The Hunga Tonga-Hunga Ha'apai Hydration of the Stratosphere.

Following the 15 January 2022 Hunga Tonga-Hunga Ha'apai eruption, several trace gases measured by the Aura Microwave Limb Sounder (MLS) displayed anomalous stratospheric values. Trajectories and radiance simulations confirm that the H2O, SO2, and HCl enhancements were injected by the eruption. In comparison with those from previous eruptions, the SO2 and HCl mass injections were unexceptional, although they reached higher altitudes. In contrast, the H2O injection was unprecedented in both magnitude (far exceeding any previous values in the 17-year MLS record) and altitude (penetrating into the mesosphere). We estimate the mass of H2O injected into the stratosphere to be 146 ± 5 Tg, or ∼10% of the stratospheric burden. It may take several years for the H2O plume to dissipate. This eruption could impact climate not through surface cooling due to sulfate aerosols, but rather through surface warming due to the radiative forcing from the excess stratospheric H2O.

Introduction

Hunga Tonga‐Hunga Ha'apai (HT‐HH), a submarine volcano in the South Pacific (20.54°S, 175.38°W), reached its climactic eruption phase on 15 January 2022. The blast sent a volcanic plume into the mesosphere to altitudes of up to 57 km—a record in the satellite era (Carr et al., 2022; Proud et al., 2022). It also triggered tsunami alerts across the world (Carvajal et al., 2022; Ramirez‐Herrera et al., 2022), waves that propagated globally (Wright et al., 2022), and ionospheric disturbances (Themens et al., 2022). Details about the HT‐HH caldera complex, seismology, and volcanology are given by Kusky (2022) and Yuen et al. (2022).

In addition to particulate matter, volcanic eruptions can loft large quantities of gases into the stratosphere. Although around 80% of this gas volume can be magmatic H2O (Coffey, 1996; Pinto et al., 1989), up to 90% of the volcanically emitted humidity is usually removed by condensation at the cold point tropopause (Glaze et al., 1997). Considerable amounts of CO2 and SO2 are also often found in volcanic plumes, along with HCl and other trace gases (e.g., Carn et al., 2016). SO2 reacts with H2O and OH to form submicron sulfate aerosols that reflect solar radiation, lowering surface temperature. For example, the radiative influence of the 1991 Mount Pinatubo eruption “put an end to several years of globally warm surface temperature” (McCormick et al., 1995), illustrating the capacity of volcanic eruptions to substantially alter global climate.

The composition of the HT‐HH plume is unprecedented, as the eruption injected vast amounts of H2O directly into the stratosphere. The high moisture content of the plume is perhaps not surprising since the HT‐HH caldera was situated 150 m below sea level (Cronin et al., 2017), where water in contact with the erupting magma (at temperatures of ∼1100–1470 K) was superheated, resulting in explosive steam.

The Microwave Limb Sounder (MLS) onboard NASA's Aura satellite provides measurements of 15 trace gases, among them H2O, HCl, and enhanced volcanic SO2. MLS measures thermal emission from the Earth's limb, covering spectral regions near 118, 190, 240, and 640 GHz (Waters et al., 2006). MLS is well suited to observe volcanic plumes since microwave radiances are largely unaffected by sulfate aerosols. Moreover, the MLS two‐dimensional retrieval exploits overlapping limb observations to better constrain trace gas gradients (Livesey et al., 2006), allowing the spatial heterogeneity of the plume to be captured.

Here, we use MLS version 4 (v4) data, instead of the most recent version (v5). Estimates of instrument pointing are required for atmospheric composition profile retrievals. V4 obtains pointing purely from O2 signals, whereas v5 also uses the H2O line. Poor fits to H2O signals in regions with extremely enhanced humidity, such as those discussed here, lead to discrepancies in pointing as large as ∼2.5 km, degrading the accuracy of some v5 products.

Validity of MLS Measurements After the Eruption

Ten hours after the eruption on 15 January, MLS measured enhanced values of H2O at altitudes up to 0.46 hPa (∼53 km), well above the stratopause (Figure 1c). Most of these measurements of enhanced H2O did not pass the MLS quality screening (QS) criteria defined by Livesey et al. (2020), indicating that the retrieval achieved only a poor fit to the radiances. The poor performance of the standard data processing algorithms is unsurprising, as the largest H2O values are more than an order of magnitude greater than any previously observed by MLS and more than 100 standard deviations above background levels. Here, data points with values greater than 7 standard deviations above the climatological January–February–March 2005–2021 average are identified as enhancements.

