As a consequence of extreme heat and drought,
record-breaking wildfires developed and ravaged south-eastern Australia
during the fire season 2019–2020. The fire strength reached its paroxysmal
phase ...at the turn of the year 2019–2020. During this phase, pyrocumulonimbus clouds (pyroCb) developed and injected biomass burning aerosols and gases into the
upper troposphere and lower stratosphere (UTLS). The UTLS aerosol layer was
massively perturbed by these fires, with aerosol extinction increased by a
factor of 3 in the visible spectral range in the Southern Hemisphere, with
respect to a background atmosphere, and stratospheric aerosol optical depth
reaching values as large as 0.015 in February 2020. Using the best available
description of this event by observations, we estimate the radiative forcing
(RF) of such perturbations of the Southern Hemispheric aerosol layer. We use
offline radiative transfer modelling driven by observed information of the
aerosol extinction perturbation and its spectral variability obtained from
limb satellite measurements. Based on hypotheses on the absorptivity and the
angular scattering properties of the aerosol layer, the regional (at three
latitude bands in the Southern Hemisphere) clear-sky TOA (top-of-atmosphere)
RF is found varying from small positive values to relatively large negative
values (up to −2.0 W m−2), and the regional clear-sky surface RF is
found to be consistently negative and reaching large values (up to −4.5 W m−2). We argue that clear-sky positive values are unlikely for this
event, if the ageing/mixing of the biomass burning plume is mirrored by the
evolution of its optical properties. Our best estimate for the area-weighted
global-equivalent clear-sky RF is -0.35±0.21 (TOA RF) and
-0.94±0.26 W m−2 (surface RF), thus the strongest documented for
a fire event and of comparable magnitude with the strongest volcanic
eruptions of the post-Pinatubo era. The surplus of RF at the surface, with
respect to TOA, is due to absorption within the plume that has contributed
to the generation of ascending smoke vortices in the stratosphere. Highly
reflective underlying surfaces, like clouds, can nevertheless swap negative
to positive TOA RF, with global average RF as high as +1.0 W m−2
assuming highly absorbing particles.
Wave‐induced Lagrangian fluctuations of temperature and vertical velocity in the lower stratosphere are quantified using measurements from superpressure balloons (SPBs). Observations recorded every ...minute along SPB flights allow the whole gravity wave spectrum to be described and provide unprecedented information on both the intrinsic frequency spectrum and the probability distribution function of wave fluctuations. The data set has been collected during two campaigns coordinated by the French Space Agency in 2010, involving 19 balloons over Antarctica and 3 in the deep tropics. In both regions, the vertical velocity distributions depart significantly from a Gaussian behavior. Knowledge on such wave fluctuations is essential for modeling microphysical processes along Lagrangian trajectories. We propose a new simple parameterization that reproduces both the non‐Gaussian distribution of vertical velocities (or heating/cooling rates) and their observed intrinsic frequency spectrum.
Key Points
Long‐duration balloon observations are used to characterize Lagrangian temperature fluctuations
Intrinsic frequency spectra and PDFs are derived for temperature and cooling rates
A parameterization of gravity wave temperature fluctuations in the lower stratosphere is developed
Most chemistry‐climate models show an intensification of the Brewer‐Dobson circulation (BDC) in the stratosphere associated with increasing greenhouse gas emissions and ozone depletion in the last ...decades, but this trend remains to be confirmed in observational data. In this work the evolution of the advective BDC for the period 1979–2012 is evaluated and compared in three modern reanalyses (ERA‐Interim, MERRA, and JRA‐55). Three different estimates of the BDC are computed for each reanalysis, one based on the definition of the residual circulation and two indirect estimates derived from momentum and thermodynamic balances. The comparison among the nine estimates shows substantial uncertainty in the mean magnitude (∼40%) but significant common variability. The tropical upwelling series show variability linked to the stratospheric quasi‐biennial oscillation and to El Niño–Southern Oscillation (ENSO) and also reflect extreme events such as major sudden stratospheric warmings and volcanic eruptions. The trend analysis suggests a strengthening of tropical upwelling of around 2–5%/decade throughout the layer 100–10 hPa. The global spatial structure of the BDC trends provides evidence of an overall acceleration of the circulation in both hemispheres, with qualitative agreement among the estimates. The global BDC trends are mainly linked to changes in the boreal winter season and can be tracked to long‐term increases in the resolved wave drag in both hemispheres.
