Contrasts in Oxygen Variability of SE Pacific Oxygen Deficient Zone and Subtropical Gyre

Two decades of observations in the South Pacific show changes in oxygen parameters – concentrations and apparent oxygen utilizations (AOUs) in the lower thermocline 26.5-27.0 σ_θ. The 2013 GEOTRACES data along ~12°S from the oxygen deficient zone (ODZ), defined as < 50 μmol/kg, into the equatorward part of the central gyre at 150°W, are compared with historical data from the 1990s to the 2010s. Transient tracer ratio ages from 2013 data suggest renewal times of over two decades from the water mass sources to the core of the ODZ – as compared with being at least 7 years younger in the equatorward part of the subtropical gyre. Similar to earlier studies, subtropical gyre oxygen concentrations increase and AOUs decrease – due to increases in ventilation between 1990s and 2013 corresponding to increased circulation related to changes in southern hemisphere westerlies. On the other hand, in the core of the ODZ, oxygen concentrations decrease and AOUs increase between 1990s and 2013, and the 5 μmol/kg oxygen contour expands in depth. ODZ changes are estimated to be due to increased biological consumption, which is consistent with increased upwelling corresponding to increased trade winds for that period (shown by other work). Differences in oxygen between the 1990s and 2013 are due to - proximity of the ODZ to coastal upwelling and ventilation time scale differences between the ODZ and gyre. Even in the equatorward part of the gyre, water masses are closer to their sources and ages are younger. Whereas, the ODZ is more isolated from water mass source regions, and it takes more than three decades for the ventilated gyre waters to reach the ODZ. In the South Pacific, changes in oxygen parameters in the ODZ and subtropical gyre – to date – appear to be due to independent processes and forcing acting in each region. Continuing ocean observations are needed to monitor changes in gases such as oxygen that are critical to life.