3.1 Cross Equatorial Transport associated with the QBO
Unrelated to the HT eruption, during the 2022 spring and summer, the tropical stratospheric winds switched from easterly to westerly due to the quasi-biennial oscillation (QBO) (see review by Baldwin et al., 2001). The descending westerly phase QBO produces a secondary circulation with downwelling at the equator – roughly the locus of the zero-wind line - and upwelling north and south of the equator (Plumb and Bell,1982). This secondary circulation will alter the distribution of trace gases such as ozone and water vapor. The induced circulation contributes to the mixing of the lower stratospheric trace gases within the tropics, and between the hemispheres as is evident in observational data sets (Anstey et al., 2022; Baldwin et al., 2001; Randel et al., 1998). The simple models of the QBO assume that the secondary circulation is symmetric about the equator so cross equatorial transport would not be possible in that framework, but the observed structure of the QBO circulation is not equatorially symmetric and the cross-equatorial circulation can be quite strong (Randel et al., 1999). The QBO circulation asymmetry is likely due to hemispheric differences in the upward gravity wave momentum flux that contributes to the QBO (Anstey et al., 2022; Baldwin et al., 2001).
Figure 1a-f shows the evolution of the OMPS-LP aerosol extinction (Taha et al, 2021) and MLS zonal mean water vapor. The MERRA2 zonal mean wind is also shown along with the residual circulation streamlines. The observations are shown at the first of each month except for August where we show the 12th, because OMPS-LP was offline at beginning of the month. We begin in March when the HT water vapor field becomes zonally well mixed as indicated by the MLS observations (Fig. 2a). The initial water vapor and aerosol distribution is primarily south of 10°N. The figure shows that the water vapor is concentrated mostly above 20 km where the warmer stratosphere can support higher concentrations (S22). The aerosols are initially distributed from the tropopause to approximately the same altitude as the water vapor, but the two distributions slowly separate in time with the water vapor anomaly rising while the peak altitude of the aerosol anomaly descends as noted in S22.
The Fig. 1 sequence shows the descent of the tropical QBO westerlies as see in the downward propagation of the zero-wind line. Between March 1 and April 1 there is little descent of the equatorial westerlies above about 30 km. Then, beginning in April, the westerlies begin to descend rapidly. By May 1, the top of the aerosol distribution has spread deeper into the SH and a secondary maximum in water vapor has appeared in the NH (see arrow). The residual streamlines shown overlaid on the water vapor plots provide an explanation for the changing aerosol and water vapor distributions. In March, the ~20°S upward transport of water vapor is consistent with the residual circulation (S22). In April, the streamlines shift, and the residual circulation begins to transport water vapor toward the north. By May 1 (Fig. 1c), a lobe of water vapor has formed in the Northern Hemisphere (NH) moving north of 15°N. The northward residual circulation is still present on May 1 but has weakened, although the water vapor anomaly continues to slowly expand northward. At lower altitudes the southern branch of the residual circulation is transporting the aerosol distribution further south.
By July, above the tropical zero-wind line within the westerly wind regime, the ascending branch of the residual circulation in the NH tropics reinforces a descending branch in the SH tropics. This circulation cell transports dry air downward into the HT anomaly while pulling the northern edge of the anomaly upward. This transport creates the U-shaped structure in water vapor seen in July and August. The aerosol anomaly, which has continued to settle throughout this sequence, does not show the cross-equatorial transport seen in the water vapor field. The residual circulation at the lower altitude does not have a northward (poleward) component during this period, so the aerosols do not spread north of 15°N.