Zonal Hydrological-Cycle Variations in Idealized Model Experiments

Stationary or low-frequency Rossby waves are the primary drivers of midlatitude weather on monthly and longer timescales. They take the form of quasi-stationary highs which can lead to droughts or persistent extratropical lows which guide storms into the coastal midlatitudes. We investigate the stationary Rossby wave response to topography and ocean heating in an idealized GCM, simulating a wide range of climates by varying the longwave optical depth. Associated with changes in stationary Rossby waves are changes in the zonal asymmetry of the hydrological cycle. Particularly in the subtropics, our experiments show decreasing zonal variation of P - E in climates much warmer than modern despite the ``wet gets wetter, dry gets drier" effect associated with increasing atmospheric moisture content. We develop a scaling for zonally anomalous P - E based on zonal-mean surface specific humidity and stationary-eddy vertical motion that disentangles the roles of thermodynamic and dynamic changes. This scaling is then applied to investigate if there is a robust departure of zonal P - E variation from Clausius-Clapeyron in the CMIP5 RCP8.5 scenario.