Abstract
Abstract. Chemistry-Climate Model (CCM) simulations are commonly used to project the past and future development of the dynamics and chemistry of the stratosphere, and in particular the ozone layer. So far, CCMs are usually not interactively coupled to an ocean model, so that sea surface temperatures (SSTs) and sea ice coverage are prescribed in the simulations. While for future integrations SSTs have to be taken from precalculated climate model projections, for CCM experiments resembling the past either modelled or observed SSTs can be used. This study addresses the question to which extent atmospheric climatologies and long-term trends for the recent past simulated in the CCM E39C-A differ when choosing either observed or modelled SSTs. Furthermore, the processes of how the SST signal is communicated to the atmosphere, and in particular to the stratosphere are examined. Two simulations that differ only with respect to the prescribed SSTs and that span years 1960 to 1999 are used. Significant differences in temperature and ozone climatologies between the model simulations are found. The differences in ozone are attributed to differences in the meridional circulation, which are in turn driven by weaker wave forcing in the simulation with generally lower SSTs. The long-term trends over 40 years in annual mean temperature and ozone differ only in the troposphere, where temperatures are directly influenced by the local SST trends. Differences in temperature and ozone trends are only found on shorter time scales. The trends in tropical upwelling, as a measure of the strength of the Brewer-Dobson circulation (BDC), differ strongly between the simulations. A reverse from negative to positive trends is found in the late 1970s in the simulation using observed SSTs while trends are positive throughout the simulation when using modelled SSTs. The increase in the BDC is a robust feature of the simulations only after about 1980 and is evident mainly in the tropics in the lower stratosphere.
Highlights
Chemistry-Climate Models (CCMs) were used extensively in the last decade to study the evolution of the global ozone layer and to identify processes leading to changes in ozone in the past and future
While in REF1, the boundary conditions including natural and anthropogenic variability are mainly deduced from observations, the REF2 or SCN2 scenarios are subjected to the use of future projections of the development of the boundary conditions
These HadGEM1 simulations are part of the World Climate Research Programme’s (WCRP’s) Coupled Chemistry Climate Intercomparison Project phase 3 (CMIP-3) multimodel dataset used for the 4th Intergovernmental Panel on Climate Change (IPCC) Assessment Report and were provided by the Program for Climate Model Diagnosis and Intercomparison (PCMDI, available at: http://www-pcmdi.llnl.gov)
Summary
Chemistry-Climate Models (CCMs) were used extensively in the last decade to study the evolution of the global ozone layer and to identify processes leading to changes in ozone in the past and future. In Deckert and Dameris (2008a), SSTs were linked to tropical upwelling via wave generation by deep convection, implying that tropical upwelling is sensitive to changes in SSTs. While this study is based on CCM simulations, similar evidence of a linkage between tropical SSTs and the lower stratosphere was found from radiosonde observational data by Rosenlof and Reid (2008). Since for future predictions only modelled SSTs can be used, the issue of how the SST differences between observed and modelled data sets affect both the climatological mean state and trends and variability of the modelled atmosphere is highly relevant to assess the uncertainty of future projections. When comparing a past period of the REF1 simulation with a future period from REF2/SCN2, one has to be aware of the fact that the changing pattern is a superimposition of changes due to changing boundary conditions with time (“climate change”), and differences due to the use of a different SST data basis.
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