Figure 3 Cross sections for June showing zonal mean zonal winds (10 m/s, filled contours) and temperatures (5K, gray contours (a and b): a) Blue contours: record cold regions where the contours (1 K) denote how much June 2022 is below the previous record low temperature. Red contours: record strong zonal mean winds where the contours (5 m/s) denote how much June 2022 is above the previous record strong wind; b) Blue contours: standard deviations of temperature below the mean (contour interval of 1, starting at -2) and red contours: standard deviations of zonal wind above the mean (contour interval of 1, starting at 2); c) Residual mean stream function (-2, -5, -10, -20\(\times\)108 kg/s) for 1980-2021 average (gray) and 2022 (black); d) residual mean circulation stream function greater than the past maximum value (magenta, contour interval 1\(\times\)108 kg/s) with wind and temperature record as in a); e) Blue contours: standard deviations of residual mean meridional wind below the mean (contour levels of -3 and -2) and red contours: standard deviations of residual mean vertical velocity above the mean (contour level of 2). Dashed lines denote 20oS and 20 hPa.
This cooling is not uniform over the globe but is strongest near 30oS and 20 hPa. In June 2022 record low temperatures for the month stretch from 55oS to 15oS (Fig. 3a). 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 (Fig. 3b) 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 counter-clockwise flow of the residual mean stream function for 2022 (Fig. 3c) shows large distortions in the region near the wind and temperature anomalies compared to the 1980-2022 averaged June residual mean stream function. In particular, the strong vertical gradient in the stream function at 30oS and 30 hPa denotes a stronger that average poleward (negative) flow in 2022. This can be represented as a clockwise anomaly in the residual mean stream function (Fig. 3d). In Fig. 2d, the stream function plotted is greater than any of the previous years in that region. This means that the distortion of the stream function from the mean seen in Fig. 3c is larger than in previous years.
The residual mean circulation can also be expressed in terms of residual mean meridional and vertical velocities (Fig. 3e). The residual mean meridional velocity is particularly striking with negative (poleward) values over three standard deviations below the mean from 10oS-30oS near 30 hPa. The upward mean vertical wind anomalies are over two standard deviations above the mean on the poleward side of the stream function anomaly. This upward anomaly does not correspond to an actual upward circulation but expresses the weaker downward circulation than average as seen in the nearly horizontal stream function regions in Fig. 3c.
4 Conclusions
Anomalous temperatures and circulation patterns analyzed by MERRA-2 in the southern hemisphere during June 2022 can be forensically attributed to the stratospheric water vapor injection from the January 2022 eruption of the Hunga Tonga-Hunga Ha’apai underwater volcano. These anomalies can be traced back to March 2022. Their consistency in space and time suggests a realistic response to a geophysical event rather than a yearly random dynamical fluctuation. In June the record winds are part of an unusual secondary jet maximum at 10 hPa, 30oS-20oS.
These wind and temperature anomalies (Fig. 3a, b) develop from the assimilation of data, mainly routine, satellite based, nadir viewing radiometers and geostrophic balance and are likely to be very realistic. Note that MERRA-2 does assimilate MLS temperatures, but only at pressures of 5hPa and lower. The residual mean circulation might be more difficult to interpret. If the cooling analysis temperature increments mainly reflect the missing water vapor cooling then they can be considered to be the missing cooling term from the lack of stratospheric water vapor in the assimilation system. Then the calculated residual circulation (Fig. 3c, d, e) should realistically capture the perturbed residual circulation. If, however, the analysis temperature increments also contain cooling induced by circulations in the atmosphere’s response to the water vapor perturbation, then it is possible that the residual circulation may adjust in an unphysical manner. However, the good SH 20 hPa agreement between MERRA-2 and M2-SCREAM seen in the sum of the analysis and radiative temperature tendencies (Fig. 2) suggests that the MERRA-2 analysis tendencies are representative of the missing water vapor cooling. Future work is planned for model simulations that include a realistic representation of the stratospheric water perturbations. These calculations should provide a more complete picture of the atmospheric response to the volcanic perturbation.