Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 29 (2015): 793-811, doi:10.1002/2014GB005001.

Description: Mesoscale eddies in Oxygen Minimum Zones (OMZs) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N-loss as well as the ocean's N isotope budget. They also represent “natural tracer experiments” with intensified biogeochemical signals that can be exploited to understand the large-scale processes that control N-loss and associated isotope effects (ε; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO3−), nitrite (NO2−), and biogenic N2 associated with an anticyclonic mode-water eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO3− was nearly exhausted, we measured the highest δ15N values for both NO3− and NO2− (up to ~70‰ and 50‰) ever reported for an OMZ. Correspondingly, N deficit and biogenic N2-N concentrations were also the highest near the eddy's center (up to ~40 µmol L−1). δ15N-N2 also varied with biogenic N2 production, following kinetic isotopic fractionation during NO2− reduction to N2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N-loss. We found apparent variable ε for NO3− reduction (up to ~30‰ in the presence of NO2−). However, the overall ε for N-loss was calculated to be only ~13–14‰ (as compared to canonical values of ~20–30‰) assuming a closed system and only slightly higher assuming an open system (16–19‰). Our results were similar whether calculated from the disappearance of DIN (NO3− + NO2−) or from the appearance of N2 and changes in isotopic composition. Further, we calculated the separate ε values for NO3− reduction to NO2− and NO2− reduction to N2 of ~16–21‰ and ~12‰, respectively, when the effect of NO2− oxidation could be removed. These results, together with the relationship between N and O of NO3− isotopes and the difference in δ15N between NO3− and NO2−, confirm a role for NO2− oxidation in increasing the apparent ε associated with NO3− reduction. The lower ε for N-loss calculated in this study could help reconcile the current imbalance in the global N budget if representative of global OMZ N-loss.

Description: This work was supported by the Deutsche Forschungsgemeinschaft- project SFB-754 (www.sfb754.de), SOPRAN II (grant FKZ 03F0611A; www.sopran.pangaea.de), NSF grants OCE 0851092 and OCE 1154741 to M.A.A., and a NSERC Postdoctoral Fellowship to A.B.

Description: 2015-12-06

Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Ocean 119 (2014): 1068–1083, doi:10.1002/2013JC009470.

Description: In the tropical eastern South Pacific the Stratus Ocean Reference Station (ORS) (∼20°S, 85.5°W) is located in the transition zone between the oxygen minimum zone (OMZ) and the well-oxygenated subtropical gyre. In February/March 2012, extremely anomalous water mass properties were observed in the thermocline at the Stratus ORS. The available eddy oxygen anomaly was −10.5 × 1016 µmol. This anomalous water was contained in an anticyclonic mode-water eddy crossing the mooring site. This eddy was absorbed at that time by an anticyclonic feature located south of the Stratus mooring. This was the largest water property anomaly observed at the mooring during the 13.5 month deployment period. The sea surface height anomaly (SSHA) of the strong mode-water eddy in February/March 2012 was weak, and while the lowest and highest SSHA were related to weak eddies, SSHA is found not to be sufficient to specify the eddy strength for subsurface-intensified eddies. Still, the anticyclonic eddy, and its related water mass characteristics, could be tracked backward in time in SSHA satellite data to a formation region in April 2011 off the Chilean coast. The resulting mean westward propagation velocity was 5.5 cm s−1. This extremely long-lived eddy carried the water characteristics from the near-coastal Chilean water to the open ocean. The water mass stayed isolated during the 11 month travel time due to high rotational speed of about 20 cm s−1 leading to almost zero oxygen in the subsurface layer of the anticyclonic mode-water eddy with indications of high primary production just below the mixed layer.

Description: Financial support was received through Woods Hole Oceanographic Institution (R.A.W. and S.B.) and the GEOMAR (L.S. and R.C). The Stratus Ocean Reference Station is supported by the National Oceanic and Atmospheric Administration’s (NOAA) Climate Observation Program (NA09OAR4320129). This work is a contribution of the DFG-supported project SFB754 (http://www.sfb754.de) which is supported by the Deutsche Forschungsgemeinschaft.

Description: 2014-08-12

Description: Simulated transient-tracer distributions (tritium, 3H3, freons) on the isopycnal horizons σ0=26.5 and 26.8 kg m−3 are presented for the East Atlantic, 10° −40°N. Tracer transport is modeled by employing a baroclinic flow field based on empirical data in a kinematic isopycnal advection-diffusion numerical model, in which winter convection is taken as the mechanism of communication with the ocean surface layer, and the isopycnal diffusivity is a free parameter. Diapucnic transport is ignored. The simulations employ time-dependent tracer boundary conditions, which are constructed on the basis of available observations. Simulations are compared to data obtained on a meridional section in 1981 (F/S Meteor, cruise 56/5). Best simulations were obtained by means of a subjective optimization procedure. On both levels, the observed distributions and the best simulated distributions agree well. The fact that the surface boundary conditions and interior distributions of the tracers are distinctly different leads us to the conclusion that our model provides a consistent description of upper main-thermocline ventilation and interior transport Surface-water densities in February are found to represent adequately the winter outcrop boundaries with an uncertainty of about ±300 km across. The required isopycnal diffusivity south of 29°N is 1700 m2 s−1, and 2900 m2 s−1 further north (+70/−40%). Interior transport is found to be predominantly advective. Advective ventilation across 30.5°N east of 33°W amounts to only 12% and 40% for the 26.5 and 26.8 horizons of the total ventilation rates reported by Sarmiento. The North Atlantic/South Atlantic Central Water boundary near 15°N is found to be predominantly determined by advection.

