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

Karstensen, Johannes; Schütte, Florian; Pietri, Alice; Krahmann, Gerd; Fiedler, Björn; Grundle, Damian S.; Hauss, Helena; Körtzinger, Arne; Löscher, Carolin R.; Testor, Pierre; Vieira, Nuno; Visbeck, Martin

doi:10.5194/bg-2016-34

Cited by 28 publications

(63 citation statements)

References 5 publications

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“…In the ETNA both eddy types have in common that in their center the ML base rises towards shallow depth (50 to 100 m), which in turn favors biological productivity in the euphotic zone (Falkowski et al, 1991;McGillicuddy et al, 1998). In addition, an enhanced vertical flux of nutrients within or at the periphery of the eddies due to submesoscale instabilities is expected to occur (Brannigan et al, 2015;Karstensen et al, 2016;Lévy et al, 2012;Martin and Richards, 2001;Omand et al, 2015). As a consequence the eddies establish a specific ecosystem of high primary production, particle load and degradation processes, and even unexpected nitrogen loss processes (Löscher et al, 2015).…”

Section: Discussionmentioning

confidence: 99%

Characterization of “dead-zone” eddies in the eastern tropical North Atlantic

Schütte¹,

Karstensen²,

Krahmann³

et al. 2016

Biogeosciences

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102

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Abstract. Localized open-ocean low-oxygen "dead zones" in the eastern tropical North Atlantic are recently discovered ocean features that can develop in dynamically isolated water masses within cyclonic eddies (CE) and anticyclonic modewater eddies (ACME). Analysis of a comprehensive oxygen dataset obtained from gliders, moorings, research vessels and Argo floats reveals that "dead-zone" eddies are found in surprisingly high numbers and in a large area from about 4 to 22 • N, from the shelf at the eastern boundary to 38 • W. In total, 173 profiles with oxygen concentrations below the minimum background concentration of 40 µmol kg −1 could be associated with 27 independent eddies (10 CEs; 17 ACMEs) over a period of 10 years. Lowest oxygen concentrations in CEs are less than 10 µmol kg −1 while in ACMEs even suboxic (< 1 µmol kg −1 ) levels are observed. The oxygen minimum in the eddies is located at shallow depth from 50 to 150 m with a mean depth of 80 m. Compared to the surrounding waters, the mean oxygen anomaly in the core depth range (50 and 150 m) for CEs (ACMEs) is −38 (−79) µmol kg −1 . North of 12 • N, the oxygen-depleted eddies carry anomalously low-salinity water of South Atlantic origin from the eastern boundary upwelling region into the open ocean. Here water mass properties and satellite eddy tracking both point to an eddy generation near the eastern boundary. In contrast, the oxygen-depleted eddies south of 12 • N carry weak hydrographic anomalies in their cores and seem to be generated in the open ocean away from the boundary. In both regions a decrease in oxygen from east to west is identified supporting the en-route creation of the low-oxygen core through a combination of high productivity in the eddy surface waters and an isolation of the eddy cores with respect to lateral oxygen supply. Indeed, eddies of both types feature a cold sea surface temperature anomaly and enhanced chlorophyll concentrations in their center. The low-oxygen core depth in the eddies aligns with the depth of the shallow oxygen minimum zone of the eastern tropical North Atlantic. Averaged over the whole area an oxygen reduction of 7 µmol kg −1 in the depth range of 50 to 150 m (peak reduction is 16 µmol kg −1 at 100 m depth) can be associated with the dispersion of the eddies. Thus the locally increased oxygen consumption within the eddy cores enhances the total oxygen consumption in the open eastern tropical North Atlantic Ocean and seems to be an contributor to the formation of the shallow oxygen minimum zone.

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Section: Discussionmentioning

confidence: 99%

Characterization of “dead-zone” eddies in the eastern tropical North Atlantic

Schütte¹,

Karstensen²,

Krahmann³

et al. 2016

Biogeosciences

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102

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“…Shipborne SST measurements recorded at 5 m depth during M105 reveal colder temperatures within the eddy when compared to outside conditions. A full description of the eddies' physical structure is given in Karstensen et al (2016).…”

