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A 423-Year Record of the Atlantic Multidecadal Oscillation (AMO)
Reference
Gray, S.T., Graumlich, L.J., Betancourt, J.L. and Pederson, G.T. 2004. A tree-ring based reconstruction of the Atlantic Multidecadal Oscillation since 1567 A.D. Geophysical Research Letters 31: 10.1029/2004GL019932.

Background
The authors note that "natural, low-frequency variations" in near-surface air temperature may "mask or amplify secular trends in the climate system." Hence, it is important - nay, necessary - to study such phenomena in order to correctly determine the cause(s) of recent climate change.

What was done
Gray et al. developed a reconstruction of the leading mode of low-frequency North Atlantic (0-70N) SST (sea surface temperature) variability - denominated the Atlantic Multidecadal Oscillation (AMO) - for the period AD 1567-1990, based on tree-ring records from regions known to border on strong centers of AMO variability: eastern North America, Europe, Scandinavia and the Middle East.

What was learned
In terms of both duration and magnitude, it was determined, in the words of the authors, that "AMO variability observed in late-19th and 20th century instrumental records is typical of North Atlantic multidecadal behavior over longer periods." It was also observed that a new warming phase began in the mid-1990s and that the most intense warm phase of the AMO record occurred between 1580 and 1596.

What it means
Gray et al. say that the first of these findings suggests that "the mechanisms driving AMO variability have operated in a similar fashion for (at least) the previous 500 years," and that "trace-gas forcing has yet to significantly affect the low-frequency component of THC [thermohaline circulation] variability." This latter conclusion also suggests that trace-gas forcing has yet to significantly affect near-surface air temperature, while the identification of the start of a new warming phase in the mid-1990s suggests that the supposedly record temperatures of the past decade or so may well have been more driven by natural AMO variability than increasing greenhouse gas concentrations.

Last of all, but by no means of least interest, we note that the most intense warm phase of the AMO record occurred near the end of the time frame associated with the evidences of the "Little" Medieval Warm Period we describe in our Editorial of 30 June 2004, when global air temperature was likely significantly warmer than it has been at any subsequent time, the tail-end of the fingerprint of which unmatched heat is clearly evident in Gray et al.'s reconstruction of North Atlantic SST anomalies. Hence, the evidence continues to mount for the existence of a warm period in the general vicinity of the 1500s (when the air's CO2 content was much less than it is today) that experienced temperatures that were unprecedented for perhaps the entire past millennium.


Reviewed 11 August 2004