Observed and simulated multidecadal variability in the Northern Hemisphere

Analyses of proxy based reconstructions of surface temperatures during the past 330 years show the existence of a distinct oscillatory mode of variabi lity with an approximate time scale of 70 years. This variability is also s een in instrumental records, although the oscillatory nature of the variabi lity is difficult to assess due to the short length of the instrumental rec ord. The spatial pattern of this variability is hemispheric or perhaps even global in scale, but with particular emphasis on the Atlantic region. Inde pendent analyses of multi century integrations of two versions of the GFDL coupled atmosphere-ocean model also show the existence of distinct multidec adal variability in the North Atlantic region which resembles the observed pattern. The model variability involves fluctuations in the intensity of th e thermohaline circulation in the North Atlantic. It is our intent here to provide a direct comparison of the observed variability to that simulated i n a coupled ocean-atmosphere model, making use of both existing instrumenta l analyses and newly available proxy based multi-century surface temperatur e estimates. The analyses demonstrate a substantial agreement between the s imulated and observed patterns of multidecadal variability in sea surface t emperature (SST) over the North Atlantic. There is much less agreement betw een the model and observations for sea level pressure. Seasonal analyses of the variability demonstrate that for both the model and observations SST a ppears to be the primary carrier of the multidecadal signal.