Observed North Atlantic Multidecadal Variability Michael E. Mann Department of Environmental Sciences University of Virginia Tom Delworth Geophysical Fluid Dynamics Laboratory Princeton, New Jersey NASA-IPRC-CLIVAR Workshop on Decadal Climate Variability Univ. of Hawaii, Manoa Hawaii January 8, 2001 Collaborators: R.J. Allan, C.K.Folland, D.E. Parker, N. Rayner

REFERENCE Delworth, T.L., Mann, M.E., Observed and Simulated Multidecadal Variability in the Northern Hemisphere, Climate Dynamics, 16, , 2000.

OVERVIEW Evidence for multidecadal variability in the instrumental record Longer-term evidence from proxy- based climate reconstructions Comparison with coupled model evidence Conclusions

North Atlantic Oscillation Explains enhanced warming in certain regions of Northern Hemisphere in past couple decades

There is evidence of multidecadal natural variability in the North Atlantic ocean circulation Atlantic Multidecadal Oscillation

Secular Warming Pattern

Secondary Secular Pattern

DJF MAM JJA SON SECULAR 1: f=0 [73%]SECULAR 2: f= (68.5 yrs) [23%]

CONCLUSIONS Evidence from instrumental record for a multidecadal pattern of climate variability centered in the North Atlantic, in addition to the (likely, in large part, anthropogenic) secular trend Evidence from paleoclimate reconstructions for the persistence of this climate signal over the past several centuries. Possible interactions with long-term natural (solar) radiative forcing Evidence for a similar intrinsic multidecadal climate signal in the GFDL coupled model