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
It is well established that the shallow branch of the Brewer‐Dobson circulation accelerates in a warming climate due to enhanced wave drag in the subtropical lower stratosphere. This has been linked ...to the strengthening of the upper flanks of the subtropical jets. However, the seasonality of the zonal wind trends, peaking in the winter hemisphere, is opposite to that of the Eliassen‐Palm flux convergence trends, peaking in summer. We investigate the seasonality in the wave drag trends and find a different behavior for each hemisphere. The Shepherd and McLandress (2011, https://doi.org/10.1175/2010jas3608.1) mechanism, involving transient wave dissipation at higher levels following the rise of the critical lines, is found to maximize in austral summer. On the other hand, in the Northern Hemisphere the wave drag increase peaks in summer primarily due to the changes in the stationary planetary waves (monsoonal circulations) associated with enhanced deep convection.
Plain Language Summary
The Brewer‐Dobson circulation, responsible for mass, heat and constituents global transport in the stratosphere, is projected to accelerate in a warming climate. This circulation is driven by the momentum transferred by dissipating waves. We explore the seasonality of trends in wave dissipation in the subtropical lower stratosphere. First, we show that the largest changes in the wave dissipation take place in the summer hemisphere, opposite to the largest changes in the zonal wind, which is known to control wave dissipation conditions. We investigate this apparent contradiction and find that (a) the conditions are particularly favorable for the waves to be affected by the changing wind in summer, due to their spectral characteristics and the structure of the background zonal wind, in particular the proximity of the zero wind line; and (b) in the Northern Hemisphere the changes are primarily associated with stationary waves triggered by enhanced deep convection in a warmer climate.
Key Points
Future subtropical trends in lower stratospheric wave drag are strongest in the summer hemisphere, whereas zonal wind trends peak in winter
The largest changes in transient wave drag due to critical line shift are found in the Southern Hemisphere summer
The Northern Hemisphere summer trends are mainly due to changes in stationary wave drag linked to stronger and higher deep convection
It is well established that increasing greenhouse gases, notably CO2, will cause an acceleration of the stratospheric Brewer‐Dobson circulation (BDC) by the end of this century. We here present ...compelling new evidence that ozone depleting substances are also key drivers of BDC trends. We do so by analyzing and contrasting small ensembles of “single‐forcing” integrations with a stratosphere resolving atmospheric model with interactive chemistry, coupled to fully interactive ocean, land, and sea ice components. First, confirming previous work, we show that increasing concentrations of ozone depleting substances have contributed a large fraction of the BDC trends in the late twentieth century. Second, we show that the phasing out of ozone depleting substances in coming decades—as a consequence of the Montreal Protocol—will cause a considerable reduction in BDC trends until the ozone hole is completely healed, toward the end of the 21st century.
Key Points
Ozone depleting substances are a major driver of Brewer‐Dobson circulation trends
Emission of these substances has been phased out by the Montreal Protocol
That phasing out will result in considerably reduced BDC trends in the 21st century
The Brewer–Dobson Circulation in CMIP6 Abalos, Marta; Calvo, Natalia; Benito-Barca, Samuel ...
Atmospheric chemistry and physics,
09/2021, Letnik:
21, Številka:
17
Journal Article
Recenzirano
Odprti dostop
The Brewer–Dobson circulation (BDC) is a key feature of the stratosphere that models need to accurately represent in order to simulate surface climate variability and change adequately. For the first ...time, the Climate Model Intercomparison Project includes in its phase 6 (CMIP6) a set of diagnostics that allow for careful evaluation of the BDC. Here, the BDC is evaluated against observations and reanalyses using historical simulations. CMIP6 results confirm the well-known inconsistency in the sign of BDC trends between observations and models in the middle and upper stratosphere. Nevertheless, the large uncertainty in the observational trend estimates opens the door to compatibility. In particular, when accounting for the limited sampling of the observations, model and observational trend error bars overlap in 40 % of the simulations with available output. The increasing CO2 simulations feature an acceleration of the BDC but reveal a large spread in the middle-to-upper stratospheric trends, possibly related to the parameterized gravity wave forcing. The very close connection between the shallow branch of the residual circulation and surface temperature is highlighted, which is absent in the deep branch. The trends in mean age of air are shown to be more robust throughout the stratosphere than those in the residual circulation.
