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

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

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Resumé

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 70km and bounded by maxima in N2. The upper N2 maximum (3-5 × 10-4s-2) coincides with the mixed layer base and the lower N2 maximum (0.4 × 10-4s-2) is found at about 200m 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 5μmolkg-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 (∼0.1ms-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 140m just outside the maximum mesoscale flow but also in a layer underneath the eddy centre, between 250 and 450m. 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 6μmolkg-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.

OriginalsprogEngelsk
TidsskriftBiogeosciences
Vol/bind14
Udgave nummer8
Sider (fra-til)2167-2181
Antal sider15
ISSN1726-4170
DOI
StatusUdgivet - 27. apr. 2017

Fingeraftryk

eddy
upwelling
nitrates
nitrate
oxygen
euphotic zone
salinity
mixed layer
wind power
temperature
wave energy
vorticity
energy
rendering
nitrogen
ships
denitrification
oxygen consumption
recycling
gas exchange

Citer dette

Karstensen, J., Schütte, F., Pietri, A., Krahmann, G., Fiedler, B., Grundle, D., ... Visbeck, M. (2017). Upwelling and isolation in oxygen-depleted anticyclonic modewater eddies and implications for nitrate cycling. Biogeosciences, 14(8), 2167-2181. https://doi.org/10.5194/bg-14-2167-2017
Karstensen, Johannes ; Schütte, Florian ; Pietri, Alice ; Krahmann, Gerd ; Fiedler, Björn ; Grundle, Damian ; Hauss, Helena ; Körtzinger, Arne ; Löscher, Carolin R. ; Testor, Pierre ; Vieira, Nuno ; Visbeck, Martin. / Upwelling and isolation in oxygen-depleted anticyclonic modewater eddies and implications for nitrate cycling. I: Biogeosciences. 2017 ; Bind 14, Nr. 8. s. 2167-2181.
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title = "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 70km and bounded by maxima in N2. The upper N2 maximum (3-5 × 10-4s-2) coincides with the mixed layer base and the lower N2 maximum (0.4 × 10-4s-2) is found at about 200m 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 5μmolkg-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 (∼0.1ms-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 140m just outside the maximum mesoscale flow but also in a layer underneath the eddy centre, between 250 and 450m. 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 6μmolkg-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.",
author = "Johannes Karstensen and Florian Sch{\"u}tte and Alice Pietri and Gerd Krahmann and Bj{\"o}rn Fiedler and Damian Grundle and Helena Hauss and Arne K{\"o}rtzinger and L{\"o}scher, {Carolin R.} and Pierre Testor and Nuno Vieira and Martin Visbeck",
year = "2017",
month = "4",
day = "27",
doi = "10.5194/bg-14-2167-2017",
language = "English",
volume = "14",
pages = "2167--2181",
journal = "Biogeosciences",
issn = "1726-4170",
publisher = "Copernicus GmbH",
number = "8",

}

Karstensen, J, Schütte, F, Pietri, A, Krahmann, G, Fiedler, B, Grundle, D, Hauss, H, Körtzinger, A, Löscher, CR, Testor, P, Vieira, N & Visbeck, M 2017, 'Upwelling and isolation in oxygen-depleted anticyclonic modewater eddies and implications for nitrate cycling', Biogeosciences, bind 14, nr. 8, s. 2167-2181. https://doi.org/10.5194/bg-14-2167-2017

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; Hauss, Helena; Körtzinger, Arne; Löscher, Carolin R.; Testor, Pierre; Vieira, Nuno; Visbeck, Martin.

I: Biogeosciences, Bind 14, Nr. 8, 27.04.2017, s. 2167-2181.

Publikation: Bidrag til tidsskriftTidsskriftartikelForskningpeer review

TY - JOUR

T1 - Upwelling and isolation in oxygen-depleted anticyclonic modewater eddies and implications for nitrate cycling

AU - Karstensen, Johannes

AU - Schütte, Florian

AU - Pietri, Alice

AU - Krahmann, Gerd

AU - Fiedler, Björn

AU - Grundle, Damian

AU - Hauss, Helena

AU - Körtzinger, Arne

AU - Löscher, Carolin R.

AU - Testor, Pierre

AU - Vieira, Nuno

AU - Visbeck, Martin

PY - 2017/4/27

Y1 - 2017/4/27

N2 - 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 70km and bounded by maxima in N2. The upper N2 maximum (3-5 × 10-4s-2) coincides with the mixed layer base and the lower N2 maximum (0.4 × 10-4s-2) is found at about 200m 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 5μmolkg-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 (∼0.1ms-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 140m just outside the maximum mesoscale flow but also in a layer underneath the eddy centre, between 250 and 450m. 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 6μmolkg-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.

AB - 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 70km and bounded by maxima in N2. The upper N2 maximum (3-5 × 10-4s-2) coincides with the mixed layer base and the lower N2 maximum (0.4 × 10-4s-2) is found at about 200m 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 5μmolkg-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 (∼0.1ms-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 140m just outside the maximum mesoscale flow but also in a layer underneath the eddy centre, between 250 and 450m. 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 6μmolkg-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.

U2 - 10.5194/bg-14-2167-2017

DO - 10.5194/bg-14-2167-2017

M3 - Journal article

AN - SCOPUS:85018269842

VL - 14

SP - 2167

EP - 2181

JO - Biogeosciences

JF - Biogeosciences

SN - 1726-4170

IS - 8

ER -

Karstensen J, Schütte F, Pietri A, Krahmann G, Fiedler B, Grundle D et al. Upwelling and isolation in oxygen-depleted anticyclonic modewater eddies and implications for nitrate cycling. Biogeosciences. 2017 apr 27;14(8):2167-2181. https://doi.org/10.5194/bg-14-2167-2017