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.