Water vapor plays an important role in many aspects of the climate system, by affecting radiation, cloud formation, atmospheric chemistry and dynamics. Even the low stratospheric water vapor content ...provides an important climate feedback, but current climate models show a substantial moist bias in the lowermost stratosphere. Here we report crucial sensitivity of the atmospheric circulation in the stratosphere and troposphere to the abundance of water vapor in the lowermost stratosphere. We show from a mechanistic climate model experiment and inter-model variability that lowermost stratospheric water vapor decreases local temperatures, and thereby causes an upward and poleward shift of subtropical jets, a strengthening of the stratospheric circulation, a poleward shift of the tropospheric eddy-driven jet and regional climate impacts. The mechanistic model experiment in combination with atmospheric observations further shows that the prevailing moist bias in current models is likely caused by the transport scheme, and can be alleviated by employing a less diffusive Lagrangian scheme. The related effects on atmospheric circulation are of similar magnitude as climate change effects. Hence, lowermost stratospheric water vapor exerts a first order effect on atmospheric circulation and improving its representation in models offers promising prospects for future research.
Upwelling across the tropical tropopause exhibits strong subseasonal variability superimposed on the well-known annual cycle, and these variations directly affect temperature and tracers in the ...tropical lower stratosphere. In this work, the dynamical forcing of tropical upwelling on subseasonal time scales is investigated using the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) for 1979-2011. Momentum balance diagnostics reveal that transience in lower-stratospheric upwelling is linked to the effects of extratropical wave forcing, with centers of action in the extratropical winter stratosphere and in the subtropical upper troposphere of both hemispheres. The time-dependent forcing in these regions induces a remote coupled response in the zonal mean wind and the meridional circulation (with associated temperature changes), which drives upwelling variability in the tropical stratosphere. This behavior is observed in the reanalysis, consistent with theory. Dynamical patterns reflect distinctive forcing of the shallow versus deep branches of the Brewer-Dobson circulation; the shallow branch is most strongly correlated with wave forcing in the subtropical upper troposphere and lower stratosphere, while the deep branch is mainly influenced by high-latitude planetary waves.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The circulation of the stratosphere, also known as the Brewer–Dobson
circulation, transports water vapor and ozone, with implications for
radiative forcing and climate. This circulation is typically ...quantified from
model output by calculating the tropical upwelling vertical velocity in the
residual circulation framework, and it is estimated from observations by
using time series of tropical water vapor to infer a vertical velocity.
Recent theory has introduced a method to calculate the strength of the global
mean diabatic circulation through isentropes from satellite measurements of
long-lived tracers. In this paper, we explore this global diabatic
circulation as it relates to the residual circulation vertical velocity,
stratospheric water vapor, and ozone at interannual timescales. We use a
comprehensive climate model, three reanalysis data products, and satellite
ozone data. The different metrics for the circulation have different
properties, especially with regards to the vertical autocorrelation. In the
model, the different residual circulation metrics agree closely and are well
correlated with the global diabatic circulation, except in the lowermost
stratosphere. In the reanalysis products, however, there are more differences
throughout, indicating the dynamical inconsistencies of these products. The
vertical velocity derived from the time series of water vapor in the tropics
is significantly correlated with the global diabatic circulation, but this
relationship is not as strong as that between the global diabatic circulation
and the residual circulation vertical velocity. We find that the global
diabatic circulation in the lower to middle stratosphere (up to 500 K) is
correlated with the total column ozone in the high latitudes and in the
tropics. The upper-level circulation is also correlated with the total column
ozone, primarily in the subtropics, and we show that this is due to the
correlation of both the circulation and the ozone with upper-level
temperatures.
It has recently been recognized that, in addition to greenhouse gases, anthropogenic emissions of ozone‐depleting substances (ODS) can induce long‐term trends in the Brewer‐Dobson circulation (BDC). ...Several studies have shown that a substantial fraction of the residual circulation acceleration over the last decades of the twentieth century can be attributed to increasing ODS. Here the mechanisms of this influence are examined, comparing model runs to reanalysis data and evaluating separately the residual circulation and mixing contributions to the mean age of air trends. The effects of ozone depletion in the Antarctic lower stratosphere are found to dominate the ODS impact on the BDC, while the direct radiative impact of these substances is negligible over the period of study. We find qualitative agreement in austral summer BDC trends between model and reanalysis data and show that ODS are the main driver of both residual circulation and isentropic mixing trends over the last decades of the twentieth century. Moreover, aging by isentropic mixing is shown to play a key role on ODS‐driven age of air trends.
