Dissolved oxygen variability in the Mediterranean Sea

The interannual variability and the mechanisms controlling the dissolved oxygen concentration in the Mediterranean Sea were investigated through generating gridded fields of dissolved oxygen, salinity and potential temperature. The Data-Interpolating Variational Analysis (DIVA) software was used to produce a gridded dataset for the time period 1960-2011. High oxygen concentrations for the upper and bottom layers, separated by an oxygen minimum zone at intermediate layers, are a typical structure of the dissolved oxygen in the Eastern and the Western Mediterranean sub-basins. Although an oxygen minimum zone is observed in both sub-basins, its vertical positions are different; in the Eastern Mediterranean at between 600 and 1200 m depth and in the Western Mediterranean at between 400 and 600 m. The vertical distribution of dissolved oxygen shows significant differences between the two sub-basins and their temporal evolution reveals large interannual to decadal variability. A negative correlation was observed between dissolved oxygen and surface potential temperature due to solubility changes over the whole period. However, the positive correlation between the dissolved oxygen and potential temperature in the Eastern Mediterranean deep layers is an indication that the dynamical processes are dominant and are involved in the dissolved oxygen interannual variability. The dissolved oxygen variability presents shifts with a multi-decadal signal, rather than trends as observed in the global ocean, associated with mixing processes and decadal oscillations that influence the dense water formation or biological activity.