Intermittent baroclinic instability and fluctuations in geophysical circulations

Abstract

The presence of blocking highs in atmospheric circulation, bringing periods of stable weather alternating with more variable weather patterns, is an example of two distinct modes of behaviour occurring under similar conditions in a natural system. The occurrence of two or more different flow regimes under a given external forcing is a consequence of nonlinear dynamics that allows the system to equilibrate in different modes. Each mode can be stable over a range of conditions and yet there may be sudden transitions between modes. We have now discovered in a laboratory experiment using rotating fluids a case of at least two modes of behaviour under identical conditions. In one mode, a weak continuous forcing drives a mean circulation that is steady but on which are superimposed waves whose amplitudes vacillate under the influence of viscous dissipation. Under suitable conditions there is an alternative mode in which the mean flow fluctuates and unstable waves occur only in short bursts at periodic intervals separated by relatively long periods of stable flow with no waves. The existence of multiple equilibrium states is of interest in the climatology of all geophysical and astrophysical fluid bodies. Our discovery of a mode involving intermittent bursts of instability and long-period fluctuations of mean flow (resulting purely from internal dynamics rather than from variations in external forcing) may have implications for the variability of oceanic circulation or solar radiation on timescales of a few years to centuries.