Tropical Waves and QBO Dynamics in a 7-km Global Climate Simulation

The quasi-biennial oscillation (QBO) in equatorial stratospheric zonal wind is a fairly predictable component of interannual climate variability, with documented effects on tropical cyclone activity and tropical-extratropical teleconnections. Model experiments suggest the QBO frequency and amplitude may change with changing climate, however the sign and magnitude of predicted future changes are sensitive to highly uncertain model details. Furthermore, generating a QBO in atmospheric general circulation models is not a simple task, and the QBO is extremely sensitive to many model parameters, such as horizontal and vertical resolution, gravity parameterization and dynamical core. A few models have been able to spontaneously generate a QBO without the use of parameterized gravity waves, but most models still need to rely heavily on them to get a QBO.

We have investigated the dynamics of the QBO in the global 7-km Goddard Earth Observing System model (GEOS-5) Nature Run (NR). In the tropics, there is evidence that the NR supports a broad range of convectively generated waves. For example, the NR precipitation spectrum resembles that of Tropical Rainfall Measuring Mission in many aspects, including the preference for westward propagating waves. However, even with 7-km horizontal resolution and a healthy population of resolved waves, parameterized gravity wave drag is still the main driver of the model QBO. These results offer clues about the importance of vertical resolution, and choice of diffusion scheme and dynamical core for producing a QBO in global models. We have also evaluated gravity waves in the NR globally. Gravity wave absolute momentum fluxes are put into context with those compared in the Geller et al. [2013] study that compared these fluxes in a variety of models and observational datasets. Overall the spatial distribution and magnitude of gravity wave absolute momentum fluxes resolved in the NR are similar to the fluxes of small-scale parameterized waves in coarse-resolution climate models, providing further evidence that small-scale waves in the NR are behaving realistically.