Decadal variability in the oxygen inventory of North Atlantic subtropical underwater captured by sustained, long-term oceanographic time series observations

摘要

Historical observations of potential temperature (theta), salinity (S), and dissolved oxygen concentrations (O-2) in the tropical and subtropical North Atlantic (0-500 m; 0-40 degrees N, 10-90 degrees W) were examined to understand decadal-scale changes in O-2 in subtropical underwater (STUW). STUW is observed at four of the longest, sustained ocean biogeochemical and ecological time series stations, namely, the CArbon Retention In A Colored Ocean (CARIACO) Ocean Time Series Program (10.5 degrees N, 64.7 degrees W), the Bermuda Atlantic Time-series Study (BATS; 31.7 degrees N, 64.2 degrees W), Hydrostation "S" (32.1 degrees N, 64.4 degrees W), and the European Station for Time-series in the Ocean, Canary Islands (ESTOC; 29.2 degrees N, 15.5 degrees W). Observations over similar time periods at CARIACO (1996-2013), BATS (1988-2011), and Hydrostation S (1980-2013) show that STUWO2 has decreased approximately 0.71, 0.28, and 0.37 mu mol kg(-1) yr(-1), respectively. No apparent change in STUW O-2 was observed at ESTOC over the course of the time series (1994-2013). Ship observation data for the tropical and subtropical North Atlantic archived at NOAA National Oceanographic Data Center show that between 1980 and 2013, STUW O-2 (upper similar to 300 m) declined 0.58 mu mol kg(-1) yr(-1) in the southeastern Caribbean Sea (10-15 degrees N, 60-70 degrees W) and 0.68 mu mol kg(-1) yr(-1) in the western subtropical North Atlantic (30-35 degrees N, 60-65 degrees W). A declining O-2 trend was not observed in the eastern subtropical North Atlantic (25-30 degrees N, 15-20 degrees W) over the same period. Most of the observed O-2 loss seems to result from shifts in ventilation associated with decreased wind-driven mixing and a slowing down of STUW formation rates, rather than changes in diffusive air-sea O-2 gas exchange or changes in the biological oceanography of the North Atlantic. Variability of STUWO2 showed a significant relationship with the wintertime (January-March) Atlantic Multidecadal Oscillation index (AMO, R-2 = 0.32). During negative wintertime AMO years trade winds are typically stronger between 10 degrees N and 30 degrees N. These conditions stimulate the formation and ventilation of STUW. The decreasing trend in STUW O-2 in the three decades spanning 1980 through 2013 reflects the shift from a strongly negative wintertime AMO between the mid-1980s and mid-1990s to a positive wintertime AMO observed between themid-1990s and 2013. These changes in STUWO2 were captured by the CARIACO, BATS, and Hydrostation S time series stations. Sustained positive AMO conditions could lead to further deoxygenation in tropical and subtropical North Atlantic upper waters.