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).