Plain Language Summary
Concentrations of human‐emitted ozone‐depleting substances in the stratosphere have increased substantially during the last decades of the twentieth century, which are found in the Southern Hemisphere. These substances have caused stratospheric ozone depletion, which not only has important direct impacts on human health but also leads to important changes in the atmospheric circulation and climate. Here the impact of these substances on the stratospheric mean transport circulation is examined using chemistry‐climate model simulations and reanalysis data. In addition to destroying ozone, these substances act as greenhouse gases. In this paper, we find that the radiative forcing impacts of human‐emitted ozone‐depleting substances on the stratospheric circulation over the last decades of the twentieth century are negligible. In contrast, the Antarctic ozone hole has contributed substantially to the acceleration of stratospheric tracer transport not only in the polar region but globally. The impacts, largest in the austral summer stratosphere, are examined in detail separating the overturning circulation and irreversible mixing. It is shown that trends in the model compare qualitatively well with estimates from two different reanalyses. This paper contributes to advance our understanding of the impact of human emissions on the stratospheric circulation and to untangle the forcings of recent trends.
Key Points
Increasing ODS have played a key role in observed BDC trends over the last decades of the twentieth century
Ozone depletion dominates the ODS impact on BDC trends, while the radiative effect of these substances is negligible
Including changes in mixing in addition to the residual circulation is crucial to interpreting past mean age trends
While the impact of the El Niño–Southern Oscillation
(ENSO) on the stratospheric circulation has been long recognized, its
effects on stratospheric ozone have been less investigated. In particular,
...the impact on ozone of different ENSO flavors, eastern Pacific (EP) El
Niño and central Pacific (CP) El Niño, and the driving
mechanisms for the ozone variations have not been investigated to date. This
study aims to explore these open questions by examining the anomalies in
advective transport, mixing and chemistry associated with different El
Niño flavors (EP and CP) and La Niña in the Northern Hemisphere in
boreal winter. For this purpose, we use four 60-year ensemble members of the
Whole Atmospheric Community Climate Model version 4. The results show a
significant ENSO signal on the total column ozone (TCO) during EP El Niño
and La Niña events. During EP El Niño events, TCO is significantly
reduced in the tropics and enhanced at middle and high latitudes in boreal
winter. The opposite response has been found during La Niña.
Interestingly, CP El Niño has no significant impact on extratropical TCO,
while its signal in the tropics is weaker than for EP El Niño events.
The analysis of mechanisms reveals that advection through changes in
tropical upwelling is the main driver for ozone variations in the lower
tropical stratosphere, with a contribution of chemical processes above 30 hPa. At middle and high latitudes, stratospheric ozone variations related to
ENSO result from combined changes in advection by residual circulation
downwelling and changes in horizontal mixing linked to Rossby wave breaking
and polar vortex anomalies. The impact of CP El Niño on the shallow
branch of the residual circulation is small, and no significant impact is
found on the deep branch.
The stratospheric ozone layer shields surface life from harmful ultraviolet radiation. Following the Montreal Protocol ban on long-lived ozone-depleting substances (ODSs), rapid depletion of total ...column ozone (TCO) ceased in the late 1990s, and ozone above 32 km is now clearly recovering. However, there is still no confirmation of TCO recovery, and evidence has emerged that ongoing quasi-global (60∘ S–60∘ N) lower stratospheric ozone decreases may be responsible, dominated by low latitudes (30∘ S–30∘ N). Chemistry–climate models (CCMs) used to project future changes predict that lower stratospheric ozone will decrease in the tropics by 2100 but not at mid-latitudes (30–60∘). Here, we show that CCMs display an ozone decline similar to that observed in the tropics over 1998–2016, likely driven by an increase in tropical upwelling. On the other hand, mid-latitude lower stratospheric ozone is observed to decrease, while CCMs that specify real-world historical meteorological fields instead show an increase up to present day. However, these cannot be used to simulate future changes; we demonstrate here that free-running CCMs used for projections also show increases. Despite opposing lower stratospheric ozone changes, which should induce opposite temperature trends, CCMs and observed temperature trends agree; we demonstrate that opposing model–observation stratospheric water vapour (SWV) trends, and their associated radiative effects, explain why temperature changes agree in spite of opposing ozone trends. We provide new evidence that the observed mid-latitude trends can be explained by enhanced mixing between the tropics and extratropics. We further show that the temperature trends are consistent with the observed mid-latitude ozone decrease. Together, our results suggest that large-scale circulation changes expected in the future from increased greenhouse gases (GHGs) may now already be underway but that most CCMs do not simulate mid-latitude ozone layer changes well. However, it is important to emphasise that the periods considered here are short, and internal variability that is both intrinsic to each CCM and different to observed historical variability is not well-characterised and can influence trend estimates. Nevertheless, the reason CCMs do not exhibit the observed changes needs to be identified to allow models to be improved in order to build confidence in future projections of the ozone layer.
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