125 research outputs found
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Direct and ozone-mediated forcing of the Southern Annular Mode by greenhouse gases
We assess the roles of long-lived greenhouse gases and ozone depletion in driving meridional surface pressure gradients in the southern extratropics; these gradients are a defining feature of the Southern Annular Mode. Stratospheric ozone depletion is thought to have caused a strengthening of this mode during summer, with increasing long-lived greenhouse gases playing a secondary role. Using a coupled atmosphere-ocean chemistry-climate model, we show that there is cancelation between the direct, radiative effect of increasing greenhouse gases by the also substantial indirectâchemical and dynamicalâfeedbacks that greenhouse gases have via their impact on ozone. This sensitivity of the mode to greenhouse gas-induced ozone changes suggests that a consistent implementation of ozone changes due to long-lived greenhouse gases in climate models benefits the simulation of this important aspect of Southern Hemisphere climate
Improvements to stratospheric chemistry scheme in the UM-UKCA (v10.7) model: solar cycle and heterogeneous reactions
Improvements are made to two areas of the United Kingdom Chemistry and Aerosol (UKCA) module, which forms part of the Met Office Unified Model (UM) used for weather and climate applications. Firstly, a solar cycle is added to the photolysis scheme. The effect on total column ozone of this addition was found to be around 1â2% in mid-latitude and equatorial regions in phase with the solar cycle. Secondly, reactions occurring on the surfaces of polar stratospheric clouds and sulfate aerosol are updated and extended by modification of the uptake coefficients of five existing reactions and the addition of a further eight reactions involving bromine species. These modifications are shown to reduce the overabundance of modeled total-column ozone in the Arctic during October to February, southern mid-latitudes during August, and the Antarctic during September. Antarctic springtime ozone depletion is shown to be enhanced by 25 DU on average, which now causes the ozone hole to be somewhat too deep compared to observations. We show that this is in part due to a cold bias of the Antarctic polar vortex in the model
Trisâ{hydridotris(1âpyrazolyl)borato}actinide Complexes: Synthesis, Spectroscopy, Crystal Structure, Bonding Properties and Magnetic Behaviour
The isostructural compounds of the trivalent actinides uranium, neptunium, plutonium, americium, and curium with the hydridotris(1-pyrazolyl)borato (Tp) ligand An[η-HB(NCH)] (AnTp) have been obtained through several synthetic routes. Structural, spectroscopic (absorption, infrared, laser fluorescence) and magnetic characterisation of the compounds were performed in combination with crystal field, density functional theory (DFT) and relativistic multiconfigurational calculations. The covalent bonding interactions were analysed in terms of the natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) models
Trisâ{hydridotris(1âpyrazolyl)borato}actinide Complexes: Synthesis, Spectroscopy, Crystal Structure, Bonding Properties and Magnetic Behaviour
The isostructural compounds of the trivalent actinides uranium, neptunium, plutonium, americium, and curium with the hydridotris(1-pyrazolyl)borato (Tp) ligand An[η-HB(NCH)] (AnTp) have been obtained through several synthetic routes. Structural, spectroscopic (absorption, infrared, laser fluorescence) and magnetic characterisation of the compounds were performed in combination with crystal field, density functional theory (DFT) and relativistic multiconfigurational calculations. The covalent bonding interactions were analysed in terms of the natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) models
Large-Scale Transport into the Arctic: The Roles of the Midlatitude Jet and the Hadley Cell
Transport from the Northern Hemisphere (NH) midlatitudes to the Arctic plays a crucial role in determining the abundance of trace gases and aerosols that are important to Arctic climate via impacts on radiation and chemistry. Here we examine this transport using an idealized tracer with a fixed lifetime and predominantly midlatitude land-based sources in models participating in the Chemistry Climate Model Initiative (CCMI). We show that there is a 25%-45% difference in the Arctic concentrations of this tracer among the models. This spread is correlated with the spread in the location of the Pacific jet, as well as the spread in the location of the Hadley Cell (HC) edge, which varies consistently with jet latitude. Our results suggest that it is likely that the HC-related zonal-mean meridional transport rather than the jet-related eddy mixing is the major contributor to the inter-model spread in the transport of land-based tracers into the Arctic. Specifically, in models with a more northern jet, the HC generally extends further north and the tracer source region is mostly covered by surface southward flow associated with the lower branch of the HC, resulting in less efficient transport poleward to the Arctic. During boreal summer, there are poleward biases in jet location in free-running models, and these models likely underestimate the rate of transport into the