The 1986–1989 ENSO cycle in a chemical climate model S. Brönnimann1, M. Schraner1, B. Müller1, A. Fischer1, D. Brunner1, E. Rozanov1,2, and T. Egorova2 1Institute for Atmospheric and Climate Science, ETH Zurich, Universitätsstr. 16, CH-8092 Zurich, Switzerland 2PMOD/WRC, Dorfstr. 33, CH-7260 Davos, Switzerland
Abstract. A pronounced ENSO cycle occurred from 1986 to 1989, accompanied by distinct
dynamical and chemical anomalies in the global troposphere and stratosphere.
Reproducing these effects with current climate models not only provides a
model test but also contributes to our still limited understanding of ENSO's
effect on stratosphere-troposphere coupling. We performed several sets of
ensemble simulations with a chemical climate model (SOCOL) forced with
global sea surface temperatures. Results were compared with observations and
with large-ensemble simulations performed with an atmospheric general
circulation model (MRF9). We focus our analysis on the extratropical
stratosphere and its coupling with the troposphere. In this context, the
circulation over the North Atlantic sector is particularly important.
Relative to the La Niña winter 1989, observations for the El Niño
winter 1987 show a negative North Atlantic Oscillation index with
corresponding changes in temperature and precipitation patterns, a weak
polar vortex, a warm Arctic middle stratosphere, negative and positive total
ozone anomalies in the tropics and at middle to high latitudes,
respectively, as well as anomalous upward and poleward Eliassen-Palm (EP)
flux in the midlatitude lower stratosphere. Most of the tropospheric
features are well reproduced in the ensemble means in both models, though
the amplitudes are underestimated. In the stratosphere, the SOCOL
simulations compare well with observations with respect to zonal wind,
temperature, EP flux, meridional mass streamfunction, and ozone, but
magnitudes are underestimated in the middle stratosphere. With respect to
the mechanisms relating ENSO to stratospheric circulation, the results
suggest that both, upward and poleward components of anomalous EP flux are
important for obtaining the stratospheric signal and that an increase in
strength of the Brewer-Dobson circulation is part of that signal.
Citation: Brönnimann, S., Schraner, M., Müller, B., Fischer, A., Brunner, D., Rozanov, E., and Egorova, T.: The 1986–1989 ENSO cycle in a chemical climate model, Atmos. Chem. Phys., 6, 4669-4685, doi:10.5194/acp-6-4669-2006, 2006.