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

Using the NCAR/NCEP reanalysis data, we analyse the atmospheric angular momentum M response to torques T in the 1-25 d spectral band. At these periodicities, the variations in M are equally distributed between variations in wind angular momentum M_R and mass angular momentum M_Ω. They are driven by mountain torques T_M which are substantially larger than boundary-layer torques T_B. This equipartition between M_R and M_Ω occurs because the response to T_M in most cases satisfies the geostrophic balance, and because the major mountain ranges are located in the midlatitudes. At these latitudes, an external positive zonal-mean zonal force is in good part equilibrated by a flux of mass equatorward through the Coriolis force, a process that increases M_Ω. In geostrophic balance with this mass redistribution, the zonal-mean zonal wind increases where the force is applied and M_R increases as well. This process leads to M_R ≈ M_Ω for parameters representative of the earth's atmosphere. This explanation of the equipartition between M_Ω and M_R 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 T_M hence varying the importance of the Coriolis force. When the mountain torque T_M is produced by mountains located in the Arctic and Antarctic sectors, the changes in M_Ω dominate those in M_R. It is the other way round when T_M is produced by mountains located in the equatorial sector and M_Ω ≈ M_R when T_M is due to mountains located in the subtropics or in the midlatitudes. The second is based on results from a one-layer shallow-w ater axisymmetric model on a sphere, where zonal body forces centred at different latitudes are specified. The latitudinal dependence of the repartition between M_R and M_Ω found in the data is reproduced by the model with M_Ω ≈ M_R when the force is centred in the midlatitudes. The Arctic Oscillation (AO) pattern being associated with substantial M_Ω, the significance of these results for the atmospheric circulation variability is also discussed. In the 1-25 d band, the AO variations are very significantly related to M_Ω variations driven by T_M. This result suggests that in this band the mountain ranges substantially affect the AO variability.