scholarly journals Characterization of “dead-zone” eddies in the tropical Northeast Atlantic Ocean

2016 ◽  
Author(s):  
Florian Schütte ◽  
Johannes Karstensen ◽  
Gerd Krahmann ◽  
Helena Hauss ◽  
Björn Fiedler ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Mixed Layer ◽  
Dead Zone ◽  
Open Ocean ◽  
Low Oxygen ◽  
Northeast Atlantic ◽  
Eastern Boundary ◽  
Northeast Atlantic Ocean ◽  
Oxygen Minimum ◽  
Oxygen Concentrations

Abstract. Localized open-ocean low–oxygen dead-zones in the tropical Northeast 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 revealed that eddies with low oxygen concentrations at 50–150 m depths can be found in surprisingly high numbers and in a large area (from about 4°N to 22°N, from the shelf at the eastern boundary to 38°W). Minimum oxygen concentrations of about 9 µmol kg−1 in CEs and severely suboxic concentrations (< 1 µmol kg−1) in ACMEs were observed. In total, 173 profiles with oxygen concentrations below the minimum background concentration of 40 µmol kg−1 could be associated with 27 independent “dead-zone” eddies (10 CEs; 17 ACMEs) over a period of 10 years. The eddies’ oxygen minimum is located in the eddy core beneath the mixed layer at a mean depth of 80 m. Compared to the surrounding waters, the mean oxygen anomaly between 50 and 150 m depth for CEs (ACMEs) is −38 (−79) µmol kg−1. The low oxygen concentration right beneath the mixed layer has been attributed to the combination of high productivity in the eddies’ surface waters and the isolation of their 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 locally increased consumption within these eddies represents an essential part of the total consumption in the open tropical Northeast Atlantic Ocean and might be partly responsible for the formation of the shallow oxygen minimum zone. 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. North of 12°N, eddies of both types carry anomalously low salinity water of South Atlantic Central Water origin from the eastern boundary upwelling region into the open ocean. Water mass properties and satellite eddy tracking both point to an eddy generation near the eastern boundary.

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

2016 ◽  
Vol 13 (20) ◽  
pp. 5865-5881 ◽  
Author(s):  
Florian Schütte ◽  
Johannes Karstensen ◽  
Gerd Krahmann ◽  
Helena Hauss ◽  
Björn Fiedler ◽  
...  
Keyword(s):  
North Atlantic ◽  
Dead Zone ◽  
Open Ocean ◽  
Depth Range ◽  
Low Oxygen ◽  
Eastern Boundary ◽  
Tropical North Atlantic ◽  
Minimum Zone ◽  
Core Depth ◽  
Oxygen Minimum

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 mode-water 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.

scholarly journals Contrasting effects of temperature and winter mixing on the seasonal and inter-annual variability of the carbonate system in the Northeast Atlantic Ocean

2009 ◽  
Vol 6 (5) ◽  
pp. 9701-9735
Author(s):  
C. Dumousseaud ◽  
E. P. Achterberg ◽  
T. Tyrrell ◽  
A. Charalampopoulou ◽  
U. Schuster ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Mixed Layer ◽  
Dissolved Inorganic Carbon ◽  
Mixed Layer Depth ◽  
Future Climate Change ◽  
Co2 Uptake ◽  
Bay Of Biscay ◽  
Carbonate System ◽  
Northeast Atlantic ◽  
Northeast Atlantic Ocean

Abstract. Future climate change due to the increase in atmospheric CO2 concentrations is expected to strongly affect the oceans, with shallower winter mixing and consequent reduction in primary productivity and oceanic carbon drawdown in low and mid-latitudinal oceanic regions. Here we test this hypothesis by examining the effects of cold and warm winters on the carbonate system in the surface waters of the Northeast Atlantic Ocean for the period between 2005 and 2007. Monthly observations were made between the English Channel and the Bay of Biscay using a ship of opportunity program. During the colder winter of 2005/2006, the maximum depth of the mixed layer reached 500 m in the Bay of Biscay, whilst during the warmer (by 2.6±0.5°C) winter of 2006/2007 the mixed layer depth reached only 300 m. The inter-annual differences in late winter concentrations of nitrate (2.8±1.1 μmol l−1) and dissolved inorganic carbon (22±6 μmol l−1), with higher concentrations at the end of the colder winter (2005/2006), led to differences in the dissolved oxygen anomaly and the fluorescence data for the subsequent growing season. In contrast to model predictions, the calculated air-sea CO2 fluxes (ranging from +4.5 to −5.5 mmol m−2 d−1) showed an increased oceanic CO2 uptake in the Bay of Biscay following the warmer winter of 2006/2007 associated with wind speed and sea surface temperature differences.

scholarly journals Contrasting effects of temperature and winter mixing on the seasonal and inter-annual variability of the carbonate system in the Northeast Atlantic Ocean