Description: Changes in the ventilation of the oxygen minimum zone (OMZ) of the tropical North Atlantic are studied using oceanographic data from 18 research cruises carried out between 28.5° and 23°W during 1999–2008 as well as historical data referring to the period 1972–85. In the core of the OMZ at about 400-m depth, a highly significant oxygen decrease of about 15 μmol kg−1 is found between the two periods. During the same time interval, the salinity at the oxygen minimum increased by about 0.1. Above the core of the OMZ, within the central water layer, oxygen decreased too, but salinity changed only slightly or even decreased. The scatter in the local oxygen–salinity relations decreased from the earlier to the later period suggesting a reduced filamentation due to mesoscale eddies and/or zonal jets acting on the background gradients. Here it is suggested that latitudinally alternating zonal jets with observed amplitudes of a few centimeters per second in the depth range of the OMZ contribute to the ventilation of the OMZ. A conceptual model of the ventilation of the OMZ is used to corroborate the hypothesis that changes in the strength of zonal jets affect mean oxygen levels in the OMZ. According to the model, a weakening of zonal jets, which is in general agreement with observed hydrographic evidences, is associated with a reduction of the mean oxygen levels that could significantly contribute to the observed deoxygenation of the North Atlantic OMZ.

Description: Chlorofluorocarbon (component CFC-11) and hydrographic data from 1997, 1999, and 2001 are presented to track the large-scale spreading of the Upper Labrador Sea Water (ULSW) in the subpolar gyre of the North Atlantic Ocean. ULSW is CFC rich and comparatively low in salinity. It is located on top of the denser “classical” Labrador Sea Water (LSW), defined in the density range σΘ = 27.68–27.74 kg m−3. It follows spreading pathways similar to LSW and has entered the eastern North Atlantic. Despite data gaps, the CFC-11 inventories of ULSW in the subpolar North Atlantic (40°–65°N) could be estimated within 11%. The inventory increased from 6.0 ± 0.6 million moles in 1997 to 8.1 ± 0.6 million moles in 1999 and to 9.5 ± 0.6 million moles in 2001. CFC-11 inventory estimates were used to determine ULSW formation rates for different periods. For 1970–97, the mean formation rate resulted in 3.2–3.3 Sv (Sv ≡ 106 m3 s−1). To obtain this estimate, 5.0 million moles of CFC-11 located in 1997 in the ULSW in the subtropical/tropical Atlantic were added to the inventory of the subpolar North Atlantic. An estimate of the mean combined ULSW/LSW formation rate for the same period gave 7.6–8.9 Sv. For the years 1998–99, the ULSW formation rate solely based on the subpolar North Atlantic CFC-11 inventories yielded 6.9–9.2 Sv. At this time, the lack of classical LSW formation was almost compensated for by the strongly pronounced ULSW formation. Indications are presented that the convection area needed in 1998–99 to form this amount of ULSW exceeded the available area in the Labrador Sea. The Irminger Sea might be considered as an additional region favoring ULSW formation. In 2000–01, ULSW formation weakened to 3.3–4.7 Sv. Time series of layer thickness based on historical data indicate that there exists considerable variability of ULSW and classical LSW formation on decadal scales.

Description: The Southern Hemisphere Subtropical Front (STF) is a narrow zone of transition between upper-level subtropical waters to the north and subantarctic waters to the south. It is found near 40 degrees S across the South Atlantic and South Indian Oceans and is associated with an eastward geostrophic current band, The current band in each basin is found at or just north of the surface front except near the eastern boundaries where most of the subtropical waters turn north into the eastern limbs of the subtropical gyres. The bands associated with the STF are thus distinct features separated from the strong zonal flows of the Antarctic Circumpolar Current farther south. The authors have referred to the current bands in the two respective oceans as the South Atlantic Current and the South Indian Ocean Current. In this paper the authors use the historical database from the South Pacific Ocean to investigate the geostrophic flow associated with the STF there. The STF extends across the southern Tasman Sea from south of Tasmania to southern New Zealand, and a weak eastward flow appears to be associated with it. The transport amounts to only about 3 Sv (1Sv = 10(6) m(3) s(-1)), little of which passes south of New Zealand. Mixing within the eddy-rich Tasman Sea may account for this weakness, while also setting up another more significant front in the northern Tasman Sea, the Tasman Front. It branches off from the East Australian Current toward the north of New Zealand, along which moves a flow of about 14 Sv. After passing north of New Zealand, a portion of this current flows east to contribute to a current band near 30 degrees S, while another portion turns south as the East Auckland Current and meets with subantarctic waters near Chatham Rise (44 degrees S), thus reestablishing the STF. An enhanced eastward current band is associated with the front there, one that extends across the remainder of the South Pacific and is referred to as the South Pacific Current. In comparison with its counterparts in the other basins, which typically begin by carrying 30 Sv (Atlantic) to 60 Sv (Indian) in the upper 1000 m in their western portions before weakening to 10-15 Sv in the east, the South Pacific Current is weak. Near Chatham Rise, it starts with a transport of approximately 5 Sv, and it remains near this strength as it shifts gradually north across the basin toward South America. The current appears to split into two smaller bands in the region of 115 degrees-85 degrees W, while near 28 degrees 5, 83 degrees W it begins to turn more strongly north and becomes shallower and weaker. Potential vorticity distributions indicate that this current acts as an impediment toward the northward spreading of Antarctic Intermediate Water, But why the South Pacific Current east of New Zealand should be so much weaker than its counterparts in the other basins is not particularly clear. It may be due to the presence of New Zealand and other topographic barriers to deep now east of Australia, to the axis of the subtropical gyre in the South Pacific shifting more rapidly southward with depth than those elsewhere, thus causing greater reductions in the underlying zonal velocities, and to strong poleward eddy heat and salt fluxes in the other two basins leading to smaller cross-STF gradients in the Pacific.

