Mass and wind atmospheric angular momentum responses to mountain torques in the 1--25 day band. Links with the Arctic Oscillation.

Using the NCAR/NCEP reanalysis data, we analyze the atmospheric angular momentum (M) response to torques (T) in the 1--25~day spectral band. At these periodicities, the variations in M are equally distributed between variations in wind angular momentum (Mw) and mass angular momentum (Mm). They are driven by mountain torques (Tm) which are substantially larger than the boundary layer torques (Tb). This equi-repartition between Mw and Mm follows that the zonal flow response to Tm is in geostrophic balance, the major mountain ranges being located in the mid-latitudes. At these latitudes, an external positive zonal mean zonal force is in good part equilibrated by a flux of mass equatorward through the Coriolis torque, a process that increases Mm. In geostrophic balance with this mass redistribution, the zonal mean zonal wind increases where the force applies and increases Mw as well.

The fact that this picture explains the equi-repartition between Mm and Mw is confirmed by two pieces of independent evidence. The first is based on the reanalysis data, in which we evaluate the contribution of six non-overlapping latitudinal sectors to Tm hence varying the importance of the Coriolis torque. When the mountain torque is produced by mountains located in the Arctic and Antarctic sectors, the changes in Mm dominate those in Mw. It is the other way round when Tm is produced by mountains located in the Equatorial sector and Mw is near Mm when Tm is due to mountains located in the subtropics or in the midlatitudes.

The second is based on results from a one layer shallow water axisymmetric model on a sphere, where zonal body forces centered at different latitudes are specified. The latitudinal dependence of the repartition between Mm and Mw found in the data is reproduced by the model with Mm near equals Mw when the force is centered in the mid-latitudes.

The Arctic Oscillation (AO) pattern being associated with substantial Mm, the significance of these results for the atmospheric circulation variability is also discussed. In the 1-25~day band, the AO variations are very significantly related to Mm variations driven by Tm. This result suggests that in this band the mountain ranges affect the AO variability substantially.