Figure 1

(a) Location of observed H2O enhancements on 14 and 15 January. (b) Location of maximum H2O on 15–18 January. Lines display back trajectories from these measurements to the eruption time. Triangles mark the volcano location. (c) H2O profiles associated with locations shown in (a). The temperature profile (red dashed line) is the average of the temperature profiles retrieved by the Microwave Limb Sounder (MLS) at those locations. (d) H2O profiles associated with locations shown in (b). The 2005–2021 January–February–March mean plus 100 standard deviation values (μ + 100σ) are also shown in (c) and (d). (e) Measured (solid lines) and simulated (with and without considering SO2, dotted and dashed lines, respectively) radiances at the mixing ratio maxima for the enhanced profiles shown in (d) (colored lines) as well as for background conditions at the same pressure levels (gray lines). Note that this MLS spectrometer is centered on the 183.3 GHz H2O spectral line. Most MLS spectrometers observe emissions from two separate spectral regions: the “lower sideband” (LSB) and “upper sideband” (USB) as indicated for selected channels.

The eruption injected H2O throughout a large vertical range encompassing most of the stratosphere, but on 15 January, MLS only measured the outer edge of the plume in the upper stratosphere, where strong winds advected the lofted H2O to locations sampled by MLS. Near 80 hPa on this day, MLS also measured some enhanced H2O injected by a previous, less violent, HT‐HH eruption on 14 January.

For the next several days, most of the largest enhancements failed the QS. Figure 1d shows the profiles displaying the largest mixing ratios on 15, 16, 17, and 18 January. Back trajectories (as in Livesey et al. (2015); Santee et al. (2022)) indicate that these enhancements lie downwind from the HT‐HH volcano (Figure 1b), and the measured spectral signature is well represented by radiance simulations (Figure 1e). Peaks centered on channels 5 and 22 on 16 and 17 January are SO2 spectral lines; they indicate that these lower plumes contained more SO2 than the high‐altitude plume on 15 January.

As the plume dispersed, the daily number of profiles failing the QS increased, reaching a maximum on 19 January. Retrieval performance then returned to normal by 8 February, by which time the plume had dispersed sufficiently that maximum H2O values had dropped to ∼50 ppmv, versus up to 350 ppmv immediately following the eruption (Figure 1).

Taken together, the back trajectories, radiance simulations, and return to typical retrieval quality confirm that the measured enhancements represent real volcanically enhanced H2O values. However, the absolute magnitudes of the enhancements, especially for those failing the QS screening, are still in question because of the poor radiance fits. The MLS retrievals were not optimized to handle such strong H2O enhancements. Thus, to fully quantify these injections and their uncertainties, we are developing a special retrieval for MLS measurements of the HT‐HH plume. Preliminary results suggest that H2O retrievals that better fit the radiances lie within 20% of current v4 estimates.

In addition, it is essential to account for the relatively coarse resolution of the MLS observations (∼3.2 × 230 km for H2O at these altitudes, as quantified by the averaging kernels (Livesey et al., 2020)) in the presence of strong vertically confined plumes (Schwartz et al., 2013, 2020). Accordingly, mid‐January maximum plume values of 1500 ppmv measured by radiosondes (Sellitto et al., 2022) are not necessarily inconsistent with observed MLS abundances, given the disparity in their respective resolutions.

Many chemical species measured by MLS show anomalous mixing ratios in the plume (Figure S1 in Supporting Information S1). However, only the H2O, SO2, and HCl spectral signatures can confidently be ascribed to real enhancements in those quantities; perturbations in other species are likely artifacts arising from SO2 spectral interference. SO2 is retrieved from a spectrometer that targets an O18O line but also covers many SO2 lines, the strongest of which are located in channels 5, 11, and 20. The triple‐peak structure in measured radiances within the volcanic plume (Figure 2b) can only be plausibly explained by an SO2 enhancement.

Figure 2

Profiles with maximum (a) SO2 and (c) HCl on 16 and 17 January. All of these measurements lie downwind of the HT‐HH volcano. (b) Measured (solid lines) and simulated (dashed) SO2 radiances at the mixing ratio maxima for the enhanced profiles (colored lines) as well as for background conditions at the same pressure levels (gray lines). (d) Same as (b) but for differences between measured radiances and those simulated without HCl (solid lines) as well as estimated HCl signatures (from differences between simulations, see legend; dashed lines). All enhancements shown fail the QS.

