  A Martian semiannual oscillation (SAO), similar to that in the Earth's
tropical stratosphere, is evident in the Mars Analysis Correction Data
Assimilation reanalysis dataset (MACDA) version 1.0, not only in the tropics,
but also extending to higher latitudes. Unlike on Earth, the Martian SAO is
found not always to reverse its zonal wind direction, but only manifests itself
as a deceleration of the dominant wind at certain pressure levels and
latitudes. Singular System Analysis (SSA) is further applied on the zonal-mean
zonal wind in different latitude bands to reveal the characteristics of SAO
phenomena at different latitudes. The second pair of principal components (PCs)
is usually dominated by a SAO signal, though the SAO signal can be strong
enough to manifest itself also in the first pair of PCs. An analysis of terms
in the Transformed Eulerian Mean equation (TEM) is applied in the tropics to
further elucidate the forcing processes driving the tendency of the zonal-mean
zonal wind. The zonal-mean meridional advection is found to correlate strongly
with the observed oscillations of zonal-mean zonal wind, and supplies the
majority of the westward (retrograde) forcing in the SAO cycle. The forcing due
to various non-zonal waves supplies forcing to the zonal-mean zonal wind that
is nearly the opposite of the forcing due to meridional advection above ~3 Pa
altitude, but it also partly supports the SAO between 40 Pa and 3 Pa. Some
distinctive features occurring during the period of the Mars year (MY) 25
global-scale dust storm (GDS) are also notable in our diagnostic results with
substantially stronger values of eastward and westward momentum in the second
half of MY 25 and stronger forcing due to vertical advection, transient waves
and thermal tides.
