Title: Understanding midlatitude dynamics and transport in the atmosphere

Abstract
Midlatitude dynamics and transport are important for the distribution of energy, momentum, and constituents in the atmosphere. Midlatitude dynamics is dominated by large-scale eddies, often characterized by overturning of a quasi-conservative quantity (e.g., potential vorticity, water vapor) with both large-scale patterns and fine-scale filaments, which visually link planetary-scale waves to frontogenesis. While eddy momentum and heat fluxes are often interpreted as the result of Rossby wave propagation, eddy potential vorticity (PV) fluxes are also treated by an eddy diffusive closure, which is in contrast to the up-gradient PV fluxes near the jet core. Here we will discuss eddy fluxes in a framework of finite-amplitude wave activity and effective diffusivity, which makes use of mass conservation with respect to quasi-conservative surfaces. This framework shows that the effective diffusivity of tracer transport can be well defined (i.e., non-negative) in spite of the counter-gradient PV flux near the jet core.
We will use the framework to assess the effect of horizontal resolution on the jet latitude in aquaplanet simulations. As the horizontal resolution increases from T42 to T340, the midlatitude jet moves poleward. An analysis of the wave activity budget indicates that the effective diffusivity of upper-tropospheric mixing is reduced on the jet’s poleward flank at higher resolutions but is increased on the equatorward flank of the jet. This explains the change of large-scale circulations due to a primary change in the model horizontal resolution and small-scale diffusion coefficient. The convergence of effective diffusivity with model resolution will also be discussed with implications for the model projection of jet latitude under climate change.

Speaker's website: http://sunspot.eas.cornell.edu/~gc352/