Interannual to decadal variability and trends of the oceanic oxygen content in the North Atlantic

This PhD project focuses on the oxygen (O2) variability and trends in the North Atlantic at interannual to decadal time scales. To analyze the decadal trends I performed a careful secondary quality control of the O2 measurements on a new dataset (CARINA) with many previously unpublished hydrographic data in the Atlantic Ocean. The CARINA data were augmented by GLODAP and WOD05 to achieve a good spatial coverage and cover the past five decades. The results from this analysis revealed a significant decrease of O2 concentrations in the Upper (UW), Mode (MW) and Intermediate (IW) Waters with a weighted mean decrease of−5.1±1.7 μmol kg−1 over the last 49 years. The observed decrease, however, is not homogeneously distributed, but is largely concentrated in the northeastern and central North Atlantic. Increasing O2 concentrations are observed in the western North Atlantic in the MW and IW. Over the same period the O2 concentrations increased in the Lower Intermediate Water (LIW) and Labrador Sea Water (LSW) throughout the North Atlantic (5.5±2.6 μmol kg−1). The observed changes can be attributed to the interplay between circulation and ventilation that affect the O2 concentration in the MW and IW, while changes in solubility seem to be the main driver for the observed decrease in the UW and the increase in the LIW and LSW. Overall, the three upper horizons (UW, MW and IW) have lost −58±20 Tmol of O2 between 1960 to 2009, and showed an O2 loss to heat gain ratio of −4.6±2.8 nmol J−1. The deeper horizons (LIW and LSW) have gained 70±34 Tmol, showing an O2 to heat ratio of −2.9±1.9 nmol J−1. These ratios are substantially larger than those expected from solubility changes alone, confirming that circulation, ventilation and biology changes tend to enhance the solubility driven changes. For the second analysis, focusing on the interannual to decadal O2 variability, I used 12 hydrographic repeated cruises along the eastern North Atlantic at 47◦N from 1993 to 2011 with almost yearly resolution until 2005. The analysis focused on the upper 1500 m. The results from this analysis show that the Subpolar Mode Water (SPMW), IW, and Mediterranean Overflow Water (MOW) underwent substantial decadal changes in the O2 concentrations between 1993 and 2002. These changes were associated to circulation, which during this period shows substantial variability due to the northward shift of the Subpolar front (SF) in the eastern North Atlantic. This shift caused more water of subtropical origin to enter into the eastern region, which subsequently reduced the ventilation in the SPMW and therefore lowered the O2 concentration by −1.8±0.7 μmol kg−1 yr−1. The same circulation changes affected the O2 concentration in the IW, leading to a decrease of −2.3±0.7 μmol kg−1 yr−1 from 1993 to 2002. I hypothesized that a decrease in the contribution of SAIW along with an increase in the contribution of AAIW and MOW, reduced the O2 concentration in this water mass. The O2 changes in the MOW were affected by a combination of solubility and circulation changes. The attribution of the O2 changes observed after 2002 is difficult, due to the lack of data from 2006 to 2009. Furthermore, interannual changes caused by mesoscale events may be relevant and affect the water column down to at least 1000 m depth. These events could