Mass and wind axial
angular-momentum responses to mountain torques in the 1-25 day band:
Links with the Arctic Oscillation
François
Lott and Fabio D'Andrea
Quarterly Journal of the
Royal Meteorological Society, Volume:
131
Number:
608 Page: 1483 -- 1500, 2005.
DOI: 10.1256/qj.03.168
Abstract
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 MR and
mass angular momentum M
. They are driven by mountain
torques TM which are substantially larger than
boundary-layer torques TB. This equipartition
between MR and M occurs because the response to
TM
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. In geostrophic balance with
this mass redistribution, the zonal-mean zonal wind increases where the
force is applied and MR increases as well. This
process leads to MR 
M for parameters representative
of the earth's atmosphere.
This explanation of the
equipartition between M and MR
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 force. When the mountain torque T
Mis produced by mountains located in the Arctic and Antarctic sectors,
the changes in M dominate those in MR.
It is the other way round when TM is produced by
mountains located in the equatorial sector and M MR
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 centred at different
latitudes are specified. The latitudinal dependence of the repartition
between MR and M found in the data is
reproduced by the model with M MR
when the force is centred in the midlatitudes.
The Arctic Oscillation (AO) pattern being associated with substantial M,
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 variations driven by TM.
This result suggests that in this band the mountain ranges
substantially affect the AO variability.
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