Articles | Volume 14, issue 8
Biogeosciences, 14, 2167–2181, 2017

Special issue: Hydrography, biogeochemistry, and biology of "dead-zone eddies"...

Biogeosciences, 14, 2167–2181, 2017

Research article 27 Apr 2017

Research article | 27 Apr 2017

Upwelling and isolation in oxygen-depleted anticyclonic modewater eddies and implications for nitrate cycling

Johannes Karstensen1, Florian Schütte1, Alice Pietri2, Gerd Krahmann1, Björn Fiedler1, Damian Grundle1, Helena Hauss1, Arne Körtzinger1,3, Carolin R. Löscher1,a, Pierre Testor2, Nuno Vieira4, and Martin Visbeck1,3 Johannes Karstensen et al.
  • 1GEOMAR, Helmholtz Zentrum für Ozeanforschung Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany
  • 2Sorbonne Universites (UPMC Univ. Pierre et Marie Curie, Paris 06)-CNRS-IRD-MNHN, UMR 7159, Laboratoire d'Oceanographie et de Climatologie (LOCEAN), Institut Pierre Simon Laplace (IPSL), Observatoire Ecce Terra, 4 place Jussieu, 75005 Paris, France
  • 3Kiel University, Kiel, Germany
  • 4Instituto Nacional de Desenvolvimento das Pescas (INDP), Cova de Inglesa, Mindelo, São Vicente, Cabo Verde
  • anow at: Nordic Center for Earth Evolution, University of Southern Denmark, Campusvej 555230 Odense M, Denmark

Abstract. The temporal evolution of the physical and biogeochemical structure of an oxygen-depleted anticyclonic modewater eddy is investigated over a 2-month period using high-resolution glider and ship data. A weakly stratified eddy core (squared buoyancy frequency N2  ∼  0.1  ×  10−4 s−2) at shallow depth is identified with a horizontal extent of about 70 km and bounded by maxima in N2. The upper N2 maximum (3–5  ×  10−4 s−2) coincides with the mixed layer base and the lower N2 maximum (0.4  ×  10−4 s−2) is found at about 200 m depth in the eddy centre. The eddy core shows a constant slope in temperature/salinity (TS) characteristic over the 2 months, but an erosion of the core progressively narrows down the TS range. The eddy minimal oxygen concentrations decreased by about 5 µmol kg−1 in 2 months, confirming earlier estimates of oxygen consumption rates in these eddies.

Separating the mesoscale and perturbation flow components reveals oscillating velocity finestructure ( ∼  0.1 m s−1) underneath the eddy and at its flanks. The velocity finestructure is organized in layers that align with layers in properties (salinity, temperature) but mostly cross through surfaces of constant density. The largest magnitude in velocity finestructure is seen between the surface and 140 m just outside the maximum mesoscale flow but also in a layer underneath the eddy centre, between 250 and 450 m. For both regions a cyclonic rotation of the velocity finestructure with depth suggests the vertical propagation of near-inertial wave (NIW) energy. Modification of the planetary vorticity by anticyclonic (eddy core) and cyclonic (eddy periphery) relative vorticity is most likely impacting the NIW energy propagation. Below the low oxygen core salt-finger type double diffusive layers are found that align with the velocity finestructure.

Apparent oxygen utilization (AOU) versus dissolved inorganic nitrate (NO3) ratios are about twice as high (16) in the eddy core compared to surrounding waters (8.1). A large NO3 deficit of 4 to 6 µmol kg−1 is determined, rendering denitrification an unlikely explanation. Here it is hypothesized that the differences in local recycling of nitrogen and oxygen, as a result of the eddy dynamics, cause the shift in the AOU : NO3 ratio. High NO3 and low oxygen waters are eroded by mixing from the eddy core and entrain into the mixed layer. The nitrogen is reintroduced into the core by gravitational settling of particulate matter out of the euphotic zone. The low oxygen water equilibrates in the mixed layer by air–sea gas exchange and does not participate in the gravitational sinking. Finally we propose a mesoscale–submesoscale interaction concept where wind energy, mediated via NIWs, drives nutrient supply to the euphotic zone and drives extraordinary blooms in anticyclonic mode-water eddies.

Short summary
High-resolution observational data from underwater gliders and ships are used to investigate drivers and pathways of nutrient upwelling in high-productive whirling ecosystems (eddies). The data suggest that the upwelling is created by the interaction of wind-induced internal waves with the local rotation of the eddy. Because of differences in nutrient and oxygen pathways, a low-oxygen core is established at shallow depth in the high-productive eddies.
Final-revised paper