Global climate modelling (CCMs & ESMs)

Our current understanding of the two-way interactions between the stratosphere and the troposphere is not adequate for predicting how the coupled climate system will evolve, and thus how future stratospheric changes will affect surface climate. This global modelling component will exploit recent advances in conceptual understanding, together with advances in modelling to quantify measures of the interactions between the stratosphere and the troposphere within the coupled chemistry-climate system. We will explore stratospheric trends and changes in variability and their link to climate and weather patterns. We will improve our understanding of the effects of polar ozone depletion/recovery on surface climate, as well as how climate change affects the stratospheric environment.

The research methodology will exploit novel observations and model data sets that become available during this project, e.g. the CMIP5 integrations (Taylor et al., 2009, 2012); the REF-C1 (hindcast 1960-2010), -C2 (forecast 1960-2100), and –C1SD (nudged run 1980-2010) simulations which are currently performed for the upcoming WMO ozone assessment (internationally organised SPARC/IGAC CCMI activity); the Paleoclimate Modelling Intercomparison Project (PMIP) (Otto-Bliesner et al. 2009); etc. It will further include new integrations and sensitivity studies performed using the state-of-the-art Chemistry-Climate Models (CCMs) and Earth System Models (ESMs). It will finally make use of improved process treatments developed within the project, based on the results of the tropical field campaign (WP 1), analyses of ground- and space-borne measurements (WPs 2 and 3) and connected process studies (WP 4). We will also use a range of simpler mechanistic models to explore the two-way interactions between the troposphere and stratosphere as well as global models including newly-developed, simplified ozone and aerosol schemes and CCM runs in ‘nudged’ mode, i.e. where model dynamics are relaxed towards observed meteorological data (e.g. reanalyses).

Three main areas of work will address the issues of

1. modelling the entry pathways into the stratosphere, including climate-driven changes in transport and tropospheric processes (e.g. sulfur fluxes, fluxes of biogenic emissions, water vapour transport), using our improved global models and emission modules developed in WP 4.1;
2. seasonal, interannual and long-term changes of chemical composition and thermal structure in the UTS;
3. the impact of changes in the UTS on tropospheric climate and composition.

5.1 Entry pathways

A range of models (in particular CCMs and ESMs) will be used for process-oriented sensitivity studies. This work will be closely linked with the activity on “Process and regional modelling” (WP 4) and will exploit, inter alia, data generated in our field campaign (WP 1) and ground station (WP 2).

1. Improved treatments of sulfur fluxes, based on the work described in WP 4.3 will be implemented into CCMs. Among others, questions to be answered are: What climate sensitivity to stratospheric input will model simulations reveal? How will this depend on different sulfur treatments and the treatment of microphysical processes?
2. Interactive (climate-dependent) schemes for the emission of short-lived halogens will be installed into CCMs. Investigations will focus on questions including: How will the sensitivity of transport of natural halogens into the stratosphere depend on different flux treatments?
3. Transport processes in the tropical tropopause layer (TTL) and around the extra-tropical tropopause determining the flux of both water vapour, and short-lived halogen substances, into the stratosphere will be studied using our CCMs and ESMs: What are the entry pathways? What do measurements (new and existing) of tracers with different lifetimes tell us about the transport processes?
4. Decadal variations in water vapour in the lower stratosphere will be evaluated: Which variability do our interactive CCMs and ESMs exhibit? What drives this variability? The underlying mechanisms, including interactions between large-scale dynamics and coupled chemical-radiative changes in the TTL, will be investigated using mechanistic models to complement the CCM and ESM results.

