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).