Arctic. Models using specified dynamics do not have biases in the jet location, but do have biases in the surface meridional flow, which may result in differences in transport into the Arctic. In addition to the land-based tracer, the midlatitude-to-Arctic transport is further examined by another idealized tracer with zonally uniform sources. With equal sources from both land and ocean, the inter-model spread of this zonally uniform tracer is more related to variations in parameterized convection over oceans rather than variations in HC extent, particularly during boreal winter. This suggests that transport of land-based and oceanic tracers or aerosols towards the Arctic differs in pathways and therefore their corresponding inter-model variabilities result from different physical processes
Extratropical age of air trends and causative factors in climate projection simulations
Climate model simulations show an acceleration of the
BrewerâDobson circulation (BDC) in response to climate change. While the
general mechanisms for the BDC strengthening are widely understood, there
are still open questions concerning the influence of the details of the wave driving. Mean age of stratospheric air (AoA) is a useful transport
diagnostic for assessing changes in the BDC. Analyzing AoA from a subset of
ChemistryâClimate Model Initiative part 1 climate projection simulations, we
find a remarkable agreement between most of the models in simulating the
largest negative AoA trends in the extratropical lower to middle
stratosphere of both hemispheres (approximately between 20 and 25Â geopotential kilometers (gpkm) and 20â50ââN and S). We show that the occurrence of AoA trend minima in those regions is directly
related to the climatological AoA distribution, which is sensitive to an
upward shift of the circulation in response to climate change. Also other
factors like a reduction of aging by mixing (AbM) and residual circulation
transit times (RCTTs) contribute to the AoA distribution changes by widening
the AoA isolines. Furthermore, we analyze the time evolution of AbM and RCTT trends in the extratropics and examine the connection to possible drivers
focusing on local residual circulation strength, net tropical upwelling and
wave driving. However, after the correction for a vertical shift of pressure
levels, we find only seasonally significant trends of residual circulation
strength and zonal mean wave forcing (resolved and unresolved) without a
clear relation between the trends of the analyzed quantities. This indicates
that additional causative factors may influence the AoA, RCTT and AbM
trends. In this study, we postulate that the shrinkage of the stratosphere
has the potential to influence the RCTT and AbM trends and thereby cause
additional AoA changes over time.Czech Science Foundation (GACË R) | Ref. 16- 01562JCzech Science Foundation (GACË R) | Ref. 18-01625SMinisterio de Ciencia e InnovaciĂłn | Ref. CGL2015-71575-PXunta de Galicia | Ref. ED481B 2018/103Ministerio de EconomĂa y Competitividad | Ref. RYC-2013-1456
Evaluating the relationship between interannual variations in the Antarctic ozone hole and Southern Hemisphere surface climate in chemistry-climate models
Studies have recently reported statistically significant relationships between observed year-to-year spring Antarctic ozone variability and the Southern Hemisphere Annular Mode and surface temperatures in spring-summer. This study investigates whether current chemistry-climate models (CCMs) can capture these relationships, in particular, the connection between November total column ozone (TCO) and Australian summer surface temperatures, where years with anomalously high TCO over the Antarctic polar cap tend to be followed by warmer summers. The interannual ozone-temperature teleconnection is examined over the historical period in the observations and simulations from the Whole Atmosphere Community Climate Model (WACCM) and nine other models participating in the Chemistry-Climate Model Initiative (CCMI). There is a systematic difference between the WACCM experiments forced with prescribed observed sea surface temperatures (SSTs) and those with an interactive ocean. Strong correlations between TCO and Australian temperatures are only obtained for the uncoupled experiment, suggesting that the SSTs could be important for driving both variations in Australian temperatures and the ozone hole, with no causal link between the two. Other CCMI models also tend to capture this relationship with more fidelity when driven by observed SSTs, though additional research and targeted modelling experiments are required to determine causality and further explore the role of model biases and observational uncertainty. The results indicate that CCMs can reproduce the relationship between spring ozone and summer Australian climate reported in observational studies, suggesting that incorporating ozone variability could improve seasonal predictions, however more work is required to understand the difference between the coupled and uncoupled simulations
Evaluating the relationship between interannual variations in the Antarctic ozone hole and Southern Hemisphere surface climate in chemistry-climate models
