Journal of the Atmospheric, October 2007,
In press
Summary:
The diurnal and
sub-diurnal variations of the mass and wind terms of the axial
Atmospheric Angular Momentum (AAM) are explored using a 1-year
integration of the LMDz-GCM, twelve 10-day ECMWF forecasts and
some ECMWF Analysis products. In these datasets, the wind and mass AAMs
present diurnal and semi-diurnal oscillations which tendencies far
exceed the total torque.
In the LMDz-GCM, these diurnal and semi-diurnal oscillations are
associated with axisymmetric (s=0) and barotropic circulation modes
that resemble to the second gravest (n=2) Eigensolution of the
Laplace's tidal equations. This mode induces a Coriolis conversion from
the wind AAM toward the mass AAM that far exceeds the total torque.At
the semi-diurnal period, this mode dominates the axisymmetric and
barotropic circulation. At the diurnal period, this $n=2$ mode is also
present, but the barotropic circulation also presents a mode resembling
to the first gravest ($n=1$) Eigensolution of the tidal equations. This
last mode does not produce anomalies in the mass and wind AAMs.
A shallow water axisymmetric model driven by zonal mean zonal forces
which vertical integral equal the zonal mean zonal stresses
issued from the GCM is then used to interpret these results. This model
reproduces well the semi diurnal oscillations in mass and wind
AAMs, and the semi-diurnal mode resembling to the n=2 Eigensolution
that produces them,
when the forcing is distributed barotropically in the vertical
direction. This model also reproduces diurnal modes resembling to the
n=1 and n=2 Eigensolutions when the forcings are distributed more
baroclinically. Among the dynamical forcings that produce these modes
of motion, we found that the mountain forcing and the divergence of the
AAM flux are equally important, and more efficient than the boundary
layer friction.
In geodesy, the large but opposite signals in the mass and wind
AAMs due to the n=2 modes can lead to large errors in the evaluation of
the AAM budget. The n=2 responses in surface pressure can affect
the Earth Ellipcity, and the n=1 diurnal response can affect the
geocenter position. For the surface pressure tide, our results suggest
that the dynamical forcings of the zonal mean zonal flow are a
potential cause for its $s=0$-component.