Regional to Global Evolution of Potential Vorticity Generated by Parameterized Mountain Wave Drag

Mountain wave breaking causes regional forcings in Earth’s atmosphere. However, general circulation model (GCM) experiments with and without parameterized mountain wave drag (MWD) indicate that these localized forcings generate a significant zonal-mean or global response. The objective of this research is to understand how these global responses evolve from these regional forcings in the stratosphere. The approach is to track the generation of potential vorticity by MWD and its seasonal evolution in the Whole Atmosphere Community Climate Model (WACCM).

To isolate MWD-generated PV and its attendant flow response, a single 20-year control simulation without MWD was completed. Every winter, a 90-day branch off this control simulation was conducted where the MWD parameterization was turned on. Differencing these runs and averaging over all 20 branches (i.e. winters) allow the MWD-generated influences to be tracked from regional to global scales. MWD-generated PV is advected zonally and apparently diffused meridionally, but zonally symmetric PV banners responses survive this diffusion. The anticyclonic, poleward PV banners diffuse markedly more than the cyclonic, equatorward banners in both hemisphere.

Qualitatively, the evolution from regional to global PV perturbations appears remarkably similar to a horizontal source, advection, and diffusion process with diffusivity peaked in the midlatitudes. A simple horizontal numerical model was constructed that handles a PV source, advection by a planetary-scale vortex, and diffusion with spatially variable diffusivity. Using a domain size, source intensity, and diffusivity guided by the GCM results, simple passive PV advection and diffusion describe the PV evolution in WACCM qualitatively very well. PV generated by parameterized MWD is advected zonally and increasingly horizontally diffused by 2-D resolved waves and mixing, but this diffusion is not large enough to prevent accumulation of MWD-generated PV into global banners. Diffusive fluxes into the smaller polar regions and the no-flux condition at the poles result in anti-cyclonic PV polar caps.