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.