The Impact of North Atlantic--Arctic Multidecadal Variability on Northern Hemisphere Surface Air Temperature.

The twentieth-century Northern Hemisphere surface climate exhibits a long-term warming trend largely caused by anthropogenic forcing, with natural decadal climate variability superimposed on it. This study addresses the possible origin and strength of internal decadal climate variability in the Northern Hemisphere during the recent decades. The authors present results from a set of climate model simulations that suggest natural internal multidecadal climate variability in the North Atlantic--Arctic sector could have considerably contributed to the Northern Hemisphere surface warming since 1980. Although covering only a few percent of the earth''s surface, the Arctic may have provided the largest share in this. It is hypothesized that a stronger meridional overturning circulation in the Atlantic and the associated increase in northward heat transport enhanced the heat loss from the ocean to the atmosphere in the North Atlantic region and especially in the North Atlantic portion of the Arctic because of anomalously strong sea ice melt. The model results stress the potential importance of natural internal multidecadal variability originating in the North Atlantic--Arctic sector in generating interdecadal climate changes, not only on a regional scale, but also possibly on a hemispheric and even a global scale. [ABSTRACT FROM AUTHOR]