Section: Eddy Characteristicsmentioning

confidence: 99%

Oxygen utilization and downward carbon flux in an oxygen-depleted eddy in the eastern tropical North Atlantic

et al. 2016

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Abstract. The occurrence of mesoscale eddies that develop suboxic environments at shallow depth (about 40-100 m) has recently been reported for the eastern tropical North Atlantic (ETNA). Their hydrographic structure suggests that the water mass inside the eddy is well isolated from ambient waters supporting the development of severe near-surface oxygen deficits. So far, hydrographic and biogeochemical characterization of these eddies was limited to a few autonomous surveys, with the use of moorings, underwater gliders and profiling floats. In this study we present results from the first dedicated biogeochemical survey of one of these eddies conducted in March 2014 near the Cape Verde Ocean Observatory (CVOO). During the survey the eddy core showed oxygen concentrations as low as 5 µmol kg −1 with a pH of around 7.6 at approximately 100 m depth. Correspondingly, the aragonite saturation level dropped to 1 at the same depth, thereby creating unfavorable conditions for calcifying organisms. To our knowledge, such enhanced acidity within near-surface waters has never been reported before for the open Atlantic Ocean. Vertical distributions of particulate organic matter and dissolved organic matter (POM and DOM), generally showed elevated concentrations in the surface mixed layer (0-70 m), with DOM also accumulating beneath the oxygen minimum. With the use of reference data from the upwelling region where these eddies are formed, the oxygen utilization rate was calculated by determining oxygen consumption through the remineralization of organic matter. Inside the core, we found these rates were almost 1 order of magnitude higher (apparent oxygen utilization rate (aOUR); 0.26 µmol kg −1 day −1 ) than typical values for the open North Atlantic. Computed downward fluxes for particulate organic carbon (POC), were around 0.19 to 0.23 g C m −2 day −1 at 100 m depth, clearly exceeding fluxes typical for an oligotrophic open-ocean setting. The observations support the view that the oxygen-depleted eddies can be viewed as isolated, westwards propagating upwelling systems of their own, thereby represent re-occurring alien biogeochemical environments in the ETNA.

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“…ACMEs are particularly efficient in transporting isolated water mass anomalies over large distances of several hundreds to thousands of kilometers (Karstensen et al, ; Schütte, Brandt, & Karstensen, ). In particular, negative oxygen anomalies were found in both CEs and ACMEs (Karstensen et al, ; Karstensen et al, ; Schütte, Karstensen, et al, ). Averaged over the ETNA region, a reduction of the oxygen concentration by ~7 μmol/kg in the 50‐ to 100‐m depth range could be associated to the enhanced biological consumption along the CE and ACME pathways (Schütte, Karstensen, et al, ).…”

Section: Introductionmentioning

confidence: 99%

Subsurface Fine‐Scale Patterns in an Anticyclonic Eddy Off Cap‐Vert Peninsula Observed From Glider Measurements

et al. 2018

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Glider measurements acquired along four transects between Cap-Vert Peninsula and the Cape Verde archipelago in the eastern tropical North Atlantic during March-April 2014 were used to investigate fine-scale stirring in an anticyclonic eddy. The anticyclone was formed near 12°N off the continental shelf and propagated northwest toward the Cape Verde islands. At depth, between 100 and -400 m, the isolated anticyclone core contained relatively oxygenated, low-salinity South Atlantic Central Water, while the surrounding water masses were saltier and poorly oxygenated. The dynamical and thermohaline subsurface environment favored the generation of fine-scale horizontal and vertical temperature and salinity structures in and around the core of the anticyclone. These features exhibited horizontal scales of O(10-30 km) relatively small with respect to the eddy radius of O(150 km). The vertical scales of O(5-100 m) were associated to density-compensated gradient. Spectra of salinity and oxygen along isopycnals revealed a slope of around k À2 in the 10-to 100-km horizontal scale range. Further analyses suggest that the fine-scale structures are likely related to tracer stirring processes. Such mesoscale anticyclonic eddies and the embedded fine-scale tracers in and around them could play a major role in the transport of South Atlantic Central Water masses and ventilation of the North Atlantic Oxygen Minimum Zone.Ubiquitous surface and subsurface cyclonic and anticyclonic eddies of various origin and nature act as a major transport agent between the coastal waters and the open ocean (e.g., Chelton et al.

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

Cited by 28 publications

References 5 publications

Characterization of “dead-zone” eddies in the eastern tropical North Atlantic

Characterization of “dead-zone” eddies in the eastern tropical North Atlantic

Oxygen utilization and downward carbon flux in an oxygen-depleted eddy in the eastern tropical North Atlantic

Subsurface Fine‐Scale Patterns in an Anticyclonic Eddy Off Cap‐Vert Peninsula Observed From Glider Measurements

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