Figure Sequence of zonal mean 997 nm aerosol extinction and water vapor
plots starting March 1 (a), April 1, (b), etc. Because OMPS-LP was not
operational on August 1, we plot August 12 in part f. The plots are the
individual days; the data is averaged over 3 adjacent days. The zonal
wind is shown overlaid on the aerosol plots as white contours. The ‘W’
and ‘E’ indicate westerly and easterly regimes. The residual circulation
streamlines (black) are overlaid on the water vapor figures along with
the zero-wind line (white contour). The arrow in Fig. 1c shows the
enhanced spreading of the water vapor below the QBO zero-wind line.
Vertical white and red lines indicate 0° and 15°N for reference.
The upward propagating tropical waves that produce QBO deposit their
momentum in the shear zone centered on the zero-wind line. As wave
momentum is deposited in the shear zone, the zonal wind speed changes,
moving the shear zone downward. Observations and models show that the
secondary circulation surrounding the QBO momentum deposition region
extends ~ 5 km below the shear zone (Baldwin et al.,
2001) and QBO wind anomalies extend horizontally to ~15°
on either side of the equator (Dunkerton and Delisi, 1985). We can
interpret the changes in water vapor in terms of the QBO induced
transport circulation as follows: Between March 1 and April 1, the QBO
descent is very slow, which means that there is little wave momentum
being deposited at upper levels. The QBO secondary circulation is weak,
and the stratospheric circulation is dominated by the seasonal
Brewer-Dobson circulation. The HT water vapor anomaly is confined mostly
to the SH at this stage. Starting in April, the westerlies begin to
descend, the meridional residual circulation below the zero-wind line
begins to transport water vapor northward across the equator. Note that
the residual circulation in the tropics, which is a combination of
seasonal and QBO circulations, is not symmetric across the equator and
the northward transport cell extends into the SH (Randel et al., 1999.
In 2022, this asymmetry may have been amplified by additional water
vapor cooling in the SH (S22). As the zero-wind line continues to
descend into the HT plume, the residual circulation weakens, and
transport slows (June, July). This weakening can be partly attributed to
a seasonal change in the Brewer-Dobson circulation which is strongest
during boreal winter (Plumb, 2002). Thus, the observed changes in the HT
water vapor distribution are broadly consistent with the circulation
surrounding the descending QBO (Plumb and Bell, 1982, Baldwin et al.,
2001) combined with the seasonally changing Brewer-Dobson circulation
(Randel et al., 1999, Gray and Dunkerton, 1990).