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Abstract

In this contribution the modelling activities of the climate research group
at the Max—Planck—Institut fuer Meteorologie (MPI) concerning the El
Nifio/Southern Oscillation (ENSO) phenomenon are briefly reviewed. The
studies described encompass the investigation of the atmospheric response to
observed sea surface temperature (SST) distributions, the oceanic response to
observed wind stress and studies with a coupled ocean—atmosphere model
investigating large scale air—sea interactions in the tropics.
It is shown that the atmosphere model simulates realistically the Southern
Oscillation during an extended range integration using near global SSTs for
the period 1970 — 1985. In particular, low frequency changes of sea level
pressure (SLP) as expressed by the Southern Oscillation Index (SOI) show a
good correspondence with the observed changes. Furthermore, the spatial
patterns of surface wind stress anomalies are simulated correctly, while the
variance is somewhat underestimated by the atmosphere model.
The equatorial oceanic general circulation model captures basic aspects of
observed SST variability in the Equatorial Pacific, when forced with observed
wind stress patterns. Associated sea level and current changes are consistent
with the available measurements.
In the next step the two models have been coupled together. Results are
shown from three different experiments. In the first experiment the response
of the coupled model to a westerly wind burst over the Western Pacific was
investigated. It will be shown that the coupling of ocean and atmosphere is
the most important factor for the persistence of SST anomalies in the
Equatorial Pacific. The second experiment is an extended range integration
with the coupled model over ten years. Low frequency variability is
significantly underestimated in this run. The coupled model shows a climate
drift consisting of a gradual cooling of the upper equatorial ocean. The
coupled model was then used to study the possible role of anomalous snow cover
over Eurasia for the initiation of ENSO events. By doubling the snow fall rate
within the atmosphere model a weak El Nifio event was induced in the Tropical
Pacific Ocean.
Finally a simplified coupled ocean—atmosphere model was used for ENSO
hindcast experiments. It will be shown that such a coupled system is quite
successful in predicting the onset and the evolution of the 1982/1983 ENSO
event.