HCl is currently measured by a spectrometer that targets an O3 line but covers HCl lines in channels 3 and 25. The ∼5 K HCl radiance signature overlaps with an ∼180 K O3 signal. The differences between the measurements and the simulations with and without accounting for contributions from HCl reveal its expected ∼5 K signature, suggesting that the observed enhancements represent real geophysical signals (Figure 2d). The HCl spectral signature is similar to that of the background because the HCl enhancements are not as dramatic as those of H2O or SO2.

MLS estimates of ice water content (IWC) are based on the differences between the measured radiances and the expected clear‐sky radiances, with the residuals attributed to ice scattering and/or ice absorption. The clear‐sky radiances are calculated using the retrieved atmospheric states; since most retrievals in the volcanic plume fail the QS in the days following the eruption, the derived IWC estimates are unreliable. In contrast, the quality of the MLS temperature, CO, and O3 measurements is not affected by the plume.

Unprecedented Stratospheric H2O Injection

Figure 3 compares the HT‐HH HCl, SO2, and H2O stratospheric injections to other stratospheric injections (volcanic or otherwise) observed by MLS. Large injections are marked individually.

Figure 3

Time series of quality‐screened maximum H2O, SO2, and HCl mixing ratios at different pressure levels. SO2 maxima at 14 hPa and HCl maxima at 31 hPa disregarding QS after the HT‐HH eruption are shown in pink. Similarly, H2O maxima disregarding QS are shown in pink for each level. Prior to the HT‐HH eruption, we show QS data to avoid displaying retrieval artifacts, but no other injections failing the QS are found in the record.

The HT‐HH eruption did not inject vast amounts of either HCl or SO2 into the stratosphere. The total injected mass of stratospheric SO2 (calculated as described by Pumphrey et al. (2021)) was 0.41 ± 0.02 Tg, which pales in comparison to that from previous eruptions measured by MLS, such as the 2008 Kasatochi, the 2009 Sarychev, or the 2019 Raikoke eruptions, which each emitted ∼1 Tg (de Leeuw et al., 2021; Pumphrey et al., 2015). The mass of SO2 injected by HT‐HH is even less noteworthy in the context of the 17 Tg injected by the 1991 Pinatubo eruption (Read et al., 1993).

The only unusual aspect of the SO2 plume is its injection height. SO2 plumes are typically injected at altitudes no higher than 46 hPa (∼21 km) (Carn et al., 2016; Pumphrey et al., 2015). HT‐HH is the only injection observed by MLS that produced maximum values of SO2 at 14 hPa (∼29 km), with enhanced values detected up to 6.8 hPa (∼35 km)—outside the normally recommended pressure range for MLS SO2. By 27 January, the SO2 plume dropped below background levels (Figure S1 in Supporting Information S1).

The HCl injection was similarly unremarkable, with only 8 profiles during 16–18 January (barely) exceeding the threshold for enhancement (Figure 2c; Figure S1 in Supporting Information S1). As with SO2, the only unusual aspect of the HCl plume is its injection height of 31.6 hPa (∼24 km), whereas previous eruptions reached no higher than 68 hPa (∼18.6 km).

In contrast, the magnitude of the HT‐HH H2O injection is unprecedented. Three natural pathways for direct injection of H2O into the stratosphere exist: overshooting convection, pyrocumulonimbus (pyroCb) storms, and volcanic eruptions. The previous stratospheric H2O record measured by MLS was 26.3 ppmv at 100 hPa associated with an overshooting convective event in August 2019 that spanned thousands of square kilometers and persisted for several hours (Werner et al., 2020). Two pyroCbs stand out in the MLS H2O record: the 2017 Pacific Northwest (Pumphrey et al., 2021) and the 2019/2020 Australian New Year's (Kablick et al., 2020; Khaykin et al., 2020; Schwartz et al., 2020) events. Only the Australian pyroCbs injected enough H2O to allow an accurate estimate of mass (19 ± 3 Tg).

