North-Atlantic Subpolar Gyre: oxygen variability

Using hydrographic datas I study the interannual variability of oxygen concentration in the North-Atlantic subpolar gyre, especially in the Irminger and the Iceland Basin. Oxygen is an indicator of water mass ventilation because when a water column overturned and is mixed, it downwells the surface oxygen-satured water parcel deeper in the column (oxygen concentration is also increased by photosynthesis). Since below the euphotic layer oxygen can only decreases (because of the oxidation of particulate organic matter, the remineralization process), its concentration is an indicator of circulation and water mass ages.

Presently I'm focusing on the OVIDE section shown here:

from which differences in various oxygen related fields are shown in the following figure (click to enlarge):

An integration over areas of interest in the section is much easier to interpret. The following figure shows the same fields averaged between 600 to 900km from Greenland, ie right over the Reykjanes Ridge Mode Water (Thierry et al, JGR 2009). In blue is the solubility, in red the actual concentration and in green the apparent oxygen utilization (AOU):
We can see the largest interannual variability peaking around 500m, associated with the bulk of Reykjanes Ridge Mode Water. Between 2006 and 2002 the solubility doesn't change much between 200m and 600m (the core of the mode water). However the concentration sees a large decrease driven by changes between 2004 and 2002 (partially cancelled by an increase between 2004 and 2006). Wether these changes are due to dynamical processes or a change in the biogeochemical activity remains to be determined and constitutes one of my research subjects...