NOTES AND CORRESPONDENCE The Effect of Changes in Observational Coverage on the Association between Surface Temperature and the Arctic Oscillation

The effect of changes in observational coverage on the association between the Arctic oscillation (AO) and extratropical Northern Hemisphere surface temperature is examined. A coupled atmosphere–ocean model, which produces a realistic simulation of the circulation and temperature patterns associated with the AO, is used as a surrogate for the real climate system. The association between the AO and spatial mean temperature, as quantified by regressing the latter on the AO index, is subject to a positive bias due to the incomplete spatial coverage of the observational network. The bias is largest during the early part of the twentieth century and decreases, but does not vanish, thereafter.