Ukaji 2003), at Oxford (Read et al 1992;Read 1997, 1998;Wordsworth et al 2008), in Bremen/Cottbus (von Larcher and Egbers 2005;Harlander et al 2011) and in Budapest (Jánosi et al 2010). Furthermore, researchers have introduced the β-effect in rotating tank experiments through modifying the configuration to mimic the planetary curvature (Mason 1975;Bastin and Read 1997;von Larcher et al 2013;Read et al 2015a;Yadav et al 2016) and zonally asymmetric topography (Leach 1981;Li et al 1986;Bernardet et al 1990;Risch and Read 2015). Very recently, Scolan and Read (2017) proposed a new experimental configuration to add the forcing thermal convection in the cylindrical rotating annulus through heating the bottom near the external wall and cooling the circular disk near the axis at the top surface of the annulus.…”

Thermal versus mechanical topography: an experimental investigation in a rotating baroclinic annulus

“…Most notably, both experiments evidently exhibit a lack of dominant stationary wave activity, both show evidence of topographic interactions and jet-shifting effects as the wave structures drift over the topographic peak or patch of heating elements, creating flows similar to the blocked and zonal states found with periodic bottom topography (such as found by Read and Risch 2011, Risch and Read 2015and Marshall and Read 2015. Both systems were shown to have the same general shape of regime diagram, separated into two separate and distinct regions.…”