Impact of prescribed SSTs on climatologies and long-term trends in CCM simulations

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, but 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. However, differences in trends are found on shorter time scales and the results suggest that these differences in trends are induced by associated SST trends. The SST trends lead to modifications in planetary wave activity and therefore a modulation of the Brewer-Dobson Circulation (BDC). This results in time series of tropical upwelling, as a measure of the strength of the BDC, differing strongly between the simulations. A reverse from negative to positive trends is found in the simulation using observed SSTs while trends are positive throughout the simulation when using modelled SSTs.