Upper-Ocean Thermal Variations in the North Pacific during 1970–1991

Abstract

A newly available, extensive compilation of upper-ocean temperature profiles was used to study the vertical structure of thermal anomalies between the surface and 400-m depth in the North Pacific during 1970–1991. A prominent decade-long perturbation in climate occurred during this time period: surface waters cooled by ∼1°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. Comparison with data from COADS 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 North Pacific shows a series of cold pulses beginning in the fall of 1976 and continuing until late 1988 that appear to originate at the surface and descend with time into the main thermocline to at least 400-m depth. Individual cold events descend rapidly (∼100 m yr⁻¹), superimposed upon a slower cooling (∼15 m yr⁻¹). The interdecadal climate change, while evident at the surface, is most prominent below ∼150 m where interannual variations are small. Unlike the central North Pacific, the temperature changes along the west coast of North America appear to be confined to approximately the upper 200–250 m. The structure of the interdecadal thermal variations in the eastern and central North Pacific appears to be consistent with the dynamics of the ventilated thermocline. In the western North Pacific, strong cooling is observed along the axis of the Kuroshio Current Extension below ∼200 m depth during the 1980s. Changes in mixed layer depth accompany the SST variations, but their spatial distribution 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 ∼20 m deepening of the mixed layer in winter, but no significant changes in mixed layer depth were found along the west coast of North America. It is suggested that other factors such as stratification beneath the mixed layer and synoptic wind forcing may play a role in determining the distribution of mixed layer depth anomalies.