Secular trends in water properties at Station P in the northeast Pacific: An updated analysis

Abstract

Oceanographic observations collected at Station P (145°W, 50°N) in the northeast Pacific extend for over six decades, representing one of the longest available records of subsurface ocean water properties. As such, this record is well suited to examine secular trends in properties of the subarctic waters of the North Pacific. In this paper, previously published trends are reviewed and updated, based on a newly compiled dataset for the station. Vertically integrated quantities such as ocean heat content and steric height are examined, as well as local water properties through the water column including temperature, salinity and, in particular, oxygen. Consideration is also given to upper-ocean stratification, along with depths of the mixed layer and isopycnal surfaces.The results provide a comprehensive view of long-term changes to ocean conditions in the deep waters of the subarctic Pacific. Increases in 0/2000 dbar ocean heat content and steric height are evident, due primarily to ocean warming through the upper 500 dbar of the water column. However, about a third of the overall 0/2000 dbar steric height trend is due to halosteric effects. A significant freshening trend is evident through the upper layer to the top of the permanent pycnocline. Significant changes are also found below the permanent pycnocline, in particular warming on isopycnal surfaces along with downward migration of isopycnals. On the other hand, no robust trend emerges in the strength of the stratification associated with the permanent pycnocline, a result that likely has implications for turbulent exchanges between the surface mixed layer and the deep ocean, including the supply of nutrients to the euphotic zone.Statistically significant declines in dissolved oxygen are observed on isopycnals below the pycnocline, down to great depth. Column-integrated dissolved oxygen has declined at a rate of 0.72 mol m−2 y−1, representing a net loss over six decades of 11.7 ± 3.5% in total oxygen content per square metre. This is substantially greater than the global average of 2%, underscoring the northeast Pacific as a region of comparatively rapid deoxygenation. Changes in solubility due to local warming have made only a minor contribution to the observed decline.