3.2 Cross Equatorial Transport Shortly after the Eruption
Figure 3 shows maps of water vapor and streamlines at 26.8 km for selected days following the eruption. Rather than average the data over three days, we show the location of MLS profiles and the water vapor mixing ratio. The maximum water vapor is shown at the lower left of each figure. Figure 3a shows the distribution on Jan 16. As noted by Millán et al. (2022), MLS scans do not completely catch the locally concentrated plume. Figure 3b (Jan. 20) shows the anomaly moving toward the equator roughly following the streamlines. By Jan 23 the anomaly has crossed the equator and reached 10°N even though streamlines are mostly zonal. The MERRA2 meridional flow at this altitude is < 2 m/s at ±15°N which means that it would take ~10 days for the plume to transit from 5°S to 10°N, but this transit took place in about 3-4 days. On Jan. 26 the anomaly has reached 10°N. Because of the strong meridional wind shear, and faster winds at the equator, move the equatorial portion ahead of the slower moving higher latitude component (Figs. 3d-3f).
Why did the HT water vapor anomaly move more rapidly to the north between Jan. 20 and Jan. 23? One possible explanation for the movement of the plume toward the equator is that the IR cooling from the water vapor anomaly excited a Rossby wave that advected the water vapor anomaly toward the equator. The simple circulation models of thermally forced equatorial Rossby waves provided by Gill (1980, Fig. 3) would apply. In this scenario, the IR cooling by the water vapor anomaly creates a local pressure anomaly which excites a Rossby wave, creates cross equatorial flow, which advects part of the anomaly across the equator. Because this cooling is not included in the MERRA2 reanalysis (because the MLS water vapor is not assimilated), the strength of the MERRA2 meridional wind is probably underestimated. We have computed the additional IR cooling for Jan 19, using the RTM, and at 27.5 km it is ~3K/day reaching ~5K/day at 30 km. Our estimate of the radiative forcing is in agreement with Silletto et al. (2022) who also noted that the aerosol plume has almost no net radiative impact. This magnitude of localized cooling just off the equator is sufficient to force the Rossby wave (Gill, 1980). After the plume is advected toward the equator and the water vapor distribution becomes more zonal, the non-zonal cooling rate would decrease and the Rossby wave amplitude would decrease as well.
A zonal spectral analysis of the temperature fields provides more insight. Figure 4 shows a zonal wavenumber spectrum at 26.8 km using 3-day average MLS perturbation temperatures. Fig. 4a shows the pre-eruption wave amplitudes vs. latitude on Jan. 13, indicating that the ambient waves are weak, with a ~1K amplitude Kelvin wave centered on the equator. On Jan. 20 (Figs. 4b, 3b), just following the eruption, conditions are immediately different. The thermal amplitude of wave one has nearly doubled north of the HT eruption latitude. The thermal disturbance associated with the spatially narrow plume spreads energy into the higher wavenumbers at 20°S. By Jan. 26, (Fig. 4c, 3c) wave one has increased to 1.5K at about 5°S, and a wave two disturbance has also formed at the HT latitude. By Jan. 26 (Fig. 4d, 3d), the wave one amplitude has increased to > 2K and wave 2 has reached 1.5 K. The waves subsequently begin to decrease in amplitude as seen on Jan. 30 (Fig. 4e, 3e). Wave amplitudes continue to decrease during February (not shown).
The thermal wavenumber analysis is consistent with the idea that H2O IR cooling generates equatorial Rossby waves shortly after the eruption. We can make a rough estimate of the enhanced meridional circulation (v’) generated by the wave using the thermal wind equation and assuming that the heating anomaly has the vertical scale of a scale height (~ 7km). v’ is given by v’=mRT’/f, where f is the Coriolis frequency at 15°S, R is the dry air gas constant, m is the zonal wavenumber and T’ is the temperature. Using T’ = 2 K, v’ ~2.5 m/s. Adding this to the background meridional flow of 2 m/s, the transit time to move the water vapor from 5°S to 15°N is 4.5 days. This is much closer to the observed anomaly transit time from Jan 20-23 period. Finally, to connect with the QBO discussion in section 3.1, Fig. 4f shows the wave amplitudes on April 1. The figure clearly shows wave amplification as the QBO shear line approaches 26 km when compared to Figure 4a.