5.2 Seasonal, interannual and long-term changes of composition and thermal structure in the UTS

Multi-decadal transient simulations with ESMs and CCMs together with long-term observations (especially from space-borne instruments, including the new products produced in WP 3) will be used to investigate changes in the variability of the UTS region in a changing climate. We will examine how the underlying processes have changed in the past and will likely alter in a changing climate, e.g. through modifications in stratospheric dynamics (with focus on the Brewer-Dobson circulation, BDC) or explicit treatment of aerosol microphysics:

1. The recovery of the ozone layer in response to control of emissions of ozone depleting substances (ODSs): The sensitivity of recovery to greenhouse gas scenarios (IPCC RCP scenarios) and to possible climate-related variations in halogen loading of the stratosphere will be explored as well as changes in Arctic lower stratospheric temperatures and climate change (connection to point 2., below).
2. Mechanisms of possible long term changes in the BDC and impact on stratospheric composition changes will be examined in diagnostic studies of the ESM and CCM simulations together with suitable complementary mechanistic model studies.
3. The impact of changes in the stratospheric aerosol loading on stratospheric circulation and chemistry will be investigated. The use of ESM and CCM which explicitly treat aerosol microphysical processes, permits a consistent calculation of the impact on changes in sources (e.g. volcanoes, marine sources (WP 4.1), entry pathways (WP 5.1) and circulation (see point 2 above) on aerosol size and optical properties.
4. The role of El Niño/Southern Oscillation (ENSO) and the quasi-biennial oscillation (QBO) in determining variations in the BDC and in the Arctic ozone layer will be analysed.

5.3 Impact of changes in the UTS on tropospheric climate and composition

The impact of stratospheric changes (thermal structure, amount of water vapour, ozone depletion/recovery) on tropospheric composition and surface climate on different time scales will be explored. Transient simulations (i.e. with variable boundary conditions) with ESMs and CCMs of the past and future as well as new sensitivity simulations, including idealised studies with mechanistic models, will be analysed to investigate the troposphere-stratosphere coupling, to understand the responsible feedback mechanisms, and to predict its future evolution. Also, for these purposes improved diagnostics for the stratospheric variability and the troposphere-stratosphere coupling will be developed thereby increasing the signal to noise ratio. Variations and changes in cold spell events in the northern hemisphere associated with stratosphere changes will be used to evaluate socio-economic impacts of these changes over Europe (WP 6). Four main topics are considered here:

1. Modes of variability and dynamical connections between stratosphere and troposphere: We will investigate the mechanisms linking changes in the stratosphere, on various timescales, to tropospheric modes of variability including the Northern Annular Mode (NAM), the North Atlantic Oscillation (NAO), ENSO, tropospheric blockings, etc. Mechanistic models in particular will be important here in examining relationships between the stratosphere and troposphere that are apparent from ESM and CCM simulations to determine cause-and-effect relations.
2. Seasonal to decadal variations and climate: We will study links between extreme conditions in the northern hemisphere winter stratosphere and anomalous surface weather events. We will make use of extended seasonal prediction models to include fast, simplified ozone and aerosol chemistry schemes. Links between decadal variations in the strength of the northern hemisphere stratospheric vortex and the oceanic circulation will be examined. The climatic impact of the stratospheric sulfate aerosol layer under perturbed (large volcanic eruption, geo-engineering) and unperturbed conditions will be assessed for decadal climate variability. Global aerosol simulation will be used to study the possible impact of potential future eruption on European and global climate.
3. Multi-decadal to centennial changes: Using various long-term CCM and ESM integration ensembles, we will diagnose the impact of changes in stratospheric chemical and aerosol composition on surface climate.
4. Changes in the stratospheric BDC and in lower stratospheric ozone can impact tropospheric composition (incl. oxidation capacity) through changes in stratosphere-troposphere exchange and through modifications in transmitted UV radiation. Changes in tropospheric oxidation can themselves impact troposphere to stratosphere transport. These dynamical-chemical-radiative feedbacks will be investigated, including the sensitivity to greenhouse gas emission scenarios.

Topics 1, 2 and 3 will use the full range of our models, from mechanistic models, including large-scale dynamical processes but otherwise highly simplified physics, through atmospheric general circulation models, models with specified fields of radiatively active gases, to CCMs and ESMs. Topics 3 and 4 will exploit our improved CCMs and ESMs including fast model tools that allow to fully include the interactive feedbacks from UTS ozone and aerosol on surface climate (WP 4.3). Topic 2 will also be addressed with CCMs and ESMs using either a prescribed aerosol climatology or an explicit treatment of aerosol microphysical processes.