Key Points
Common variability among estimates of the BDC in three modern reanalyses
Qualitatively consistent BDC strengthening trends for 1979–2012 in reanalyses
Transit properties across the tropical tropopause layer are studied using extensive forward and backward Lagrangian diabatic trajectories between cloud tops and the reference surface 380 K. After ...dividing the tropical domain into 11 subregions according to the distribution of land and convection, we estimate the contribution of each region to the upward mass flux across the 380 K surface and to the vertical distribution of convective sources and transit times over the period 2005–2008. The good agreement between forward and backward statistics is the basis of the results presented here. It is found that about 85 % of the tropical parcels at 380 K originate from convective sources throughout the year. From November to April, the sources are dominated by the warm pool which accounts for up to 70 % of the upward flux. During boreal summer, the Asian monsoon region is the largest contributor with similar contributions from the maritime and continental parts of the region; however, the vertical distributions and transit times associated with these two subregions are very different. Convective sources are generally higher over the continental part of the Asian monsoon region, with shorter transit times. We estimate the monthly averaged upward mass flux on the 380 K surface and show that the contribution from convective outflow accounts for 80 % on average and explains most of its seasonal variations. The largest contributor to the convective flux is the South Asian Pacific region (warm pool) at 39 % throughout the year followed by oceanic regions surrounding continental Asia at 18 % and Africa at 10.8 %. Continental Asian lowlands account for 8 %. The Tibetan Plateau is a minor overall contributor (0.8 %), but transport from convective sources in this region is very efficient due to its central location beneath the Asian upper level anticyclone. The core results are robust to uncertainties in data and methods, but the vertical source distributions and transit times exhibit some sensitivity to the representations of cloud tops and heating rates. The main sensitivity is to the radiative heating rates which vary among reanalyses.
We use a combination of spaceborne instruments to study the unprecedented stratospheric plume after the Tonga eruption of 15 January 2022.
The aerosol plume was initially formed of two clouds at 30 ...and 28 km, mostly composed of submicron-sized sulfate particles, without ash, which is washed out within the first day following the eruption.
The large amount of injected water vapour led to a fast conversion of SO2 to sulfate aerosols and induced a descent of the plume to 24–26 km over the first 3 weeks by radiative cooling.
Whereas SO2 returned to background levels by the end of January, volcanic sulfates and water still persisted after 6 months, mainly confined between 35∘ S and 20∘ N until June due to the zonal symmetry of the summer stratospheric circulation at 22–26 km.
Sulfate particles, undergoing hygroscopic growth and coagulation, sediment and gradually separate from the moisture anomaly entrained in the ascending branch Brewer–Dobson circulation.
Sulfate aerosol optical depths derived from the IASI (Infrared Atmospheric Sounding Interferometer) infrared sounder show that during the first 2 months, the aerosol plume was not simply diluted and dispersed passively but rather organized in concentrated patches. Space-borne lidar winds suggest that those structures, generated by shear-induced instabilities, are associated with vorticity anomalies that may have enhanced the duration and impact of the plume.
The tropical tropopause layer (TTL) is the transition region between the well-mixed convective troposphere and the radiatively controlled stratosphere with air masses showing chemical and dynamical ...properties of both regions. The representation of the TTL in meteorological reanalysis data sets is important for studying the complex interactions of circulation, convection, trace gases, clouds, and radiation. In this paper, we present the evaluation of climatological and long-term TTL temperature and tropopause characteristics in the reanalysis data sets ERA-Interim, ERA5, JRA-25, JRA-55, MERRA, MERRA-2, NCEP-NCAR (R1), and CFSR. The evaluation has been performed as part of the SPARC (Stratosphere-troposphere Processes and their Role in Climate) Reanalysis Intercomparison Project (S-RIP).
The Tonga eruption of 15 January 2022 has released a long‐lived stratospheric plume of sulfate aerosols. More than 17 months after, we focus on the high quality data series of SAGE III (Stratospheric ...Aerosol and Gas Experiment) on board the International Space Station (ISS) to determine the mean radius and size distribution of the aerosols and their total mass. The persisting volcanic aerosols—with a mode width of 1.25 and an effective radius of 0.4 μm—differ from the significantly smaller background aerosols and from those measured during recent stratospheric eruptions. The sulfuric acid mass between 50°S and 30°N is estimated to be very stable in spite of considerable redistribution in latitude at a value of 0.66 ± 0.1 Tg, corresponding to an initial sulfur dioxide emission of 0.44 Tg. Such properties are expected to facilitate the persistence of a climate warming due to the volcanic water vapor.