The 2008 Kasatochi (Schwartz et al., 2013) and the 2015 Calbuco (Sioris et al., 2016) volcanic eruptions were the only others in the MLS record that injected appreciable amounts of H2O into the stratosphere. Neither deposited H2O at altitudes higher than 68 hPa (∼18.6 km), and both injections were too small for a reliable H2O mass estimate.

The HT‐HH eruption injected at least 146 ± 5 Tg of H2O into the stratosphere, not only surpassing the magnitudes of all other injections in the MLS record, but also eclipsing a theoretical estimate of 37.5 Tg from Pinatubo (Pitari & Mancini, 2002). This stratospheric H2O injection is unique in the satellite record (1979 to date). To put the HT‐HH injection into perspective, the enhancement represents ∼10% of the estimated stratospheric H2O burden of 1400 Tg (Glaze et al., 1997). Further, the H2O plume injection height far exceeded that of any other injections in the MLS record (Figure 3).

Evolution of the H2O Plume

To study the development of the H2O plume, Figure 4 shows maps for selected days after the eruption and meridional and zonal mean anomalies based on all data points as well as only those that pass the QS criteria. On 15 January, the plume reached 0.46 hPa (∼53 km), with most of the MLS retrievals failing QS. On 16 January, two separate plumes are visible, one in the upper stratosphere (between 1 and 8 hPa) and the other in the lower stratosphere (between 10 and 80 hPa), where most of the H2O volume was injected. On this day, the effects on the plume of strong wind shear between 1 and 8 hPa are already apparent.

Figure 4

(a) Maps of H2O at selected pressure levels for illustrative days after the eruption. Stippling indicates regions where a majority of the retrievals do not pass the QS. The volcano location is indicated by a triangle. (b) Meridional (30°S to 5°N) and (c) zonal mean anomalies for the same days. Colored contours show anomalies using all Microwave Limb Sounder H2O retrievals, while line contours display the same anomalies based only on QS data; differences indicate regions where many measurements do not pass QS. The volcano location is shown by dashed vertical lines; dashed horizontal lines indicate the level of the map on each day.

By 22 January, the plume had almost entirely circled the globe at 2.1 hPa, while only traveling halfway around at 26 hPa. On average, through January and February, the plume moved ∼37° longitude per day at 2.1 hPa, but only ∼18° longitude per day from 31 to 6 hPa, consistent with winds from meteorological analyses (see Figure S2 in Supporting Information S1) interpolated to the MLS measurement times and locations as described by Manney et al. (2007). By 5 February, the plume covered all longitudes, with the largest enhancements from 38 to 21 hPa (∼22–26 km). By 31 March, the plume around 4.6 hPa had dropped to near background values.

Measurements from 31 March show the persistence of the H2O plume in the lower and middle stratosphere. Concurrent with encircling the globe, the H2O plume broadened slowly, spreading mostly northward around 26 hPa. This plume will require further monitoring as the eruption signal propagates into the upper stratosphere and toward the poles in the Brewer‐Dobson Circulation (BDC).

Discussion and Summary

The importance of stratospheric H2O is well established; it affects stratospheric chemistry and dynamics as well as atmospheric radiation. For example, excess stratospheric H2O could lead to enhanced OH concentrations, slightly enhancing O3 production through the CH4 oxidation cycle but worsening O3 depletion through the HO cycle, leading to a net decrease in O3 (e.g., Dvortsov & Solomon, 2001; Stenke & Grewe, 2005). The enhanced OH concentrations could also increase the loss of CH4, resulting in a decrease in its lifetime (e.g., Ko et al., 2013; Stevenson et al., 2020) and thus reducing its long‐term effect on climate. In addition, if enhanced H2O concentrations were to be entrained into the developing Antarctic vortex to an extent sufficient to raise the formation temperature of polar stratospheric clouds, then the earlier onset of heterogeneous processing would exacerbate cumulative chemical O3 loss. In terms of transport, a study of the dynamical response to a uniform doubling of stratospheric H2O concluded that such moistening could reduce stratospheric temperature and increase the strength of the BDC; it could also result in the tropospheric westerly jets becoming stronger and storm tracks shifting poleward (Maycock et al., 2013). Since the HT‐HH injection is ∼10% of the stratospheric H2O burden, a dynamical response of lesser magnitude than that found by Maycock et al. (2013) would be expected.

