A study of temperature changes in the upper North Atlantic: 1950-94

In this payer, decadal evolution of warm and cold anomalies in the subtropi cal 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 acid the east Atlantic and subtropic al return flow. Major warm and cold anomalies in the 1950s and 1970s, respe ctively, 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 anoma ly in 1966-72 and an intense warm anomaly in 1988-94, that show the greates t temperature anomalies in the North Atlantic during the period of the stud y. Interisotherm thickness anomalies are shown for the subtropical gyre dur ing these periods. In the warm period. mode waters are both warmer (18 degr ees-19 degrees C) and of greater volume than on average, and lie in a narro w band south of the Gulf Stream above a depressed thermocline with warm tem perature anomalies to at least 800-m depth. In the cold period, the predomi nant mode water temperature is closer to 17 degrees C, but there is reduced water formation overall with a raised thermocline and cold temperature ano malies down to 600-m depth. The bowl of the gyre is hat during the cold per iod, and the implied recirculation may be weaker and extend farther to the south. The changes appear to he consistent with the intensification of the subtropical gyre in the warm period and a spindown in the cold period, alth ough the relative roles of wind stress and air-sea heat fluxes in these cha nges need to be determined.