Decadal Variability in the Oxygen Inventory of North Atlantic Subtropical Underwater Captured by Sustained, Long-Term Oceanographic Time Series Observations

Enrique Montes, Frank E Muller-Karger, Andrés Cianca, Michael W Lomas, Laura Lorenzoni, Sennai Habtes

Research output: Contribution to journalArticlepeer-review

Abstract

<p> Historical observations of potential temperature ( <em> &theta; </em> ), salinity ( <em> S </em> ), and dissolved oxygen concentrations (O <sub> 2 </sub> ) in the tropical and subtropical North Atlantic (0&ndash;500&thinsp;m; 0&ndash;40&deg;N, 10&ndash;90&deg;W) were examined to understand decadal&hyphen;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&deg;N, 64.7&deg;W), the Bermuda Atlantic Time&hyphen;series Study (BATS; 31.7&deg;N, 64.2&deg;W), Hydrostation &ldquo;S&rdquo; (32.1&deg;N, 64.4&deg;W), and the European Station for Time&hyphen;series in the Ocean, Canary Islands (ESTOC; 29.2&deg;N, 15.5&deg;W). Observations over similar time periods at CARIACO (1996&ndash;2013), BATS (1988&ndash;2011), and Hydrostation S (1980&ndash;2013) show that STUW O <sub> 2 </sub> has decreased approximately 0.71, 0.28, and 0.37&thinsp;&micro;mol&thinsp;kg <sup> &minus;1 </sup> &thinsp;yr <sup> &minus;1 </sup> , respectively. No apparent change in STUW O <sub> 2 </sub> was observed at ESTOC over the course of the time series (1994&ndash;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&thinsp;m) declined 0.58&thinsp;&micro;mol&thinsp;kg <sup> &minus;1 </sup> &thinsp;yr <sup> &minus;1 </sup> in the southeastern Caribbean Sea (10&ndash;15&deg;N, 60&ndash;70&deg;W) and 0.68&thinsp;&micro;mol&thinsp;kg <sup> &minus;1 </sup> &thinsp;yr <sup> &minus;1 </sup> in the western subtropical North Atlantic (30&ndash;35&deg;N, 60&ndash;65&deg;W). A declining O <sub> 2 </sub> trend was not observed in the eastern subtropical North Atlantic (25&ndash;30&deg;N, 15&ndash;20&deg;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&hyphen;driven mixing and a slowing down of STUW formation rates, rather than changes in diffusive air&hyphen;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&ndash;March) Atlantic Multidecadal Oscillation index (AMO, <em> R </em> <sup> 2 </sup> &thinsp;=&thinsp;0.32). During negative wintertime AMO years trade winds are typically stronger between 10&deg;N and 30&deg;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&hyphen;1980s and mid&hyphen;1990s to a positive wintertime AMO observed between the mid&hyphen;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 languageAmerican English
JournalGlobal Biogeochemical Cycles
Volume30
DOIs
StatePublished - Mar 1 2016

Keywords

  • Atlantic Multidecadal Oscillation
  • biogeochemical time series stations
  • climate change
  • dissolved oxygen
  • North Atlantic salinity maximum
  • subtropical underwater

Disciplines

  • Life Sciences

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