Delworth and Mann analyze observed and simulated multidecadal variability in the Northern Hemisphere. Proxy-based reconstructions of surface temperatures over the past 330 years show a distinct oscillatory mode with a 70-year time scale. This variability is also seen in instrumental records, though its oscillatory nature is hard to assess due to the short length of the instrumental record. The spatial pattern of this variability is hemispheric or global, with emphasis on the Atlantic region. Independent analyses of two versions of the GFDL coupled atmosphere-ocean model also show distinct multidecadal variability in the North Atlantic, resembling the observed pattern. The model variability involves fluctuations in the intensity of the thermohaline circulation in the North Atlantic. The study compares observed variability with simulated variability in a coupled ocean-atmosphere model, using both existing instrumental data and newly available proxy-based multi-century surface temperature estimates. The analyses show substantial agreement between simulated and observed patterns of multidecadal variability in sea surface temperature (SST) over the North Atlantic. There is less agreement for sea level pressure. Seasonal analyses show that SST is the primary carrier of the multidecadal signal. The study highlights the importance of understanding internal, unforced variability of the climate system on decadal and longer time scales. The coupled ocean-atmosphere system is modeled as a first-order Markov process, with enhanced internal variability at longer time scales due to ocean thermal inertia. However, there are notable departures from this model, such as the El Niño-Southern Oscillation (ENSO) and greater variability in instrumental records than expected from red noise. Proxy data provide a longer-term perspective on multidecadal and century-scale climate variability, but have been limited by spatial extent and reliability. Recent proxy-based reconstructions are now available, allowing more robust comparisons with model data. The study uses numerical models of the Earth's climate system to analyze extended integrations, offering insights into model-generated variability and underlying physics. Numerical models allow separation of forced signals from internal variability through designed experiments. The study compares observational (instrumental and proxy-based) and model-based results, providing a direct comparison of observed and simulated multidecadal-to-centennial variability in the Northern Hemisphere. The analysis shows a clear resemblance between simulated and observed patterns of multidecadal variability in the North Atlantic, with SST being the primary carrier of the signal. The study also highlights the importance of understanding the mechanisms behind multidecadal variability, such as thermohaline circulation in the North Atlantic. The results suggest that the observed multidecadal variability is robust over several centuries, with a dominant time scale of approximately 70 years. The study concludes that the simulated and observed patterns of multidecadal variability are similar, with the largest resemblance in the North Atlantic. However, notable differences in details and amplitudes are also clear. The study emphasizes the importance of understanding the seasonal dependenceDelworth and Mann analyze observed and simulated multidecadal variability in the Northern Hemisphere. Proxy-based reconstructions of surface temperatures over the past 330 years show a distinct oscillatory mode with a 70-year time scale. This variability is also seen in instrumental records, though its oscillatory nature is hard to assess due to the short length of the instrumental record. The spatial pattern of this variability is hemispheric or global, with emphasis on the Atlantic region. Independent analyses of two versions of the GFDL coupled atmosphere-ocean model also show distinct multidecadal variability in the North Atlantic, resembling the observed pattern. The model variability involves fluctuations in the intensity of the thermohaline circulation in the North Atlantic. The study compares observed variability with simulated variability in a coupled ocean-atmosphere model, using both existing instrumental data and newly available proxy-based multi-century surface temperature estimates. The analyses show substantial agreement between simulated and observed patterns of multidecadal variability in sea surface temperature (SST) over the North Atlantic. There is less agreement for sea level pressure. Seasonal analyses show that SST is the primary carrier of the multidecadal signal. The study highlights the importance of understanding internal, unforced variability of the climate system on decadal and longer time scales. The coupled ocean-atmosphere system is modeled as a first-order Markov process, with enhanced internal variability at longer time scales due to ocean thermal inertia. However, there are notable departures from this model, such as the El Niño-Southern Oscillation (ENSO) and greater variability in instrumental records than expected from red noise. Proxy data provide a longer-term perspective on multidecadal and century-scale climate variability, but have been limited by spatial extent and reliability. Recent proxy-based reconstructions are now available, allowing more robust comparisons with model data. The study uses numerical models of the Earth's climate system to analyze extended integrations, offering insights into model-generated variability and underlying physics. Numerical models allow separation of forced signals from internal variability through designed experiments. The study compares observational (instrumental and proxy-based) and model-based results, providing a direct comparison of observed and simulated multidecadal-to-centennial variability in the Northern Hemisphere. The analysis shows a clear resemblance between simulated and observed patterns of multidecadal variability in the North Atlantic, with SST being the primary carrier of the signal. The study also highlights the importance of understanding the mechanisms behind multidecadal variability, such as thermohaline circulation in the North Atlantic. The results suggest that the observed multidecadal variability is robust over several centuries, with a dominant time scale of approximately 70 years. The study concludes that the simulated and observed patterns of multidecadal variability are similar, with the largest resemblance in the North Atlantic. However, notable differences in details and amplitudes are also clear. The study emphasizes the importance of understanding the seasonal dependence