2010 ◽  
Vol 7 (5) ◽  
pp. 1481-1492 ◽  
Author(s):  
C. Dumousseaud ◽  
E. P. Achterberg ◽  
T. Tyrrell ◽  
A. Charalampopoulou ◽  
U. Schuster ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Mixed Layer ◽  
Dissolved Inorganic Carbon ◽  
Mixed Layer Depth ◽  
Future Climate Change ◽  
Co2 Uptake ◽  
Bay Of Biscay ◽  
Carbonate System ◽  
Northeast Atlantic ◽  
Northeast Atlantic Ocean

Abstract. Future climate change as a result of increasing atmospheric CO2 concentrations is expected to strongly affect the oceans, with shallower winter mixing and consequent reduction in primary production and oceanic carbon drawdown in low and mid-latitudinal oceanic regions. Here we test this hypothesis by examining the effects of cold and warm winters on the carbonate system in the surface waters of the Northeast Atlantic Ocean for the period between 2005 and 2007. Monthly observations were made between the English Channel and the Bay of Biscay using a ship of opportunity program. During the colder winter of 2005/2006, the maximum depth of the mixed layer reached up to 650 m in the Bay of Biscay, whilst during the warmer (by 2.6 ± 0.5 °C) winter of 2006/2007 the mixed layer depth reached only 300 m. The inter-annual differences in late winter concentrations of nitrate (2.8 ± 1.1 μmol l−1) and dissolved inorganic carbon (22 ± 6 μmol kg−1, with higher concentrations at the end of the colder winter (2005/2006), led to differences in the dissolved oxygen anomaly and the chlorophyll α-fluorescence data for the subsequent growing season. In contrast to model predictions, the calculated air-sea CO2 fluxes (ranging from +3.7 to −4.8 mmol m−2 d−1) showed an increased oceanic CO2 uptake in the Bay of Biscay following the warmer winter of 2006/2007 associated with wind speed and sea surface temperature differences.

scholarly journals Elevated iron to nitrogen recycling by mesozooplankton in the Northeast Atlantic Ocean

2012 ◽  
Vol 39 (12) ◽  
pp. n/a-n/a ◽  
Author(s):  
Sarah L. C. Giering ◽  
Sebastian Steigenberger ◽  
Eric P. Achterberg ◽  
Richard Sanders ◽  
Daniel J. Mayor
Keyword(s):  
Atlantic Ocean ◽  
Nitrogen Recycling ◽  
Northeast Atlantic ◽  
Northeast Atlantic Ocean

scholarly journals Open ocean dead zones in the tropical North Atlantic Ocean

2015 ◽  
Vol 12 (8) ◽  
pp. 2597-2605 ◽  
Author(s):  
J. Karstensen ◽  
B. Fiedler ◽  
F. Schütte ◽  
P. Brandt ◽  
A. Körtzinger ◽  
...  
Keyword(s):  
North Atlantic ◽  
North Atlantic Ocean ◽  
Dead Zone ◽  
Marine Ecosystem ◽  
High Surface ◽  
Open Ocean ◽  
Depth Range ◽  
Low Oxygen ◽  
Dead Zones ◽  
Tropical North Atlantic

Abstract. Here we present first observations, from instrumentation installed on moorings and a float, of unexpectedly low (<2 μmol kg−1) oxygen environments in the open waters of the tropical North Atlantic, a region where oxygen concentration does normally not fall much below 40 μmol kg−1. The low-oxygen zones are created at shallow depth, just below the mixed layer, in the euphotic zone of cyclonic eddies and anticyclonic-modewater eddies. Both types of eddies are prone to high surface productivity. Net respiration rates for the eddies are found to be 3 to 5 times higher when compared with surrounding waters. Oxygen is lowest in the centre of the eddies, in a depth range where the swirl velocity, defining the transition between eddy and surroundings, has its maximum. It is assumed that the strong velocity at the outer rim of the eddies hampers the transport of properties across the eddies boundary and as such isolates their cores. This is supported by a remarkably stable hydrographic structure of the eddies core over periods of several months. The eddies propagate westward, at about 4 to 5 km day−1, from their generation region off the West African coast into the open ocean. High productivity and accompanying respiration, paired with sluggish exchange across the eddy boundary, create the "dead zone" inside the eddies, so far only reported for coastal areas or lakes. We observe a direct impact of the open ocean dead zones on the marine ecosystem as such that the diurnal vertical migration of zooplankton is suppressed inside the eddies.

scholarly journals Availability of iron and major nutrients for phytoplankton in the northeast Atlantic Ocean

2004 ◽  
Vol 49 (6) ◽  
pp. 2095-2104 ◽  
Author(s):  
Stèphane Blain ◽  
Cècile Guieu ◽  
Hervè Claustre ◽  
Karine Leblanc ◽  
Thierry Moutin ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Northeast Atlantic ◽  
Northeast Atlantic Ocean ◽  
Major Nutrients
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