Paper Z 



contributing to the development of surrmer EL Nino conditions in 

 the eostern equatorial zone. 



Over the i\iorth American and Atlantic sectors the key development 

 was the westward retrogression of the dominant centers of action 

 in summerr with a general shortening of wavelengths in the upper 

 westerlies and amplification of the mean flow. From Europe a 

 powerful preexisting block moved westward to settle tenaciously 

 over Britain and assume a dominating role in the Atlantic 

 circulation (Fig. 2.4). With this event and with intensifica- 

 tion of trougfting activity off the Pacific Northwest/ the western 

 Atlantic riage was encouraged to move inland to east-central 

 North America^ leaving only a weak subsidiary cell over the 

 western Atlantic. In the north the persistent mean trough over 

 Greenland also participated in this general retrogressions moving 

 westward while retaining its spring intensity/^ to become 

 centered/ on average^ over the Davis Strait. Coupling between 

 this cell and the separated centers of positive height anomaly to 

 its southwest and southeast induced strong/ anomalous 

 northwesterly winds from arctic Canada to the Laborador Sea and 

 vigorous/ anomalous southwesterly flow at 7C0 mb from southern 

 Greenland to Iceland and the Norwegian-Greenland Sea. Thus/ 

 while the circulation in this sector was much amplified compared 

 to earlier seasons/ the polar westerlies retained great vigor in 

 the strong barocLinic zone along the Arctic fringes. In August/ 

 for example/ when the subpolar trough and the British ridge were 

 both more than three standard deviations from normal intensity/ 

 1 ZjZ mb level wind speeds were 14 m/s stronger than normal over 

 Iceland iDicKson/ 1976b). abrw-nesDO bsunrJfioo gnrnr 



witn the withdrawal of the preexisting western Atlantic ridge to 

 North America/ the causes of ocean warming off the middle 

 Atlantic states and cooling south of Newfoundland were both 

 simultaneously removed so that surface temperatures there became 

 more nearly normal than in former seasons --< JeMTffemo st rong 

 northwesterly component of airflow from the Canadian arctic was/ 

 however/ responsible for maintaining the intense cooling off the 

 Labrador coast Canomaly of -3.5F (-1.9C) in the seasonal meanDiJ'"^ 



The events of fall. (Fig. 2.5) were apparently of great climatic 

 significance/ bringing an end (at least temporarily) to long- 

 established regimes in the atmospheric circulation and in the 

 underlying surface temperature field. Hitherto/ for example/ the 

 1970's had been characterized by the extreme vigor of the 

 temperate westerlies over both oceans. In fall/ however/ the 

 pronressive amplification of the circulation was completed with 

 the establishment ot a full train of meridional troughs and 

 ridges at mi d- 1 at i tudes of the fjorthern Hemisphere. While a 

 deep/ f u 1 1- la t i tude trough developed in intensity over the 

 central North Pacific/ the northerly anomalous circulation along 

 its western flank brought a further increment of cooling to the 



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