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Stratospheric Processes And their Role in Climate
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The Future Development of SPARC

Alan O’Neill1 ( alan@met.reading.ac.uk), and A.R. Ravishankara2 (ravi@al.noaa.gov), SPARC co-Chairs (on behalf of the SPARC SSG)

1Data Assimilation Research Centre, Reading, UK
2NOAA-Aeronomy LAboratory, Boulder CO, USA

In a companion article in this Newsletter, M.-L. Chanin summarises the first decade of SPARC as a project in the World Climate Research Programme (WCRP) (see report). As the first co-chairs of SPARC, M.-L. Chanin and M. Geller played a central role in guiding SPARC’s development and encouraging numerous enthusiastic scientists to become involved with it. We have heard members of WCRP’s Joint Scientific Steering Committee (JSC) say on several occasions that they were impressed by SPARC’s approach and effectiveness.

Briefly put, SPARC’s approach has been first, to be responsive to the need for scientific input to international scientific assessments; secondly, to identify manageable projects where co-ordination at international level can make a difference; and thirdly, to have clear deliverables for each project, such as scientific reviews which summarise the state of knowledge, facilitate and stimulate new directions for research.

M.-L. Chanin’s article has noted that SPARC organised its activities around particular foci, or themes concerned with observed changes in the stratosphere, atmospheric processes relevant to those changes, and modelling those processes. Our intention is to retain this thematic structure, but to evolve it in response to progress during the first phase of SPARC, and to respond to new issues that have recently emerged about the role of the stratosphere in climate. The main aim of SPARC will continue to be: to bring stratospheric expertise to bear on scientific issues concerned with climate processes and climate prediction, for the benefit of climate science as a whole, and specifically for WCRP, the WMO/UNEP Ozone Assessment exercises, the Intergovernmental Panel on Climate Change (IPCC), and the Space Agencies, which seek guidance on stratospheric issues for mission planning. The effective approach developed by our predecessors will remain: to deal with manageable scientific tasks, with a well-defined outcome, over a relatively short period of time (to bring about closure and to maintain momentum in the project), while seeking to anticipate the needs of the wider community.

We see collaboration with other international projects, both within and outside the current WCRP, as essential for promoting SPARC science. The JSC of the WCRP has recommended the following key objectives for SPARC: to lead a collaboration on chemistry-climate interactions with the IGAC project, a project in the International Geosphere Biosphere Programme (IGBP); to focus on issues raised by recent studies of the Arctic Oscillation (AO); to liaise with SCOSTEP on solar radiative forcing and temperature trends; to work with the WMO’s Global Atmospheric Watch (GAW) project on the penetration of ultra-violet radiation; and to contribute to international planning and mission planning. The above list is merely a subset of possible areas of collaboration. We should add, for instance CLIVAR, with which much stronger links are essential on long-term climate variability and predictability.

In response to the stimulus of the JSC, and with the guidance of our colleagues in SPARC (particularly the SSG), we propose the following outline of SPARC’s scientific objectives for the next five years. Implicit in each of them is the goal, now being re-emphasised within WCRP’s developing strategy, of making better predictions of changes to the climate system. The scientific questions raised under each of the themes set out below give a preliminary view of some of the key questions as seen by SPARC’s Scientific Steering Group (SSG). Working groups are being established for each theme to refine the issues, to chart the way forward and to widen the participation of the scientific community.

SPARC Scientific Themes and Associated Key Questions

The following themes are meant to encapsulate, in brief, SPARC’s future programme. The associated questions posed with each theme are certainly not exhaustive; they aim to identify primary foci for our activities, at least in the immediate future.

1. Detection, Attribution and Prediction of Stratospheric Changes

This theme is a continuation, synthesis and extension of earlier SPARC themes on long-term variability and trends in the stratosphere. The extension of previous work should be a greater emphasis on attribution and prediction, which will require a concerted, collaborative research programme involving, in many instances, coupled chemistry-climate models. A report, in this Newsletter, on the SPARC Workshop on Understanding Seasonal Temperature Trends in the Stratosphere summarises some essential priorities concerned with the acquisition of observational data and the needs for numerical modelling (see report). It is now clear that confidence in attribution and prediction will demand statistically significant results based on large ensembles of integrations with numerical models (or approaches that can be shown to be statistically equivalent). SPARC can play an important role in co-ordinating experimental design by different groups to facilitate meaningful comparison of results.

