Evaluation of the North Atlantic Oscillation as simulated by a coupled climate model |
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Authors: | T J Osborn K R Briffa S F B Tett P D Jones R M Trigo |
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Institution: | (1) Climatic Research Unit, University of East Anglia, Norwich, United Kingdom E-mail: t.osborn@uea.ac.uk, GB;(2) Hadley Centre for Climate Prediction and Research, Meteorological Office, Bracknell, United Kingdom, GB |
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Abstract: | The realism of the Hadley Centre’s coupled climate model (HadCM2) is evaluated in terms of its simulation of the winter North
Atlantic Oscillation (NAO), a major natural mode of the Northern Hemisphere atmosphere that is currently the subject of considerable
scientific interest. During 1400 y of a control integration with present-day radiative forcing levels, HadCM2 exhibits a realistic
NAO associated with spatial patterns of sea level pressure, synoptic activity, temperature and precipitation anomalies that
are very similar to those observed. Spatially, the main model deficiency is that the simulated NAO has a teleconnection with
the North Pacific that is stronger than observed. In a temporal sense the simulation is compatible with the observations if
the recent observed trend (from low values in the 1960s to high values in the early 1990s) in the winter NAO index (the pressure
difference between Gibraltar and Iceland) is ignored. This recent trend is, however, outside the range of variability simulated
by the control integration of HadCM2, implying that either the model is deficient or that external forcing is responsible
for the variation. It is shown, by analysing two ensembles, each of four HadCM2 integrations that were forced with historic
and possible future changes in greenhouse gas and sulphate aerosol concentrations, that a small part of the recent observed
variation may be a result of anthropogenic forcing. If so, then the HadCM2 experiments indicate that the anthropogenic effect
should reverse early next century, weakening the winter pressure gradient between Gibraltar and Iceland. Even combining this
anthropogenic forcing and internal variability cannot explain all of the recent observed variations, indicating either some
model deficiency or that some other external forcing is partly responsible.
Received: 20 August 1998 / Accepted: 12 May 1999 |
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