H2O enters the stratosphere primarily in the tropics, where it freeze‐dries at the cold point tropopause (Brewer, 1949). This mechanism gives rise to the “tape recorder,” whereby the annual cycle in tropopause temperatures is imprinted in alternating bands of dry and moist air rising in the tropical stratosphere (Mote et al., 1996). By short‐circuiting the pathway through the cold point, HT‐HH has disrupted this “heartbeat” signal (Figure 5a).

Figure 5

(a) The atmospheric tape recorder (zonal mean H2O anomalies in the tropics). (b) Time series of near‐global (60°S to 60°N) H2O at 100 and 31 hPa. H2O abundances are based on GOZCARDS (Froidevaux et al., 2015) and Microwave Limb Sounder data.

Consistent with the freeze‐drying mechanism, unusually low tropopause temperatures around 2001 led to a sharp drop in the amount of H2O entering the stratosphere (e.g., Randel et al., 2006; Rosenlof & Reid, 2008; Figure 5). This dry anomaly propagated via the BDC (Randel et al., 2006; Urban et al., 2014), slowly rising through the stratosphere and moving toward the poles. Using the propagation of the 2001 H2O drop as described by Brinkop et al. (2016) as an analog for the transport of the HT‐HH plume, we expect that ascent could carry volcanic H2O to 10 hPa within ∼9 months. The excess H2O could arrive in northern and southern midlatitudes in ∼18 and ∼24 months, respectively, over a broad domain in the upper stratosphere. Since part of the plume has entered the lower branch of the BDC, the elevated H2O may reach lower stratospheric midlatitudes within a few months. The timescale for complete dissipation of the plume may be 5–10 years (Hall & Waugh, 1997).

Radiative calculations of the sudden drop in H2O of ∼0.4 ppmv (at 100 hPa) in 2001 (Figure 5b) demonstrated that the radiative forcing from even small variations in lower stratospheric H2O could induce decadal‐scale changes in global‐mean surface temperature (e.g., Solomon et al., 2010). The unprecedented HT‐HH enhancement would correspond to ∼1.5 ppmv (at 31 hPa) if averaged over 60°S–60°N.

Previous studies of the radiative effects of stratospheric H2O perturbations, including direct volcanic injection, have shown that they can cause surface warming (e.g., Joshi & Jones, 2009; Rind & Lonergan, 1995). As established in Section 3, the HT‐HH eruption was unusual in that it injected extremely large amounts of H2O. Preliminary climate model simulations (see Supporting Information S1 for details) suggest an effective radiative forcing (e.g., Forster et al., 2001; Myhre et al., 2013; Smith et al., 2020; Wang et al., 2017) at the tropopause of +0.15 Wm−2 due to the stratospheric H2O enhancement (Figure S3b in Supporting Information S1). For comparison, the radiative forcing increase due to the CO2 growth from 1996 to 2005 was about +0.26 Wm−2 (Solomon et al., 2010).

The HT‐HH H2O enhancement will exert a positive radiative forcing on the surface, offsetting the surface cooling caused by the aerosol radiative forcing (e.g., Sellitto et al., 2022; Zhang et al., 2022). Given the extraordinary magnitude of the HT‐HH H2O injection and the fact that its anticipated stratospheric residence time exceeds the typical 2–3 years timescale for sulfate aerosols to fall out of the stratosphere (Joshi & Jones, 2009), HT‐HH may be the first volcanic eruption observed to impact climate not through surface cooling caused by volcanic sulfate aerosols, but rather through surface warming caused by excess H2O radiative forcing.

In summary, MLS measurements indicate that an exceptional amount of H2O was injected directly into the stratosphere by the HT‐HH eruption. We estimate that the magnitude of the injection constituted at least 10% of the total stratospheric H2O burden. On the day of the eruption, the H2O plume reached ∼53 km altitude. The H2O injection bypassed the cold point tropopause, disrupted the H2O tape recorder signal, set a new record for H2O injection height in the 17‐year MLS record, and could alter stratospheric chemistry and dynamics as the long‐lived H2O plume propagates through the stratosphere in the BDC. Unlike previous strong eruptions in the satellite era, HT‐HH could impact climate not through surface cooling due to sulfate aerosols, but rather through surface warming due to the excess stratospheric H2O forcing. Given the potential high‐impact consequences of the HT‐HH H2O injection, it is critical to continue monitoring volcanic gases from this eruption and future ones to better quantify their varying roles in climate.

Supporting Information S1

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