UPPER-OCEAN THERMAL VARIATIONS IN THE NORTH PACIFIC DURING 1970-1991

A newly available, extensive compilation of upper-ocean temperature pr ofiles was used to study the vertical structure of thermal anomalies b etween the surface and 400-m depth in the North Pacific during 1970-19 91. A prominent decade-long perturbation in climate occurred during th is time period: surface waters cooled by similar to 1 degrees C in the central and western North Pacific and warmed by about the same amount along the west coast of North America from late 1976 to 1988. Compari son with data from GOADS suggests that the relatively sparse sampling of the subsurface data is adequate for describing the climate anomaly. The vertical structure of seasonal thermal anomalies in the central N orth Pacific shows a series of cold pulses beginning in the fall of 19 76 and continuing until late 1988 that appear to originate at the surf ace and descend with time into the main thermocline to at least 400-m depth. Individual cold events descend rapidly (similar to 100 m yr(-1) ), superimposed upon a slower cooling (similar to 15 m yr(-1)). The in terdecadal climate change, while evident at the surface, is most promi nent below similar to 150 m where interannual variations are small. Un like the central North Pacific, the temperature changes along the west coast of North America appear to be confined to approximately the upp er 200-250 m. The structure of the interdecadal thermal variations in the eastern and central North Pacific appears to be consistent with th e dynamics of the ventilated thermocline. In the western North Pacific , strong cooling is observed along the axis of the Kuroshio Current Ex tension below similar to 200 m depth during the 198Os. Changes in mixe d layer depth accompany the SST variations, but their spatial distribu tion is not identical to the pattern of SST change. In particular, the decade-long cool period in the central North Pacific was accompanied by a similar to 20 m deepening of the mixed layer in winter, but no si gnificant changes in mixed layer depth were found along the west coast of North America. It is suggested that other factors such as stratifi cation beneath the mixed layer and synoptic wind forcing may play a ro le in determining the distribution of mixed layer depth anomalies.