Description: Ocean observations carried out in the framework of the Collaborative Research Center 754 (SFB 754) "Climate-Biogeochemistry Interactions in the Tropical Ocean" are used to study (1) the structure of tropical oxygen minimum zones (OMZs), (2) the processes that contribute to the oxygen budget, and (3) long-term changes in the oxygen distribution. The OMZ of the eastern tropical North Atlantic (ETNA), located between the well-ventilated subtropical gyre and the equatorial oxygen maximum, is composed of a deep OMZ at about 400 m depth with its core region centred at about 20° W, 10° N and a shallow OMZ at about 100 m depth with lowest oxygen concentrations in proximity to the coastal upwelling region off Mauritania and Senegal. The oxygen budget of the deep OMZ is given by oxygen consumption mainly balanced by the oxygen supply due to meridional eddy fluxes (about 60%) and vertical mixing (about 20%, locally up to 30%). Advection by zonal jets is crucial for the establishment of the equatorial oxygen maximum. In the latitude range of the deep OMZ, it dominates the oxygen supply in the upper 300 to 400 m and generates the intermediate oxygen maximum between deep and shallow OMZs. Water mass ages from transient tracers indicate substantially older water masses in the core of the deep OMZ (about 120–180 years) compared to regions north and south of it. The deoxygenation of the ETNA OMZ during recent decades suggests a substantial imbalance in the oxygen budget: about 10% of the oxygen consumption during that period was not balanced by ventilation. Long-term oxygen observations show variability on interannual, decadal and multidecadal time scales that can partly be attributed to circulation changes. In comparison to the ETNA OMZ the eastern tropical South Pacific OMZ shows a similar structure including an equatorial oxygen maximum driven by zonal advection, but overall much lower oxygen concentrations approaching zero in extended regions. As the shape of the OMZs is set by ocean circulation, the widespread misrepresentation of the intermediate circulation in ocean circulation models substantially contributes to their oxygen bias, which might have significant impacts on predictions of future oxygen levels.

Description: 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.

Description: Ocean models predict a decline in the dissolved oxygen inventory of the global ocean of one to seven per cent by the year 2100, caused by a combination of a warming-induced decline in oxygen solubility and reduced ventilation of the deep ocean1, 2. It is thought that such a decline in the oceanic oxygen content could affect ocean nutrient cycles and the marine habitat, with potentially detrimental consequences for fisheries and coastal economies3, 4, 5, 6. Regional observational data indicate a continuous decrease in oceanic dissolved oxygen concentrations in most regions of the global ocean1, 7, 8, 9, 10, with an increase reported in a few limited areas, varying by study1, 10. Prior work attempting to resolve variations in dissolved oxygen concentrations at the global scale reported a global oxygen loss of 550 ± 130 teramoles (1012 mol) per decade between 100 and 1,000 metres depth based on a comparison of data from the 1970s and 1990s10. Here we provide a quantitative assessment of the entire ocean oxygen inventory by analysing dissolved oxygen and supporting data for the complete oceanic water column over the past 50 years. We find that the global oceanic oxygen content of 227.4 ± 1.1 petamoles (1015 mol) has decreased by more than two per cent (4.8 ± 2.1 petamoles) since 1960, with large variations in oxygen loss in different ocean basins and at different depths. We suggest that changes in the upper water column are mostly due to a warming-induced decrease in solubility and biological consumption. Changes in the deeper ocean may have their origin in basin-scale multi-decadal variability, oceanic overturning slow-down and a potential increase in biological consumption11, 12.

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

Description: 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.

Description: Oxygen minimum zones (OMZs) in the open ocean occur below the surface in regions of weak ventilation and high biological productivity. Very low levels of dissolved oxygen affect marine life and alter biogeochemical cycles. One of the most intense but least understood OMZs in the world is located in the Arabian Sea in a depth range between 300 to 1000 m. Within the last decades observations suggest a decreasing oxygen trend. Thus, an improved understanding of the crucial processes is necessary for a reliable assessment of the future development of the Arabian Sea OMZ. This study uses a combination of observational data as well as reanalysis velocity fields from the ocean model Hycom (Hybrid Coordinate Ocean Model) to explore the ventilation dynamics of the Arabian Sea OMZ. Our results show that the OMZ features a strong seasonal cycle with regional differences that is correlated with the monsoon system: In the eastern basin, the OMZ is strongest during the winter monsoon with a core thickness of 1000 m depth and oxygen values of less than 5 µmol/kg. Ventilation during that phase is dominated by Persian Gulf water, that clockwise circles the perimeter of the basin and enters the OMZ from the north. During the summer monsoon ventilation from the southeast leads to higher oxygen values indicating a reverse flow along the Indian coast in the intermediate layer compared to the southeastward surface currents. The seasonal cycle in the western basin has the same seasonality as the one in the eastern basin with a core thickness of 900 m during the winter monsoon. The oxygen supply during the summer monsoon is weaker compared to the eastern basin and correlates with the ventilation of Persian Gulf (Red Sea) water during the summer monsoon (autumn inter-monsoon) phase. As the interior exchange between the eastern and western basin is weak, the more pronounced OMZ in the eastern basin is explained by prolonged ventilation time scales. For the eastern (western) basin Persian Gulf water needs 2–3 (1–2) years and Red Sea water 7–8 (3–4) years to ventilate the OMZ.

Description: Goal 14 of the United Nations' Sustainable Development Goals (SDGs) is dedicated to conserving and using the oceans and their resources for sustainable development. We suggest that a 'gross marine product' (GMP) index — a measure of the oceans' natural capital — would be invaluable for achieving this goal. The seas provide us with food, materials, livelihoods and recreation. Managing these ecosystem services effectively can help us to eradicate poverty, develop sustainable economies and adapt to global environmental changes. Yet international-resource experts and national strategies still focus largely on goods and services delivered by terrestrial ecosystems (see go.nature.com/2bcqjr0). A GMP index would provide a measure of marine ecosystem goods and services on a national or global scale, derived from estimates for individual oceans. More international research will be necessary to underpin these estimates. The results would inform decision-makers, the private sector and the public on how they could help to achieve goal 14, as well as the 60 targets across most of the 17 SDGs that are relevant to the sustainable development of coastal zones. An integrated programme that measures, monitors and assesses the health of human–ocean systems should oversee their sustainability.

Description: Occupying about 14 % of the world's surface, the Southern Ocean plays a fundamental role in ocean and atmosphere circulation, carbon cycling and Antarctic ice-sheet dynamics. Unfortunately, high interannual variability and a dearth of instrumental observations before the 1950s limits our understanding of how marine–atmosphere–ice domains interact on multi-decadal timescales and the impact of anthropogenic forcing. Here we integrate climate-sensitive tree growth with ocean and atmospheric observations on southwest Pacific subantarctic islands that lie at the boundary of polar and subtropical climates (52–54° S). Our annually resolved temperature reconstruction captures regional change since the 1870s and demonstrates a significant increase in variability from the 1940s, a phenomenon predating the observational record. Climate reanalysis and modelling show a parallel change in tropical Pacific sea surface temperatures that generate an atmospheric Rossby wave train which propagates across a large part of the Southern Hemisphere during the austral spring and summer. Our results suggest that modern observed high interannual variability was established across the mid-twentieth century, and that the influence of contemporary equatorial Pacific temperatures may now be a permanent feature across the mid- to high latitudes.