A Note on Bjerknes' Hypothesis . . .

Bryan. K. and Stouffer. R.. 1991. A note on Bjerknes' hypothesis for North Atlantic VOiriOibililY. J. ~IOIr. SYSI.. 1: 229-241 On decadal time-scales the historical surface temperature record over land in the Northern Hemisphere is dominated by polar amplified variations. These variations are coherent ~.ith SST anomalies concentrated in the Northwest Atlantic. but extending with lesser amplitude into the North Pacific as well. Bjerknes suggested that multi-year SST anomalies in the subpolar North Atlantic were due to irregular changes in the intensity of the thermohaline circulation. In support of the Bjerknes hypothesis there is evidence that winter overturning in the Labrador Sea was suppressed for a brief period from 1967-1969 by a cap of relative fresh water at the surface. Cause and effect ;ue unclear. but this event was associated ~'ith a marked cooling of the entire Northern Hemisphere. The difference in SST averaged over the Northern Hemisphere oceans aJ"ld SST averaged over the Southern Hemisphere oceans from the equator to 40 0 S is coherent with Sahel summer rainfall on decadal time scales. Empirical evidence is supported by numerical experiments with the British Meteorological Office atmospheric climate model which simulate augmented monSoonal rainfall in the Sahel region of Africa in response to realistic warm SST anomalies in the Northwest Atlantic. A coupled ocean-atmosphere global model exhibits two equilibrium climate states. One has an acti\e thermohaline circulation in the North Atlantic and the other does not. The two climate states provide an extreme example ~'hich illustrates the type of large scale air sea interaction Bjerknes \isualized as a mechanism for North Atlantic climate \'ariability on decadal