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 relativ e roles of midlatitude and tropical SSTs in producing atmospheric vari ability. 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 reprod uced some aspects of the observed circulation variability, the results from the two experiments including tropical SSTs performed best by mo st measures, particularly in showing temporal signals that agreed with those seen in the observations. The results indicate that the respons e of the observed atmospheric circulation to North Pacific SST anomali es is much smaller and much less systematic than the response to tropi cal SSTs. It is suggested that the apparent links between North Pacifi c SSTs and the observed winter circulation are due primarily to the ef fects on oceanic forcing by the recurrent patterns of atmospheric vari ability remotely forced by changes in tropical SSTs. The results are c onsistent with the idea that the major shift in the winter circulation during the mid-1970s was forced by changes in tropical SSTs.