Atmospheric dynamics of the zonally symmetric extratropical response to ENSO

It has been recognized for decades that extratropical responses to variations in ocean temperatures and atmospheric convective activity in the equatorial Pacific Ocean (denoted ENSO, El Niño-Southern Oscillation) are characterized by a stationary train of Rossby waves emanating from that region. Recently, however, studies of observations and of the output of general circulation models have revealed that the extratropical response to ENSO includes a component projecting strongly on the zonal mean, and, that this symmetric component is important for organizing global precipitation anomalies associated with ENSO. Increased precipitation occurs in zonal bands of rising motion; in these bands the atmosphere cools, in marked contrast to the global temperature increase associated with ENSO's warm phase.

The observed and GCM modeled responses to ENSO are readily reproduced in simple dry models and are readily understood using the quasigeostrophic theory of eddy-zonal flow interactions. ENSO's warm phase induces an intensification and equatorward shift of the zonally averaged storm track and eddy-driven jet. The climatological influence of baroclinic eddies is, counter intuitively, to increase the equatorward temperature gradient across the latitudes of greatest eddy generation; thus more intense eddy generation leads to poleward cooling.

Implications of these results for global climate change will also be discussed.