Journal of Oceanography, Vol. 60 (No. 1), pp. 139-147, 2004
Steven Emerson1*, Yutaka W. Watanabe2, Tsuneo Ono3 and Sabine Mecking4
1School of Oceanography, University of Washington, Seattle, WA 98144, U.S.A.
2Graduate School of Environmental and Earth Science, Hokkaido University, Sapporo 060-0810, Japan
3Hokkaido National Fisheries Research Institute, Kushiro 085-0802, Japan
4Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.
(Received 1 October 2003; in revised form 27 November 2003; accepted 27 November 2003)
Abstract: We present a compilation of apparent oxygen utilization (AOU) changes observed in the upper pycnocline of the North Pacific Ocean over the last several decades. The goal here is to place previously-published data in a common format, and assess the causes of the observed changes. The general trend along repeat cross sections of the eastern and western subtropical ocean and the subarctic ocean is an increase in AOU from the mid 1980s to the mid 1990s. AOU has also been increasing in a time-series study in the northwest subarctic Ocean off of Japan since the late 1960s. Observed AOU changes south of 35°N in the subtropical ocean are 10-20 mmol kg-1, with much greater changes, reaching 60-80 mmol kg-1 in isolated areas, in the subtropical/subarctic boundary and the subarctic ocean. Analysis of changes in both AOU and salinity on isopycnals suggests that there are significant salinity-normalized increases that must be due to alteration in the rate of ventilation or organic matter degradation. A common feature in the data is that the maximum increase in AOU is centered near the density horizon sq = 26.6. Time series results from the Oyashio Current region near the winter outcrop area of this density horizon indicate that surface waters there have become fresher with time, which may mean this density surface has ceased to outcrop in the latter decades of the 20th century. Whether this is due to natural decadal-scale changes or anthropogenic influences can be decided by determining future trends in AOU on these density surfaces.