How Stationary Eddies Shape Changes in the Hydrological Cycle: Zonally Asymmetric Experiments in an Idealized GCM
Stationary and low-frequency Rossby waves are the primary drivers of extratropical weather variations on monthly and longer time scales. They take the form of persistent highs and lows, which, for example, shape subtropical dry zones and guide extratropical storms. More generally, stationary-eddy circulations, including zonally anomalous tropical overturning circulations, set up large zonal variations in net precipitation (precipitation minus evaporation, P − E). This paper investigates the response of stationary eddies and the zonally asymmetric hydrological cycle to global warming in an idealized GCM, simulating a wide range of climates by varying longwave absorption. The stationary eddies are forced by two idealized zonal asymmetries: a midlatitude Gaussian mountain and an equatorial ocean heat source. Associated with changes in stationary eddies are changes in the zonal variation of the hydrological cycle. Particularly in the subtropics, these simulations show a nearly constant or decreasing amplitude of the zonally anomalous hydrological cycle in climates warmer than modern despite the wet gets wetter, dry gets drier effect associated with increasing atmospheric moisture content. An approximation for zonally anomalous P − E, based on zonal-mean surface specific humidity and stationary-eddy vertical motion, disentangles the roles of thermodynamic and dynamic changes. The approximation shows that changes in the zonally asymmetric hydrological cycle are predominantly controlled by changes in lower-tropospheric vertical motion in stationary eddies.
© 2016 American Meteorological Society. Manuscript received 31 October 2015, in final form 9 February 2016. This research has been supported by NSF Grant AGS-1019211. The idealized GCM simulations for this study were performed on ETH Zürich's EULER computing cluster. We thank Michael Byrne, Xavier Levine, Timothy Cronin, and an anonymous reviewer for useful comments and discussion during the development of this manuscript.
Published - jcli-d-15-0781.1.pdf