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

Abstract

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.

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

“…3a, b and 4a) water mass that matches the characteristics of South Atlantic Central Water (SACW; and is a typical composition for low oxygen eddies in the eastern tropical North Atlantic Schütte et al, 2016a, b). The properties confirm that the ACME was formed in the coastal area off Mauritania, as suggested by the SLA analysis (Schütte et al, 2016b; Fig. 1, left).…”

“…To further differentiate a positive SLA (indicative of anticyclonic-rotating eddies) into either an ACE or an ACME the SST anomaly across the eddy was additionally inspected, because ACMEs (ACEs) in the eastern tropical North Atlantic show a cold (warm) SST anomaly (Schütte et al, 2016a, b). For further evidence, Argo profile data were inspected to detect anomalously low temperature/salinity signatures also indicative of low oxygen ACMEs in the region Schütte et al, 2016b). In late December 2013 a candidate eddy was identified through this mechanism and in late January 2014 a pre-survey was initiated, making use of autonomous gliders.…”

“…Characteristics of a low oxygen ACME found in the eastern tropical North Atlantic are provided. The paper is part of a series of publications that report on different genomic, biological, and biogeochemical aspects of this eddy (Löscher et al, 2015;Hauss et al, 2016;Schütte et al, 2016b).…”

“…2b). The SLA data analysis for the eddy (see Schütte et al, 2016b, for details) suggests a formation in the Mauritanian upwelling region (Fig. 1).…”

Impact of mesoscale eddies on water mass and oxygen distribution in the eastern tropical South Pacific

“…contribute to cooling the surface layer in this region (Holte et al, 2013) For the Atlantic Ocean the low-oxygen eddy cores has been attributed to high productivity in the surface (Schütte et al, 2016b), enhanced respiration of sinking organic material at subsurface depth 10 (Fiedler et al, 2016) and a strong isolation of the eddy core (Karstensen et al, 2017). An anticyclonic mode water eddy observed at the Stratus mooring in February/March 2012 indicated high primary production just below the mixed layer (Stramma et al, 2014).…”