Dynamical Consequences of Polar Amplification on Standing Rossby Waves: a Laboratory Perspective

Abstract

Polar amplification describes the predicted reduction in the latitudinal surface temperature gradient, which will have physical implications for mid-latitude dynamics. The precise nature of these dynamical consequences remains unclear. Here we explore aspects of polar amplification by way of 24 distinct idealised laboratory experiments. The apparatus employed can independently prescribe laboratory analogues for the latitudinal temperature gradient ($Δ{T}$, which controls the stratification $N$), background zonal flow speed ($U_{b}$), and strength of the background gradient in potential vorticity ($β$). The ability to control these processes individually is beneficial as decoupling them from one another enables their influences can be examined separately. Reducing the sidewall temperature difference substantially reduces small-scale and high frequency dynamics, but does not affect the large scale features of the flow, including the north-south amplitude of standing meanders. Reducing the zonal flow speed does reduces the length-scales and amplitudes of the standing Rossby waves, while reducing the potential vorticity gradient has the opposite effect; these responses are well described by the canonical expression relating the standing Rossby wavelength to $\sqrt{U_{b}/β}$. Variability is partitioned into components that are standing and transient; the response of this variability partitioning depends on all 3 experimental parameters, and a non-dimensional term is developed ($U_{b}β/N^{2}$) which captures the behaviour of the variability. These findings suggest that the dynamical consequences of polar amplification is a tendency for mid-latitude weather to shift away from transient storms towards more persistent events, however the zonal wavelength and north-south extent of these persistent events will tend to decrease.