characterization of “dead-zone” eddies in the eastern tropical north atlantic
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2016
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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 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.
| Reference Key |
schtte2016biogeosciencescharacterization
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|---|---|
| Authors | ;F. Schütte;J. Karstensen;G. Krahmann;H. Hauss;B. Fiedler;P. Brandt;M. Visbeck;A. Körtzinger |
| Journal | tetrahedron letters |
| Year | 2016 |
| DOI |
10.5194/bg-13-5865-2016
|
| URL | |
| Keywords |
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