The Response of Idealized Stationary-Eddy Circulations to Climate Change

Abstract:

The zonal variation of temperature and precipitation is set by energy and moisture transport by zonally anomalous circulations, or stationary eddies. The magnitude of zonal variations in climate thus changes with the amplitude of stationary eddies. We investigate the response of stationary eddies to climate change across a wide range of climates in an idealized GCM using two types of zonally asymmetric forcing: an extratropical Gaussian mountain and equatorial ocean heat-flux convergence. Orographically forced stationary eddies respond non-monotonically with global-mean temperature in response to changes in zonal surface winds and surface-pressure gradients. The Walker circulation forced by equatorial ocean heat-flux convergence as well as the Rossby waves forced by this divergent flow decrease monotonically throughout the range of climates as the moisture content of the atmosphere increases. We investigate changes in zonally anomalous precipitation minus evaporation (P - E) associated with these stationary-eddy changes. We find that stationary-eddy changes are large such that 'wet gets wetter, dry gets drier' ideas do not apply locally. We find that the zonal variance of P - E is closer to following the changes in atmospheric moisture content, but departs from this expectation within the Walker circulation and in very warm climates, showing a decrease in the amplitude of stationary-eddy circulations in these cases. The ideas demonstrated with these idealized experiments apply to CMIP5 simulations as well, where the zonal variance of P - E increases slower than atmospheric moisture content throughout a range of latitudes indicating a slowdown of stationary-eddy circulations.