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.