Stratospheric Circulation Changes Associated with the Hunga Tonga Hunga Ha'apai Eruption
The 15 January 2022 eruption of the Hunga Tonga-Hunga Ha'apai underwater volcano injected an unprecedented amount of water directly into the stratosphere. This excess water vapor settled near 20 hPa (~25 km) altitude in a latitude band from 30°S to 5°N. With no major thermodynamic or photochemical sinks, the excess moisture is expected to remain in the stratosphere for two to three years. Since water vapor is radiatively active in the infrared, these large perturbations in water vapor are expected to increase the amount of radiation lost to space, locally cooling the stratosphere.
The MERRA-2 (Modern-Era Retrospective Analysis for Research and Applications, Version 2) reanalysis provides the circulation fields, temperatures and winds, shown for the years 1980-2022. The Southern Hemisphere temperature at 20 hPa (Fig. 1) depicts the 2022 cooler than average Southern Hemisphere temperatures. Here, the April through July 2022 temperatures are setting new record lows temperatures at 20 hPa in the Southern Hemisphere.
This cooling is not uniform over the globe but is strongest near 30°S and 20 hPa, the location of the water vapor anomaly (Fig 2). In June 2022, record low temperatures for the month stretch from 55°S to 15°S (Fig. 2). These temperatures break the previous low temperature record by as much 3K. In addition, the zonal mean winds are breaking records by as much as 10 m/s. The location of these record strong winds near the low temperatures is consistent with the geostrophic relation where increased cooling toward the pole requires increased vertical wind shear. In addition to setting records for the month of June, these 2022 low temperatures and strong winds were outside the standard deviation of the year-to-year variability with values greater than double the standard deviation.
These wind and temperature anomalies are likely associated with changes in the mean circulation as the atmosphere adjusts to the temperature perturbation. The mean circulation is best characterized by the residual mean, a zonally averaged quantity that incorporates wave generated transport effects. The climatological, counter-clockwise, poleward and downward circulation in the June Southern Hemisphere is found to be perturbed by a strong clockwise anomaly (Fig. 4). The magenta contour encircles the region where the June clockwise circulation is the strongest in the MERRA-2 record. The blue and red contours highlight regions of negative meridional wind and positive vertical wind (respectively) associated with the circulation anomaly.
In conclusion, the anomalous temperatures and circulation patterns analyzed by MERRA-2 in the Southern Hemisphere during June 2022 can be attributed to the anomalous stratospheric water vapor injection from the January 2022 eruption of the Hunga Tonga-Hunga Ha'apai underwater volcano. The unprecedented amount of water ejected into the stratosphere created significant disruption to the stratospheric circulation lasting up to six months after the eruption. Future work is planned to examine the effect of these changes on trace gases in the stratosphere, such as ozone, as well as global model simulations that include a realistic representation of the stratospheric water anomaly.
References
Coy, L., P. A. Newman, K. Wargan, G. Partyka, S. Strahan, and S. Pawson, 2022. Stratospheric Circulation Changes Associated with the Hunga Tonga-Hunga Ha´apai Eruption. Geophysical Research Letters. Available at https://doi.org/10.1002/essoar.10512388.1
