Ocean Deoxygenation Conference | Kiel 2018

For recent decades a rather rapid change in dissolved ocean oxygen (DO) was found in observations and numerical models respectively. Tropical and subtropical DO decreased in most regions of the world ocean. These DO trends are a combination of long-term trends superimposed with (multi-) decadal and short-term variability. The exact processes at play remain elusive. Here we show some of the identified drivers of deoxygenation separated for the upper ocean above 1200 m as well as for the deep ocean below 1200 m. The marked drivers are solubility and stratification, decline and increase in source waters, (overturning) circulation changes, nutrient stimulation via upwelling and multidecadal variability like the Pacific Decadal Oscillation (PDO). Observed nutrient changes seem to be related to oxygen changes modified by local and eddy processes. In addition the oxygen and nutrient changes in some regions of the Pacific Ocean where longer time series exist are investigated in relation to the PDO for the period since 1950. In subsurface layers (e.g. 50-300 m) in the eastern Pacific the oxygen content increased often during the cold PDO phase until 1976 and decreased after 1977. Nutrient trends are more variable related to local processes; however nutrients often show a reversed trend to the oxygen trend. El Niño and La Niña events modify the measured oxygen and nutrient content during the year of these events. Anticyclonic eddies e.g. off Peru carry oxygen-poor, nitrate-poor and nitrite-enhanced water westward into the open South Pacific Ocean. Silicate and phosphate in anticyclonic eddies show a vertical expansion of their characteristics at the depth of core layer of the eddy.