Studies have recently reported statistically significant relationships between observed year-to-year spring Antarctic ozone variability and the Southern Hemisphere Annular Mode and surface temperatures in spring-summer. This study investigates whether current chemistry-climate models (CCMs) can capture these relationships, in particular, the connection between November total column ozone (TCO) and Australian summer surface temperatures, where years with anomalously high TCO over the Antarctic polar cap tend to be followed by warmer summers. The interannual ozone-temperature teleconnection is examined over the historical period in the observations and simulations from the Whole Atmosphere Community Climate Model (WACCM) and nine other models participating in the Chemistry-Climate Model Initiative (CCMI). There is a systematic difference between the WACCM experiments forced with prescribed observed sea surface temperatures (SSTs) and those with an interactive ocean. Strong correlations between TCO and Australian temperatures are only obtained for the uncoupled experiment, suggesting that the SSTs could be important for driving both variations in Australian temperatures and the ozone hole, with no causal link between the two. Other CCMI models also tend to capture this relationship with more fidelity when driven by observed SSTs, though additional research and targeted modelling experiments are required to determine causality and further explore the role of model biases and observational uncertainty. The results indicate that CCMs can reproduce the relationship between spring ozone and summer Australian climate reported in observational studies, suggesting that incorporating ozone variability could improve seasonal predictions, however more work is required to understand the difference between the coupled and uncoupled simulations
The influence of mixing on the stratospheric age of air changes in the 21st century
Climate models consistently predict an acceleration of the
BrewerâDobson circulation (BDC) due to climate change in the 21st century.
However, the strength of this acceleration varies considerably among
individual models, which constitutes a notable source of uncertainty for
future climate projections. To shed more light upon the magnitude of this
uncertainty and on its causes, we analyse the stratospheric mean age of air
(AoA) of 10 climate projection simulations from the Chemistry-Climate Model
Initiative phase 1 (CCMI-I), covering the period between 1960 and 2100. In
agreement with previous multi-model studies, we find a large model spread in
the magnitude of the AoA trend over the simulation period. Differences
between future and past AoA are found to be predominantly due to differences
in mixing (reduced aging by mixing and recirculation) rather than differences
in residual mean transport. We furthermore analyse the mixing efficiency, a
measure of the relative strength of mixing for given residual mean transport,
which was previously hypothesised to be a model constant. Here, the mixing
efficiency is found to vary not only across models, but also over time in all
models. Changes in mixing efficiency are shown to be closely related to
changes in AoA and quantified to roughly contribute 10â% to the long-term
AoA decrease over the 21st century. Additionally, mixing efficiency
variations are shown to considerably enhance model spread in AoA changes. To
understand these mixing efficiency variations, we also present a consistent
dynamical framework based on diffusive closure, which highlights the role of
basic state potential vorticity gradients in controlling mixing efficiency
and therefore aging by mixing.Helmholtz Association | Ref. VH-NG-1014Australian Research Councilâs Centre of Excellence for Climate System Science | Ref. CE110001028Australian Governmentâs National Computational Merit Allocation Scheme | Ref. FUERZAS 4012Ministerio de Ciencia e InnovaciĂłn | Ref. CGL2015-71575-PNew Zealand Royal Society Marsden Fund | Ref. 12-NIW-00
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Implementation of U.K. Earth system models for CMIP6
We describe the scientific and technical implementation of two models for a core set of
experiments contributing to the sixth phase of the Coupled Model Intercomparison Project (CMIP6).
The models used are the physical atmosphere-land-ocean-sea ice model HadGEM3-GC3.1 and the
Earth system model UKESM1 which adds a carbon-nitrogen cycle and atmospheric chemistry to
HadGEM3-GC3.1. The model results are constrained by the external boundary conditions (forcing data)
and initial conditions.We outline the scientific rationale and assumptions made in specifying these.
Notable details of the implementation include an ozone redistribution scheme for prescribed ozone
simulations (HadGEM3-GC3.1) to avoid inconsistencies with the model's thermal tropopause, and land use
change in dynamic vegetation simulations (UKESM1) whose influence will be subject to potential biases in
the simulation of background natural vegetation.We discuss the implications of these decisions for
interpretation of the simulation results. These simulations are expensive in terms of human and CPU
resources and will underpin many further experiments; we describe some of the technical steps taken to
ensure their scientific robustness and reproducibility
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