Abstract
<p> Historical observations of potential temperature ( <em> θ </em> ), salinity ( <em> S </em> ), and dissolved oxygen concentrations (O <sub> 2 </sub> ) in the tropical and subtropical North Atlantic (0–500 m; 0–40°N, 10–90°W) were examined to understand decadal‐scale changes in O <sub> 2 </sub> 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°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). Observations over similar time periods at CARIACO (1996–2013), BATS (1988–2011), and Hydrostation S (1980–2013) show that STUW O <sub> 2 </sub> has decreased approximately 0.71, 0.28, and 0.37 µmol kg <sup> −1 </sup>  yr <sup> −1 </sup> , respectively. No apparent change in STUW O <sub> 2 </sub> 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 <sub> 2 </sub> (upper ~300 m) declined 0.58 µmol kg <sup> −1 </sup>  yr <sup> −1 </sup> in the southeastern Caribbean Sea (10–15°N, 60–70°W) and 0.68 µmol kg <sup> −1 </sup>  yr <sup> −1 </sup> in the western subtropical North Atlantic (30–35°N, 60–65°W). A declining O <sub> 2 </sub> trend was not observed in the eastern subtropical North Atlantic (25–30°N, 15–20°W) over the same period. Most of the observed O <sub> 2 </sub> 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 <sub> 2 </sub> gas exchange or changes in the biological oceanography of the North Atlantic. Variability of STUW O <sub> 2 </sub> showed a significant relationship with the wintertime (January–March) Atlantic Multidecadal Oscillation index (AMO, <em> R </em> <sup> 2 </sup>  = 0.32). During negative wintertime AMO years trade winds are typically stronger between 10°N and 30°N. These conditions stimulate the formation and ventilation of STUW. The decreasing trend in STUW O <sub> 2 </sub> 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 the mid‐1990s and 2013. These changes in STUW O <sub> 2 </sub> 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.</p>
Original language | American English |
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Journal | Global Biogeochemical Cycles |
Volume | 30 |
DOIs | |
State | Published - Mar 1 2016 |
Keywords
- Atlantic Multidecadal Oscillation
- biogeochemical time series stations
- climate change
- dissolved oxygen
- North Atlantic salinity maximum
- subtropical underwater
Disciplines
- Life Sciences