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

Historical observations of potential temperature (θ), salinity (S), and dissolved oxygen concentrations (O₂) in the subtropical North Atlantic (0–500 m; 0–40°N, 10–80°W) were examined to understand decadal-scale changes in O₂ in Subtropical Underwater (STUW). STUW is observed at four of the longest, sustained ocean biogeochemical and ecological time-series stations, namely the CARIACO Ocean Time-Series Program (10.5°N, 64.7°W), the Bermuda Atlantic Time-series Study (BATS; 31.7°N, 64.2°W), Hydrostation “S” (32.1°N, 64.4°W), and the European Station for Time-series in the Ocean, Canary Islands (ESTOC; 29.2°N, 15.5°W). Data archived by NOAA NODC show that, between 1980 and 2013, STUW O₂ (upper ~300 m) has declined 0.58 μmol kg^-1 yr^-1 in the southeastern Caribbean Sea (10–15°N, 60–70°W), and 0.68 μmol kg^-1 yr^-1 in the western subtropical North Atlantic, respectively (30–35°N, 60–65°W). Observations at CARIACO (1995–2013) and BATS (1988–2012), specifically, show that STUW O₂ has decreased approximately 0.61 and 0.21 μmol kg^-1 yr^-1, respectively. No apparent change in STUW O₂ was observed at ESTOC over the course of the time series (1994–2013). Most of the observed O₂ loss seems to result from shifts in ventilation associated with wind-driven mixing and slow down of STUW formation rates, rather than changes in diffusive air-sea O₂ gas exchange. Variability of STUW O₂ showed a strong relationship with the Atlantic Multidecadal Oscillation (AMO; R²=0.32, p < 0.001) index phase. During negative AMO years trade winds are stronger between 10°N and 30°N. These conditions stimulate the formation and ventilation of STUW. The decreasing trend in STUW O₂ in the three decades spanning 1980 through 2013 thus reflects a shift from a strongly negative AMO between mid-1980’s and mid-1990’s to a positive AMO observed between the mid-1990’s and 2013. These changes in STUW O₂ were captured by the CARIACO, BATS, and Hydrostation “S” time series stations. Sustained positive AMO conditions could lead to further de-oxygenation in tropical and sub-tropical North Atlantic upper waters.