Lower Tropospheric Eddy Momentum Fluxes in Idealized Models and Reanalysis Data

The eddy momentum fluxes (EMFs) in the lower tropospheres of a two-layer quasi-geostrophic model, a dry primitive equation model and the southern hemisphere of a reanalysis dataset are investigated. The EMF co-spectra are very similar in the two models, showing a dipole structure in the center of the jet with EMF convergence at phase speeds close to the lower layer jet speed and EMF divergence at slower phase speeds. These regions are bordered on the flanks of the jet by regions of EMF divergence and EMF convergence, respectively. Calculating the spectra as a function of wavenumber shows that long, slow waves are responsible for the EMF divergence, while short, fast waves are responsible for the EMF convergence. Many of the same features are seen in the co-spectra of the reanalysis data, particularly in austral summer. This suggests that the same fundamental dynamics are at play in the models as well as in the reanalysis data, so the two-layer model can be used to interpret the lower tropospheric dynamics.

We attribute the EMF divergence to the weak, negative potential vorticity gradient in the lower troposphere and also to the width of the disturbance which excites the baroclinic eddies. Together these mean that the lower troposphere cannot be separated into a stirred region and a propagating region. Instead, lower tropospheric eddies that are excited near their critical latitudes by the stirring break and accelerate the flow there. There is compensating deceleration in the center of the domain where there is eddy generation but no breaking. The EMF convergence at faster phase speeds is due to coupled, equivalent barotropic modes which mostly propagate on the upper tropospheric potential vorticity gradient. These modes accelerate the flow where they form and decelerate the flow where they break.