Meso- and submesoscale variability within the Peruvian upwelling regime : mechanisms of oxygen supply to the subsurface ocean

Thomsen, Sören

The role of meso- and submesoscale processes for the near-coastal circulation, physical and biogeochemical tracer distributions and oxygen minimum zone ventilation in the Peruvian upwelling regime is investigated in this thesis. A multi-platform four-dimensional observational experiment was carried out off Peru in early 2013 and is the basis for this thesis. Furthermore a high-resolution submesoscale permitting physical circulation model is used to study submesoscale frontal dynamics in more detail. The formation of a subsurface anticyclonic eddy and its impact on the near-coastal salinity, oxygen and nutrient distributions was captured by the observations. The eddy developed in the Peru-Chile Undercurrent downstream of a topographic bend, suggesting flow separation as the eddy formation mechanism. The eddy resulted in enhanced cross-shore exchange of physical and biogeochemical tracers due to along-isopycnal stirring and offshore transport of core waters. The core waters originated from the bottom boundary layer and were characterized by low potential vorticity and an enhanced nitrogen-deficit. The subduction of highly oxygenated surface water in a submesoscale cold filament is observed by glider-based measurements. The subduction ventilates the upper oxycline but does not reach into oxygen minimum zone core waters during the summer observations. Lagrangian floats are used to study the pathways of newly upwelled water in a regional submesoscale permitting model. The model analysis suggests a gradual warming of the newly upwelled waters due to surface heat fluxes. The associated density decrease prevents the floats to enter the density range of the oxygen minimum zone in summer. However, in winter a density increase is found due to surface cooling and thus it might be possible that submesoscale processes ventilate the oxygen minimum zone. In the model about 50 % of the newly upwelled floats leave the mixed layer within 5 days both in summer and winter emphazising a hitherto unrecognized importance of subduction for the ventilation of the Peruvian oxycline.

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Thomsen, Sören: Meso- and submesoscale variability within the Peruvian upwelling regime. mechanisms of oxygen supply to the subsurface ocean. 2016.

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