Holtermann, P.

Rehder, Gregor

Schmale, O.

Tanhua, Toste

Umlauf, L.

Waniek, J.

English

OceanRep

Vertical Mixing in the Tropical  North Atlantic Ocean; Results from a large scale Tracer Release ExperimentToste TanhuaDonata Banyte, Doug W.R. Wallace, Johannes Karstensen, Gerd Krahmann, Anke Schneider, Lothar Stramma, Martin VisbeckHelmholtz Centre for Ocean Research | GEOMAR, Kiel, GermanyClimate - Biogeochemistry Interactions in the Tropical Ocean SFB- 754T,S,v N,P,FeO2tThe Oxygen Minimum Zone in the Tropical North Atlantic Ocean – The Guinea Dome.Background:The “SFB 754”Climate – Biogeochemistry Interactionsin the Tropical OceanGuinea Upwelling Tracer Release Experiment (GUTRE)XHow does subsurface dissolved oxygen in the tropical oceanrespond to variability in ocean circulation and ventilation?• Dissolved Oxygen can be supplied by:1) Lateral Pathways by mean and variable currents along isopycnals2) Vertical Pathways by mixing across isopycnalsCirculation and Oxygen Concentration:XObjective: - Constrain estimates of diapycnal and isopycnal mixing in the ocean- Observe advection of “labeled” water masses- Study biogeochemical processes within the labeled water massAdvantage:- Integrated value of all processes over a certain time period over a larger area- Estimates to high accuracy is possibleChallenges:- Only limited process understandingTracer Release Experiments:10 cmThe Tracer Injection:OTIS – Ocean Tracer Injection System92 kg (470 mole) of CF3SF5 was Injected on the density surfaceσΘ = 26.88 kg m-3 and 8°N, 23°W- In the upper oxygen gradient of the Tropical North Atlantic OMZ CF3SF5 is an inert gas that does not vave any measurable backgroundconcentration in the ocean.The Scene:TopographyStratification•Seamount chain in the SE•Abyssal plain in the NW•High stratification in SE•Low stratification in NWTracer Observations:Horizontal spreading Vertical spreading Diapycnal spreadingCalculating the mixing:Normalized vertical profilesclosely resembles Gaussian distribution, so that the diffusivitycan be calculated by the second moment of the Gaussian fit.Kz = S2(t2) – S2(t1) / 2(t2 - t1)Assuming Gaussian distributionVertical advection diffusion model We did these calculations in: depth (Dz in m2 s-1)anddensity (Dρ in (kg m-3)2 s-1) Results:Vertical diffusivity: Dz = 1.18 ± 0.13 x 10-5 m2 s-1Dρ = 3.10 ± 0.28 x 10-11 (kg m-3)2 s-1Gaussian Advection/DiffusionVertical velocity: ωz = 0.6 ± 1.3 x 10-7 m s-1ωρ = 1.3 ± 2.0 x 10-10 kg m-3 s-1A significant upward velocity forthe time between survey1 and 2/31.6 ± 0.6 x 10-7 m s-1 (i.e. ~5 m y-1)Insignificant vertical gradient in diffusion (δD/δz)Regional variability:Stratification; Dz1.260.74Topography; Dρ2.443.38Discussion:The GUTRE experiment (Latitude 4° - 12° N) has somewhat higher diffusivity (dissipation rates) than predicted by Gregg et al., (2003) comparedto the NATRE experiment  (Latitude 10° - 26° N) (Ledwell et al., 1998).Enhanced mixing over rough topography mightbe an explanation for this. We have introduced the diapycnal diffusivity in density space (Dρ) with the unitsof (kg m-3)2 s-1. Dρ is a useful property; in our experiment we see higher mixing over rough Topography only in Dρ space, not in Dz (where we see the opposite pattern). Dρ thus defines the concentration changes over time for parameters like oxygen. A “tropical” TRE over the Oxygen Minimum Zone in the Atlantic Ocean• We find diapycnal diffusivities that are slightly lower than for NATRE roughly 10° further north• We find significant regional differences in Dρprobably associated with topographic “roughness”Conclusions:Dz = 1.18 ± 0.13 x 10-5 m2 s-1Dρ = 3.10 ± 0.28 x 10-11 (kg m-3)2 s-1

The Baltic Sea Tracer Release Experiment

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