How Stationary Eddies Shape Changes in the Hydrological Cycle: Zonally Asymmetric Experiments in an Idealized GCM

Abstract

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 2 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 2 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.

Local Energetic Constraints on Walker Circulation Strength

“…These simulations are described in further detail in Wills and Schneider (2016). The zonally anomalous Q flux is constrained to the equatorial regions and has the form and s f 5 88, as in Wills and Schneider (2016). Our simulations differ from those in Merlis and Schneider (2011) only by small differences in the aspect ratio (zonal vs meridional elongation) and amplitude of the Q fluxes.…”

“…Precipitation changes are small as a consequence of small changes in zonally anomalous evaporation, which is constrained energetically by the prescribed ocean heat flux convergence, and small changes in zonally anomalous precipitation minus evaporation (P 2 E), which results from a cancellation between thermodynamic and dynamic changes in the moisture budget with warming. This cancellation is also seen in comprehensive climate models (Seager et al 2010;Chadwick et al 2013;Wills et al 2016) but is a direct consequence of the reduction in circulation strength, thus making this a circular argument. The atmospheric moisture budget provides a useful link between changes in circulation and changes in precipitation, but it does not provide a way of knowing one without assuming the other.…”

“…These simulations are described in further detail in Wills and Schneider (2016). The zonally anomalous Q flux is constrained to the equatorial regions and has the form and s f 5 88, as in Wills and Schneider (2016).…”

“…Our simulations differ from those in Merlis and Schneider (2011) only by small differences in the aspect ratio (zonal vs meridional elongation) and amplitude of the Q fluxes. They were chosen as a representative example from the suite of simulations in Wills (2016). This zonally asymmetric Q flux integrates to zero in the zonal mean, such that its effect on the zonal-mean circulation (i.e., Hadley cells) is negligible.…”

“…Associated with this weakening is a reduction of tropical precipitation contrasts (Wills and Schneider 2016;Wills et al 2016) and a modification of extratropical Rossby waves, which influence midlatitude temperature and precipitation patterns (Seager et al 2005). A quantitative theory for the strength of convective mass fluxes and zonally anomalous tropical overturning circulations is needed to constrain the associated impacts on tropical precipitation and midlatitude teleconnections.…”