Effective climate sensitivity (EffCS), commonly estimated from model simulations with abrupt 4×CO2 for 150 years, has been shown to depend on the CO2 forcing level. To understand this dependency ...systematically, we performed a series of simulations with a range of abrupt CO2 forcing in two climate models. Our results indicate that normalized EffCS values in these simulations are a non-monotonic function of the CO2 forcing, decreasing between 3× and 4×CO2 in CESM1-LE (2× and 3×CO2 in GISS-E2.1-G) and increasing at higher CO2 levels. The minimum EffCS value, caused by anomalously negative radiative feedbacks, arises mainly from sea-surface temperature (SST) relative cooling in the tropical and subtropical North Atlantic. This cooling is associated with the formation of the North Atlantic Warming Hole and Atlantic Meridional Overturning Circulation collapse under CO2 forcing. Our findings imply that understanding changes in North Atlantic SST patterns is important for constraining near-future and equilibrium global warming.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The 1998-2016 ozone trends in the lower stratosphere are examined using the Modern-Era Retrospective Analysis for Research and Applications Version 2 (MERRA-2) and related National Aeronautics and ...Space Administration products. After removing biases resulting from step changes in the MERRA-2 ozone observations, a discernible negative trend of 1.67 +/- 0.54 Dobson units per decade (DU/decade) is found in the 10-km layer above the tropopause between 20 deg N and 60 deg N. A weaker but statistically significant trend of 1.17 +/- 0.33 DU/decade exists between 50 deg S and 20 deg S. In the Tropics, a positive trend is seen in a 5-km layer above the tropopause. Analysis of an idealized tracer in a model simulation constrained by MERRA-2 meteorological fields provides strong evidence that these trends are driven by enhanced isentropic transport between the tropical (20 deg S-20 deg N) and extratropical lower stratosphere in the past two decades. This is the first time that a reanalysis data set has been used to detect and attribute trends in lower stratospheric ozone.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The Brewer–Dobson Circulation in CMIP6 Abalos, Marta; Calvo, Natalia; Benito-Barca, Samuel ...
Atmospheric chemistry and physics,
09/2021, Volume:
21, Issue:
17
Journal Article
Peer reviewed
Open access
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.
Apparent hydrological sensitivity (ηa), the change in the global mean precipitation per degree K of global surface warming, is a key aspect of the climate system's response to increasing CO2 forcing. ...To determine whether ηa depends on the forcing amplitude we analyze idealized experiments over a broad range of abrupt CO2 forcing, from 2× to 8× preindustrial values, with two distinct climate models. We find little change in ηa between 2× and 4×CO2, and almost no change beyond 5×CO2. We validate this finding under transient CO2 forcing at 1%‐per‐year, up to 8×CO2. We further corroborate this result by analyzing the 1%‐per‐year output of more than 15 CMIP5/6 models. Lastly, we examine the 1,000‐year long LongrunMIP model output, and again find little change in ηa. This wealth of results demonstrates that ηa is a very weak function of CO2 forcing.
Plain Language Summary
Hydrological sensitivity (HS) is defined as the change in globally‐averaged precipitation per degree K of surface temperature increase caused by increasing concentrations of greenhouse gasses, such as CO2. It is important to understand how HS changes with different levels of CO2 in the atmosphere. To do this we analyzed model experiments with varying increases of CO2. We find little change in HS between 2× to 4× the pre‐industrial levels of CO2, and almost no change beyond 5×CO2. Additionally, we analyze model experiments where CO2 concentrations increase by 1% per year and see similar results. Finally, we validate this finding with models with significantly longer run times. We thus conclude that HS is independent of the level of CO2 in the atmosphere.
Key Points
We examine the dependence of apparent hydrological sensitivity (ηa, defined as ΔP/ΔT) on the magnitude of CO2 forcing
We find little change in ηa in abrupt 2× to 8×CO2 experiments and a transient 1%/year experiment up to 8×CO2
Little change in ηa, notably at large CO2, is also found in most CMIP5, CMIP6, and Longrun‐MIP models
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
A climatology of air‐mass origin in the tropical lower stratosphere is presented for the Goddard Earth Observing System Chemistry Climate Model. During late boreal summer and fall, air‐mass fractions ...reveal that as much as 20% of the air in the tropical lower stratosphere last contacted the planetary boundary layer (PBL) over Asia; by comparison, the air‐mass fractions corresponding to last PBL contact over North America and over Europe are negligible. Asian air reaches the extratropical tropopause within a few days of leaving the boundary layer and is quasi‐horizontally transported into the tropical lower stratosphere, where it persists until January. The rapid injection of Asian air into the lower stratosphere—and its persistence in the deep tropics through late (boreal) winter—is important as industrial emissions over East Asia continue to increase. Hence, the Asian monsoon may play an increasingly important role in shaping stratospheric composition.
Key Points
Approximately 20% of tropical lower stratospheric air originates at the Asian boundary layer
Asian air reaches the tropical stratosphere within days of leaving the surface
Origin tracers are rigorous and practical tools for assessing model transport
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The enhanced warming of the Arctic, relative to other parts of the Earth, a phenomenon known as Arctic amplification, is one of the most striking features of climate change, and has important ...climatic impacts for the entire Northern Hemisphere. Several mechanisms are believed to be responsible for Arctic amplification; however, a quantitative understanding of their relative importance is still missing. Here, using ensembles of model integrations, we quantify the contribution of ocean coupling, both its thermodynamic and dynamic components, to Arctic amplification over the 20th and 21st centuries. We show that ocean coupling accounts for ~80% of the amplification by 2100. In particular, we show that thermodynamic coupling is responsible for future amplification and sea-ice loss as it overcomes the effect of dynamic coupling which reduces the amplification and sea-ice loss by ~35%. Our results demonstrate the utility of targeted numerical experiments to quantify the role of specific mechanisms in Arctic amplification, for better constraining climate projections.
