Abstract:
Large-scale atmospheric motions near the low latitude stratopause in
the UK Meteorological Office stratospheric analyses during the
northern hemisphere winter are investigated, focusing in particular on
the 2-day wave and inertial eddies.
The 2-day wave is a westward propagating planetary wave which appears
recurrently in the summer subtropical upper stratosphere and
mesosphere. Space-time spectral analysis of the assimilated fields of
temperature and winds, and high-resolution transport of water vapour,
reveal large-amplitude breaking planetary waves in the
summer subtropics, principally a 2-day wave 3 and a wave 1 of a period
near 7 days. A region of anomalous zonal-mean meridional gradient of
potential vorticity also exists, pointing to a barotropic instability
mechanism for the origin of the 2-day wave.
By contrast, the winter subtropics are characterized by mixing on
less-than-planetary scales during the period considered, occurring
during the easterly phase of the stratospheric semi-annual
oscillation. Water vapour is mixed turbulently into gyres, and drawn
in narrow filaments out of the southern hemisphere. The potential
vorticity field displays vertically layered, zonally-asymmetric
variations.
The behaviour of the model assimilated fields is supported by
along-track water vapour distribution observed by the Microwave
Limb Sounder aboard the Upper Atmosphere Research Satellite shows
evidence for intermittent filamentary mixing in the northern
subtropics, and a 2-day wave signature in the summer subtropics.
Based on these results, a possible mechanism connecting the initiation
of the austral 2-day wave events to strong planetary wave activity in
the winter is proposed. The winter planetary waves provide the
background conditions for development of inertial instabilities at
low latitudes of the winter hemisphere, which in turn create
conditions favourable for barotropic instability of the summer
subtropical easterly jet.
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