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

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 (T∕S) characteristic over the 2 months, but an erosion of the core progressively narrows down the T∕S 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.