We provide an overview of the REF-C1SD specified-dynamics experiment that was conducted as part of phase 1 of the Chemistry-Climate Model Initiative (CCMI). The REF-C1SD experiment, which consisted ...of mainly nudged general circulation models (GCMs) constrained with (re)analysis fields, was designed to examine the influence of the large-scale circulation on past trends in atmospheric composition. The REF-C1SD simulations were produced across various model frameworks and are evaluated in terms of how well they represent different measures of the dynamical and transport circulations. In the troposphere there are large (~40 %) differences in the climatological mean distributions, seasonal cycle amplitude, and trends of the meridional and vertical winds. In the stratosphere there are similarly large (~50 %) differences in the magnitude, trends and seasonal cycle amplitude of the transformed Eulerian mean circulation and among various chemical and idealized tracers. At the same time, interannual variations in nearly all quantities are very well represented, compared to the underlying reanalyses. We show that the differences in magnitude, trends and seasonal cycle are not related to the use of different reanalysis products; rather, we show they are associated with how the simulations were implemented, by which we refer both to how the large-scale flow was prescribed and to biases in the underlying free-running models. In most cases these differences are shown to be as large or even larger than the differences exhibited by free-running simulations produced using the exact same models, which are also shown to be more dynamically consistent. Overall, our results suggest that care must be taken when using specified-dynamics simulations to examine the influence of large-scale dynamics on composition.
Understanding and modeling the large-scale transport of trace gases and aerosols is important for interpreting past (and projecting future) changes in atmospheric composition. Here we show that there ...are large differences in the global-scale atmospheric transport properties among the models participating in the IGAC SPARC Chemistry–Climate Model Initiative (CCMI). Specifically, we find up to 40% differences in the transport timescales connecting the Northern Hemisphere (NH) midlatitude surface to the Arctic and to Southern Hemisphere high latitudes, where the mean age ranges between 1.7 and 2.6 years. We show that these differences are related to large differences in vertical transport among the simulations, in particular to differences in parameterized convection over the oceans. While stronger convection over NH midlatitudes is associated with slower transport to the Arctic, stronger convection in the tropics and subtropics is associated with faster interhemispheric transport. We also show that the differences among simulations constrained with fields derived from the same reanalysis products are as large as (and in some cases larger than) the differences among free-running simulations, most likely due to larger differences in parameterized convection. Our results indicate that care must be taken when using simulations constrained with analyzed winds to interpret the influence of meteorology on tropospheric composition.
Offline chemical transport models (CTMs) have traditionally been used to perform studies of atmospheric chemistry in a fixed dynamical environment. An alternative to using CTMs is to constrain the ...flow in a general circulation model using winds from meteorological analyses. The Goddard Earth Observing System (GEOS) “replay” approach involves reading in analyzed fields every 6 h and recomputing the analysis increments, which are applied as a forcing to the meteorology at every model time step. Unlike in CTM, all of the subgrid‐scale processes are recalculated online so that they are consistent with the large‐scale analysis fields, similar in spirit to “nudged” simulations, in which the online meteorology is relaxed to the analysis. Here we compare the transport of idealized tracers in different replay simulations constrained with meteorological fields taken from The Modern‐Era Retrospective Analysis for Research and Applications, Version 2 (MERRA‐2). We show that there are substantial differences in their large‐scale stratospheric transport, depending on whether analysis fields or assimilated fields are used. Replay simulations constrained with the instantaneous analysis fields produce stratospheric mean age values that are up to 30% too young relative to observations; by comparison, simulations constrained with the time‐averaged assimilated fields produce more credible stratospheric transport. Our study indicates that care should be taken to correctly configure the model when the replay technique is used to simulate stratospheric composition.
Key Points
GEOS replay simulations produce credible large‐scale stratospheric and tropospheric transport for use in studies of atmospheric composition
Simulations constrained with analysis fields produce stratospheric mean ages that are too young, versus when assimilated fields are used
By comparison, large‐scale tropospheric transport properties are relatively insensitive to whether analysis or assimilated fields are used
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DOBA, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
Extreme cold events over North America such as the February 2021 cold wave have been suggested to be linked to stratospheric polar vortex stretching. However, it is not resolved how robustly and on ...which timescales the stratosphere contributes to the surface anomalies. Here we introduce a simple measure of stratospheric wave activity for reanalyses and model outputs. In contrast to the well-known surface influences of sudden stratospheric warmings (SSWs) that increase the intraseasonal persistence of weather regimes, we show that extreme stratospheric wave events are accompanied by intraseasonal fluctuations between warm and cold spells over North America in observations and climate models. Particularly, strong stratospheric wave events are followed by an increased risk of cold extremes over North America 5-25 days later. Idealized simulations in an atmospheric model with a well-resolved stratosphere corroborate that strong stratospheric wave activity precedes North American cold spells through vertical wave coupling. These findings potentially benefit the predictability of high-impact winter cold extremes over North America.
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