Plain Language Summary
We study the stratospheric aerosol plume produced by the Hunga Tonga–Hunga Ha'apai eruption on 15 January 2022 based on the high quality solar occultation measurements of the instrument SAGE III (Stratospheric Aerosol and Gas Experiment) onboard the International Space Station. These data reveal that the aerosol sizes are about twice as large as after other documented volcanic eruptions and that the total mass of H2SO4 in the liquid droplets of sulfate in the stratosphere has been very stable from March 2022, when it started to be well homogeneized in longitude, to November 2022, when it started to decay. The total mass of 0.66 Tg of H2SO4 is in good agreement with the early estimates of a stratospheric emission of 0.4–0.5 Tg of SO2. The implication is that the aerosol radiative impact will not mask the persisting warming effect of the water vapor injected in the stratosphere by the eruption.
Key Points
The extinction of the stratospheric plume of the 2022 Tonga eruption is well modeled by a unimodal distribution of sulfate particles
The effective radius of the aerosols is large with values close to 0.4 μm, and a mode width of 1.25, from March 2022 to June 2023
We estimate a total H2SO4 mass in stratospheric sulfate aerosols of about 0.66 corresponding to 0.44 Tg of SO2 initially injected
The stratospheric circulation determines the transport and lifetime of key
trace gases in a changing climate, including water vapor and ozone, which
radiatively impact surface climate.
The unusually ...warm El Niño–Southern Oscillation (ENSO) event aligned with a
disrupted Quasi-Biennial Oscillation (QBO) caused an unprecedented perturbation
to this circulation in 2015–2016.
Here, we quantify the impact of the alignment of these two phenomena in 2015–2016
on lower stratospheric water vapor and ozone from satellite observations. We show
that the warm ENSO event substantially increased water vapor and decreased ozone
in the tropical lower stratosphere.
The QBO disruption significantly decreased global lower stratospheric water vapor
and tropical ozone from early spring to late autumn.
Thus, this QBO disruption reversed the lower stratosphere moistening triggered
by the alignment of the warm ENSO event with westerly QBO in early boreal winter.
Our results suggest that the interplay of ENSO events and QBO phases will be
crucial for the distributions of radiatively active trace gases
in a changing future climate, when increasing El Niño-like conditions and
a decreasing lower stratospheric QBO amplitude are expected.
The Australian bushfires around the turn of the year 2020 generated an unprecedented perturbation of stratospheric composition, dynamical circulation and radiative balance. Here we show from ...satellite observations that the resulting planetary-scale blocking of solar radiation by the smoke is larger than any previously documented wildfires and of the same order as the radiative forcing produced by moderate volcanic eruptions. A striking effect of the solar heating of an intense smoke patch was the generation of a self-maintained anticyclonic vortex measuring 1000 km in diameter and featuring its own ozone hole. The highly stable vortex persisted in the stratosphere for over 13 weeks, travelled 66,000 km and lifted a confined bubble of smoke and moisture to 35 km altitude. Its evolution was tracked by several satellite-based sensors and was successfully resolved by the European Centre for Medium-Range Weather Forecasts operational system, primarily based on satellite data. Because wildfires are expected to increase in frequency and strength in a changing climate, we suggest that extraordinary events of this type may contribute significantly to the global stratospheric composition in the coming decades.
The 2019/2020 Australian wildfires generated a smoke cloud that organized itself into a persistent vortex structure and ascended to 35 km altitude through solar heating, according to satellite tracking.
An accelerating Brewer–Dobson circulation (BDC) is a robust signal
of climate change in model predictions but has been questioned by
trace gas observations. We analyse the stratospheric mean age of ...air and
the full age spectrum as measures for the BDC and its trend. Age
of air is calculated using the Chemical Lagrangian Model of the
Stratosphere (CLaMS) driven by ERA-Interim, JRA-55 and MERRA-2
reanalysis data to assess the robustness of the representation of
the BDC in current generation meteorological reanalyses. We find
that the climatological mean age significantly depends on the
reanalysis, with JRA-55 showing the youngest and MERRA-2 the
oldest mean age. Consideration of the age spectrum indicates that
the older air for MERRA-2 is related to a stronger spectrum tail,
which is likely associated with weaker tropical upwelling and stronger
recirculation. Seasonality of stratospheric transport is robustly
represented in reanalyses, with similar mean age variations
and age spectrum peaks. Long-term changes from 1989 to 2015 turn
out to be similar for the reanalyses with mainly decreasing mean age
accompanied by a shift of the age spectrum peak towards shorter
transit times, resembling the forced response in climate model
simulations to increasing greenhouse gas concentrations. For the
shorter periods, 1989–2001 and 2002–2015, the age of air changes are
less robust. Only ERA-Interim shows the hemispheric dipole pattern
in age changes from 2002 to 2015 as viewed by recent satellite
observations. Consequently, the representation of decadal
variability of the BDC in current generation reanalyses appears less
robust and is a major uncertainty of modelling the BDC.
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