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J.D. Opsteegh, R.J. Haarsma,
F.M. Selten and A. Kattenberg, 1998,
Tellus, 50A, 348-367 .
As an alternative to the frequently used mixed boundary conditions in ocean
GCM's we present a dynamic atmospheric model (ECBILT) that is simple and yet
describes the relevant dynamic and thermodynamic feedback processes to the
ocean. This provides the possibility of studying atmosphere/ocean dynamics on
very long time-scales of order thousand years. The model is two orders of
magnitude faster than AGCMs.
We have been running ECBILT with prescribed SST's for a period of 500 years
with seasonal cycle included both in the solar forcing and in the
climatological SST's. The mean state and the variability properties of ECBILT
are reasonably realistic. The simulation of the surface fluxes is quite
realistic and justifies coupling ECBILT to an ocean GCM.
We have done two SST anomaly experiments, one with a tropical SST anomaly as
observed during January 1983 and one with an SST anomaly in the northern
extra-tropical Atlantic ocean. For the tropical SST anomaly experiment the
amount of anomalous precipitation agrees well with what has been found with
atmospheric GCM's, but the resulting extra-tropical teleconnection pattern is
very weak. The atmospheric response pattern to extra-tropical SST anomalies
agrees well with similar SST anomaly experiments performed with atmospheric
GCM's.
We have tested the performance of ECBILT in coupled mode by coupling it to a
simple ocean GCM and thermodynamic sea-ice model and integrating the coupled
system for a period of thousand years after a spin up of 4000 years. The
simulation of the mean water mass distribution and the mean circulation of the
ocean resembles the observed ocean circulation. It has a warm and fresh bias
and the circulation and associated transports are too diffuse and too weak.
The ocean's variability is realistic, considering the simplicity of the ocean
model, although a bit too weak.
We have explored the covariability between the atmosphere and ocean over the
Northern Atlantic ocean by performing a singular value decomposition of SST
anomalies and 800 hPa geopotential height anomalies. The second mode shows a
peak in both spectra at a timescale of about 18 years. The time scale of this
mode is set by the ocean but the physical mechanisms that are operating are not
yet unambiguously identified. The simulation of this coupled extra tropical
decadal mode of variability, which also shows up in the observations and in
much more complex coupled models provides strong evidence for the potential
usefullness of ECBILT when studying atmosphere/ocean interaction and the
associated ocean variability on time scales ranging from decades to many
thousands of years.