Figure Maps of the MLS water vapor at 26.8 km (~ 21.5
hPa) using 3 days of data centered on the date shown. Temperatures (also
from MLS) are shown with black contours. The streamlines (white arrows)
are generated using MERRA2 winds. The dates correspond to those in Fig.
1.
From the simple models of the QBO, we expect that waves to amplify as
the shear zone approaches from above, and then wave amplitudes should
decrease as the shear zone passes. The change in wave activity occurs
due to conservation of wave action density – the wave energy divided by
the frequency (Andrews et al., 1987, Eq 4A.12). As the wave propagates
upward toward its critical line, the group velocity decreases, and the
wave amplitude increases. This should enhance the variance in trace gas
fields if a tracer gradient is present. Figure 2 shows maps of the MLS
water vapor distribution and temperatures at 26.8 km
(~21.5 hPa) along with streamlines from MERRA2 winds.
The H2O distribution on April 1 shows a wave structure
at the northern edge of the anomaly, and the temperature and streamlines
show more non-zonal structure. By May 1 the water vapor distribution
uniformly extends to 20°N and the wave structures in tropical wind and
temperature fields have decreased. The wave structure seen on April 1
might be expected from the amplification of the Kelvin wave as it
approaches the critical line. Then, in the subsequent months
(June-August), the water vapor distribution becomes more zonally uniform
along with the wind and temperature fields. We have examined the time
variation of the water vapor variance at 26.8 km and indeed it increase
as the QBO moves downward to this altitude and then abruptly decreases
with the passage of the shear zone. The equatorial seasonal upward
residual circulation also switches from ascending to descending as the
QBO shear zone passes then returns to ascending as expected from the
simple QBO models (Plumb and Bell, 1982).