Plain Language Summary
The 15 January 2022 eruption of the Hunga Tonga-Hunga Ha’apai underwater
volcano injected a record amount of water directly into the
stratosphere. This study attempts to quantify this impact on the
temperature, as well as the subsequent changes to the stratospheric
circulation, during the months following the eruption. The extreme
nature of the stratospheric temperature, wind, and circulation changes
are tracked through comparisons of the first six months of 2022 with the
past 42 years. Details of the stratospheric perturbations in latitude
and pressure are presented for June 2022, where anomalously low
temperatures are found at near 20 km altitude from
60oS to 30oS. In response to this
cooling the atmosphere adjusts by creating strong westerly winds above
the temperature anomaly and large changes to the downward and poleward
mean meridional circulation.
1 Introduction
The 15 January 2022 eruption of the Hunga Tonga-Hunga Ha’apai underwater
volcano injected an unprecedented amount of water directly into the
stratosphere (Millán et al., 2022; Xu et al., 2022; Carr et al., 2022).
While the initial injection plume at 20oS reached to
the upper stratosphere (Carr et al., 2022), Millán et al. (2022), showed
that after three months this excess water vapor settled near 20 hPa
altitude in a latitude band from 30oS to
5oN. With no major thermodynamic or photochemical
sinks, this excess moisture is expected to remain in the stratosphere
for two to three years. Water vapor is radiatively active in the
infrared, contributing to the total radiative cooling in the
stratosphere, which is dominated by the effects of carbon dioxide and
ozone (e.g., Gille and Lyjak, 1986). These large perturbations in water
vapor are expected to increase the amount of radiation lost to space,
locally cooling the stratosphere. This study attempts to quantify this
impact on the temperature, as well as the subsequent changes to the
stratospheric circulation, during the first six months after the
eruption.
The MERRA-2 (Modern-Era Retrospective Analysis for Research and
Applications, Version 2) reanalysis (Gelaro et al., 2017) provides the
circulation fields (temperatures and winds) for this study. While
MERRA-2 assimilates a number of in-situ and space-based observations
that constrain tropospheric moisture, stratospheric water vapor
observations are not assimilated. Stratospheric moisture is closely
constrained to monthly climatologies imposed by a relaxation constraint
in the GEOS model, so that the stratospheric water vapor in MERRA-2 does
not respond to the volcanic perturbation. This precludes the use of
MERRA-2 to directly infer the thermal impacts of the water vapor
increase on the circulation. However, numerous nadir- and limb-sounding
microwave observations are assimilated in the stratosphere (McCarty et
al., 2016) and these data will constrain the analyzed temperatures,
which can be used to indirectly infer the thermal and dynamical response
of the stratosphere in MERRA-2. Additionally, a more recent data
assimilation system, M2-SCREAM (described below), does contain
additional water vapor and is used to compare with the MERRA-2 analysis
and radiative temperature tendencies.
2 Assimilation Products
The 43-year (1980-2022) climate record from MERRA-2 is used to assess
the anomalies of 2022. Two sets of monthly averaged files are used: The
assimilation files for winds and temperatures (GMAO, 2015a), and the
temperature tendency files to obtain the analysis temperature tendencies
(GMAO, 2015b). The residual zonal mean circulations for each month are
calculated from the monthly averaged assimilation files as, in addition
to winds and temperatures, these files also contain the heat and
momentum fluxes needed for evaluation of the residual circulation (see
Andrews et al., 1987, page 128). For this study we examine the stream
function of the residual circulation as well as the residual mean
meridional and vertical winds.
The MERRA-2 Stratospheric Composition Reanalysis of Aura Microwave Limb
Sounder (M2-SCREAM: Wargan et al., 2022) is a new stratosphere-focused
reanalysis product developed at NASA’s Global Modeling and Assimilation
Office. M2-SCREAM assimilates stratospheric profiles of ozone, water
vapor, hydrogen chloride, nitric acid, and nitrous oxide from version
4.2 retrievals of the Microwave Limb Sounder (MLS: Waters et al., 2006;
Livesey et al., 2020) observations, the same as those used in Millán et
al. (2022), alongside total ozone observations from the Ozone Monitoring
Instrument (Levelt et al., 2006, 2018). Temperature, winds, surface
pressure, and tropospheric water vapor in M2-SCREAM are constrained by
the MERRA-2 assimilated fields. M2-SCREAM covers the MLS period from
September 2004 to May 2022. Details of the M2-SCREAM system are
described in Wargan et al., (2020; 2022). Because this reanalysis
assimilates MLS data, unlike MERRA-2, it contains a representation of
the water vapor enhancement from the Hunga Tonga eruption in agreement
with MLS. The total increase of stratospheric water vapor mass
calculated from M2-SCREAM is 10% as in Millán et al. (2022). As shown
below, the radiative transfer model in M2-SCREAM responds to the
moisture enhancement by producing long wave cooling in better agreement
with the observations.
3 Results