2. Stratospheric Chemistry and Climate

The latest assessment report of the IPCC identifies insufficient knowledge of the coupling and feedbacks between atmospheric chemistry, the biosphere and the climate – and the consequent failure to represent the relevant processes adequately in climate prediction models – as serious scientific limitations. Some of the diverse scientific challenges that must be tackled have been summarised by A.R. Ravishankara at the SSG meeting (see the full report). An interdisciplinary approach must be adopted involving laboratory measurements, field campaigns and numerical modelling. It is proposed that work under this theme will be undertaken as a strong collaboration between SPARC and the IGAC project of the IGBP. The UTLS region is a region of common interest where some of the scientific challenges are most demanding.

3. Stratosphere-Troposphere Coupling

A strong motivation for this theme is that several recent observational studies have proposed that a so-called Arctic Oscillation (or Northern Annular Mode, with an equivalent Southern Annular Mode) is a dominant component of large-scale variability in the atmosphere. The finding that anomalies in an AO index can sometimes span the stratosphere-troposphere (ST) system has revivified the long-stating issue of ST coupling. In particular, the occasional downward propagation of anomalies from the stratosphere into the troposphere implies, with support from statistical analysis of the data, that knowledge of the state of the stratosphere can enhance our ability to predict aspects of the large-scale evolution of the troposphere, which would be of practical value for weather forecasting and climate prediction. Whether the state of the stratosphere influences the evolution of the troposphere in any causal sense, and if so by what mechanisms and on what timescales, are key issues demanding numerical experimentation.

Underpinning Activities

We anticipate that research to address these scientific questions will highlight the need for underpinning activities, which will require the setting up of (possibly temporary) targeted working groups.

Three such activities are:

SPARC’s collaborations on model development have been undertaken within the GCM Reality Inter-comparison Project for SPARC (GRIPS). As M.-L. Chanin notes in her article, GRIPS has evolved through successive phases, from undertaking basic comparisons of models, to studies of mechanisms. Since numerical modelling will be an essential component of all the above scientific themes, we envisage that GRIPS will evolve to play a key role in all of them, with more targeted exercises to document and resolve model deficiencies being undertaken by specially created working groups.

We also envisage that targeted working groups will need to be established to resolve a variety of issues concerned with atmospheric processes within the context of the main scientific themes. As one of many possible examples, considerable uncertainties remain about microphysical processes in the atmosphere, in the tropopause transition layer (or tropical tropopause layer) in particular, uncertainties which seriously limit our ability to understand the transport of water vapour from the troposphere to the stratosphere, and to account for apparent long-term variability in water vapour concentrations. SPARC will contribute to resolving these uncertainties through scientific assessments aimed at producing scientific review papers, and by promoting and participating in observational campaigns and associated numerical modelling.

In connection with observational data for the scientific themes, we aim to secure a continuing role for the SPARC Data Centre as a repository of data sets associated with SPARC’s research activities. Global, quality-controlled data sets produced by assimilating diverse observations into GCMs are a vital resource for climate research. In recognition of this, a SPARC Data Assimilation Working Group has been established to provide the community with information on data availability, location and documentation; to carry out inter-comparisons of data sets focused on specific atmospheric processes and phenomena; and to consult with the space agencies on data needs. This working group aspires to close links with a related activity in WCRP’s Working Group on Numerical Experimentation (WGNE).

During the next few months, SPARC colleagues will be helping us to devise a specific set of actions to implement the scientific programme outlined above. By demanding clear thinking on how an international project like SPARC can make a difference, the very act of creating a new “implementation plan” should in itself be a stimulus to moving the programme forward, as well as to communicating our goals to potential collaborators.

We both feel privileged to be co-chairs of a well-respected project like SPARC, at a time when the stratosphere is gaining new prominence in climate science. The friendliness and enthusiasm of the growing community involved with SPARC is testimony to the wise leadership and inspiration of the co-chairs who preceded us, M.-L Chanin and M. Geller. Our unalloyed enthusiasm for SPARC is tempered only by the thought that we have a lot to live up to.

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