Mechanisms of Stationary Rossby Wave Change in a Changing Climate

Abstract:

Stationary or low-frequency Rossby waves are the primary drivers of midlatitude weather on monthly and longer timescales. They take the form of blocking highs which can lead to droughts downstream 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. The stationary Rossby wave response to warming depends on the latitude and type of forcing. We find that the primary mechanism of change is in the downstream propagation of energy which is well described by linear wave theory and thus depends primarily on the structure of the zonal mean jet. For example, as the tropopause height increases allowing the jet to get stronger with warming, the stationary wavenumber decreases, leading to, among other things, an expansion of dry zones associated with stationary wave subsidence. Additionally, as the easterlies expand, the influence of extratropical stationary wave sources is no longer as prominent in the subtropics. Deviations from these linear responses are examined in terms of the vorticity budget. Nonlinear effects remain secondary to linear theory throughout the range of climates. We compare these results with the stationary wave response in the CMIP5 RCP8.5 scenario.