Impact of prescribed SSTs on climatologies and long-term trends in CCM simulations Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
Received: 22 Dec 2008 – Published in Atmos. Chem. Phys. Discuss.: 23 Feb 2009 – Published: 20 Aug 2009Abstract. 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.
Citation: Garny, H., Dameris, M., and Stenke, A.: Impact of prescribed SSTs on climatologies and long-term trends in CCM simulations, Atmos. Chem. Phys., 9, 6017-6031, doi:10.5194/acp-9-6017-2009, 2009.