Storm-track organization and variability in a simplified atmospheric global circulation model

Abstract

A storm track is simulated in a coarse-resolution multilevel primitive-equation model with linear surface friction and heating terms. A restoration temperature distribution consisting of a dipole embedded in a zonally symmetric profile forces the model to simulate the surface heating. Three simulations, each with a different dipole orientation, are performed to test the sensitivity of storm-track organization to the external forcing field. The climatological time-mean circulation and the transient disturbances of the reference simulation agree well with observations of northern hemisphere wintertime storm tracks. Local energetics show that baroclinic instability is responsible for the enhanced eddy kinetic energy downstream of the jet, and the downstream end of the storm track results from the barotropic conversions of eddy kinetic energy to the time-mean flow. Low-frequency fluctuations with a period of about 50 days associated with a retrograding large-scale wave pattern are identified by a complex empirical-orthogonal-function analysis of the vertically averaged stream function. A composite life cycle of the low-frequency variability reveals the growth and decay of a blocking anticyclone downstream of the storm track. A cyclogenesis initiated by an eastward-propagating wave train is observed a few-days before the amplitude of the low-frequency anomaly attains its peak. The net forcing of the high-frequency eddies contributes to the growth and decay of the blocking anticyclone. The results suggest that the occurrence of the blocking-like event is part of the storm-track dynamics.