On the Roles of Tropical and Midlatitude SSTs in Forcing Interannual to Interdecadal Variability in the Winter Northern Hemisphere Circulation

Three numerical experiments have been conducted to explore the relative roles of midlatitude and tropical SSTs in producing atmospheric variability. In these experiments, anomalous observed SSTs were prescribed in 1) the tropical oceans only, 2) the midlatitude oceans only, and 3) globally. These simulations were conducted with the same atmospheric model and covered the period 1970–88. Although each simulation reproduced some aspects of the observed circulation variability, the results from the two experiments including tropical SSTs performed best by most measure particularly in showing temporal signals that agreed with those seen in the observations. The results indicate that the response of the observed atmospheric circulation to North Pacific SST anomalies is much smaller and much less systematic than the response to tropical SSTs. It is suggested that the apparent links between North Pacific SSTs and the observed winter circulation am due primarily to the effects on oceanic forcing by the recurrent patterns of atmospheric variability remotely forced by changes in tropical SSTs. The results are consistent with the idea that the major shift in the winter circulation during the mid-1970s was forced by changes in tropical SSTS.