Eddy memory as a cause of intra-seasonal variability of the Southern Hemisphere baroclinic annular mode

Abstract:

The baroclinic annular mode (BAM) is a leading-order mode of the eddy kinetic energy in the Southern Hemisphere exhibiting oscillatory behavior at intra-seasonal

time scales. The oscillation mechanism has been linked to transient eddy-mean flow interactions that remain poorly understood. Here we demonstrate that the finite

memory effect in eddy heat flux dependence on the large-scale flow can explain the origin of the BAM’s oscillatory behavior. We represent the eddy memory effect by a

delayed integral kernel that leads to a generalized Langevin equation for the planetaryscale heat equation. Considering a mathematical framework for the interactions

between planetary and synoptic scale motions, we derive a reduced dynamical model of the BAM – a stochastically-forced oscillator with a period proportional

to the geometric mean between the eddy memory timescale and the diffusive eddy equilibration timescale. Our model provides a formal justification for the previously

proposed phenomenological model of the BAM and could be used to explicitly diagnose the memory kernel and improve our understanding of transient eddy-mean flow

interactions in the atmosphere.