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  • Knight, J. R.,, 2008: The Atlantic Multidecadal Oscillation as inferred from the forced climate response in coupled general circulation models. J. Climate. Submitted.

Instrumental sea surface temperature records in the North Atlantic
Ocean are characterised by large multidecadal variability known as
the Atlantic Multidecadal Oscillation (AMO). The lack of strong
oscillatory forcing of the climate system at multidecadal time
scales and the results of long unforced climate simulations have led
to the widespread, although not ubiquitous, view that the AMO is an
internal mode of climate variability. Here, a more objective
examination of the the origins of the AMO is performed using
simulations with natural and anthropogenic forcings from the CMIP3
data base. Ensemble means derived from these data allow an estimate
of the response of models to forcings, as averaging leads to
cancellation of the internal variability between ensemble members.
In general, the means of individual model ensembles appear to be
inconsistent with observed temperatures, although small ensemble
sizes result in uncertainty in this conclusion. Combining the
ensembles from different models creates a multi-model ensemble of
sufficient size to allow a good estimate of the forced response.
This shows that the variability in observed North Atlantic
temperatures possess a clearly distinct signature to the climate
response expected from forcings. The reliability of this finding is
confirmed by sampling those models with low decadal internal
variability and by a comparison of simulated and observed trends. In
contrast to the inconsistency with the ensemble mean, the
observations are consistent with the spread of responses in the
ensemble members, suggesting they can be accounted for by the
combined effects of forcings and internal variability. In the most
recent period, the results suggest that the North Atlantic is
warming faster than expected and that the AMO entered a positive
phase in the 1990s. The differences found between observed and
ensemble mean temperatures could arise through errors in the
observational data, errors in the models' response to forcings or in
the forcings themselves, or as a result of genuine internal
variability. Each of these possibilities is discussed and it is
concluded that internal variability within the natural climate
system is the most likely origin of the differences. Finally, the
estimate of internal variability obtained using the model-derived
ensemble mean is proposed as a new way of defining the AMO which has
important advantages over previous definitions.

Last Updated: 2008-08-22

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