A Study of Temperature Changes in the Upper North Atlantic: 1950–94

Abstract

In this paper, decadal evolution of warm and cold anomalies in the subtropical and subpolar gyres of the North Atlantic in the 300‐500-m and 100‐250-m depth ranges is described. A series of pentadally averaged objective maps of upper-ocean thermal anomalies, from bathythermograph data, are presented. Warm and cold anomalies in the western subtropical gyre are succeeded by similar anomalies in the subpolar gyre and the east Atlantic and subtropical return flow. Major warm and cold anomalies in the 1950s and 1970s, respectively, are similar to those described previously in SSTs, although there is more temporal continuity in the subsurface anomalies. Two very strong events in the subtropical gyre are identified, a cold anomaly in 1966‐72 and an intense warm anomaly in 1988‐94, that show the greatest temperature anomalies in the North Atlantic during the period of the study. Interisotherm thickness anomalies are shown for the subtropical gyre during these periods. In the warm period, mode waters are both warmer (188‐198C) and of greater volume than on average, and lie in a narrow band south of the Gulf Stream above a depressed thermocline with warm temperature anomalies to at least 800-m depth. In the cold period, the predominant mode water temperature is closer to 178C, but there is reduced water formation overall with a raised thermocline and cold temperature anomalies down to 600-m depth. The bowl of the gyre is flat during the cold period, and the implied recirculation may be weaker and extend farther to the south. The changes appear to be consistent with the intensification of the subtropical gyre in the warm period and a spindown in the cold period, although the relative roles of wind stress and air‐sea heat fluxes in these changes need to be determined.