4. Summary and Discussion
The HT injection of aerosols and water into the mid-stratosphere
provides an unprecedented opportunity to examine our understanding of
tropical stratospheric dynamics and interhemispheric transport of trace
gases. Trajectory simulations of the plume spread show almost no
mod-stratospheric transport across the equator during first 5 months
after the eruption (S22); nonetheless, at least two cross equatorial
transport events occurred. The first, shortly after the eruption and the
second during April and May 2022. Explanation for these events is given
in this paper.
The initial HT plume moved ~ 30° northward within the
first few weeks after the eruption (Fig. 3) even though the pre-eruption
flow was approximately zonal with weak wave activity at tropical
latitudes. The northward advection of the plume may have resulted from
strong H2O IR cooling of the plume, and the subsequent
non-zonal radiative cooling would force an equatorial Rossby wave
response (Gill, 1980). The resulting cross equatorial flow would have
transported the plume meridionally. Wavenumber analyses of MLS
temperatures show a coincidental rapid increase in wave one and two
across throughout tropics, consistent with this hypothesis. The
meridional cross-equatorial velocity may have more than doubled due to
the presence of the wave. By the end of January, the forced Rossby wave
subsides as the water vapor plume shears out and the localized
(non-zonal) forcing decreases.
During March, the QBO shear zone began to descend through the tropics
switching the zonal winds from easterlies to westerlies in the
mid-stratosphere. The induced circulation produced by wave momentum
deposition combined with the Brewer-Dobson circulation produces a second
cross-equatorial transport event. This event is most evident at
~26 km where the meridional water vapor gradient is
large. The QBO transport both observed in the MLS water vapor mixing
ratios, and as diagnosed through the residual circulation, is consistent
with earlier analyses of QBO dynamics (Baldwin et al., 2001; Randel et
al., 1999). However, the circulation well below the QBO shear zone
appears to prevent a similar spread in the aerosol distribution.
The fact that these two transport events were not reproduced by
trajectory simulations (S22) suggests the need for additional
improvements in MERRA2 tropical dynamics, and the need for stratospheric
water vapor assimilation – at least during the HT period. Finally,
although the SH and NH tropical stratospheres appear to be relatively
isolated under normal conditions (Stolarski et al., 2014), the evolution
of the HT plume reveals that the QBO can play an important, albeit
episodic, role in trace gas exchange between the two hemispheres.