Description: In this paper, the historical hydrographic database for the south Indian Ocean is used to investigate (i) the hydrographic boundary between the subtropical gyre and the Antarctic Circumpolar Current (ACC), the subtropical front (STF), and especially (ii) the southern current band of the gyre. A current band of increased zonal speeds in the upper 1000 m is found just north of the STF in the west near South Africa and at the surface STF in the open Indian Ocean until the waters off the coast of Australia are reached. As neither any other investigation of this current nor a name for it are known, the flow has been called the South Indian Ocean Current (SIOC). This name is anologous to the same current band in the South Atlantic Ocean, the South Atlantic Current. The STF is located in the entire south Indian Ocean near 40-degrees-S. The associated current band of increased zonal speeds is the SIOC, which is found at or north of the STF. East of 100-degrees-E the SIOC separates from the STF and continues to the northeast. The zonal flow south of the STF is normally weak and serves to separate the South Indian Ocean and Circumpolar currents. Near Africa the SIOC has a typical volume transport of 60 Sv (1 Sv = 10(6) m3 s-1) in the upper 1000 m relative to deep potential density surfaces of sigma(4) = 45.87 kg m-3 (2800-3500 m) or sigma(2) = 36.94 kg m-3 (1500-2500 m). Near western Australia the SIOC is reduced to about 10 Sv as it turns to the northeast.

Description: Historical data from the region between the Greenwich meridian and the African continental shelf are used to compute the offshore geostrophic transport of the Benguela Current. At 32°S, the Benguela Current is located near the African coast, transporting about 21 Sv (1 Sv = 106 m3 s−1) of surface water toward the north relative to a potential density surface lying between the upper branch of Circumpolar Deep Water and the North Atlantic Deep Watar. Two warm core eddies of probable Agulhas Current origin an observed west of the Benguela Current at 32°S. Near 30°S, the Benguela Current turns toward the northwest and begins to separate from the eastern boundary. It carries about 18 Sv of surface water across 28°S. The current then turns mainly toward the west to flow over a relatively deep segment of the Walvis Ridge south of the Valdivia Bank. A surface current with northward surface of about 10 cm s−1 flows along the western side of the Valdivia Bank, while another northward surface current flows at about 20 cm s−1 some 300 km west of the bank. About 3 Sv of surface now do not leave the Cape Basin south of the Vaidivia Bank, but instead drift northward as a wide. sluggish flow out of the northern end of the Cape Basin. Because of the more southerly seaward extensions of most of the Benguela Current, there are no deep-reaching interactions observed between this current and the cyclonic gyre in the Angola Basin east of the Greenwich meridian. Beneath the surface layer, about 4–5 Sv of Antarctic Intermediate Water are carried northward across 32° and 28°S by the Benguela Current, essentially all of which turns westward to cross the Greenwich meridian south of 24°S.

Description: Data from a surface mooring located in the Sargasso Sea at 34°N, 70°W between May 1982 and May 1984 were compared with satellite data to investigate large diurnal sea surface temperature changes. Mooring and satellite measurements are in excellent agreement for those days on which no clouds covered the site at the time of the satellite pass. During the summer half-year at this site, there is a 20% charm of diurnal warming of more than 0.5°C, with values of up to 3.5°C observed in the two-year period. Diurnal warming observed at the mooring has been simulated well by a one-dimensional model driven by local beat and momentum fluxes. Under the conditions of very light wind and strong insolation that produce the Largest surface warming, the surface mixed-layer depth reduces to the convection depth, and wind-mixing becomes unimportant. The thermal response is then limited to depths between 1 and 2 m, making it likely that such events have been underreported in routine ship observations. In all cases observed, the spatial extent of warming events as determined by satellite data are well correlated with the corresponding atmospheric pressure patterns. Conditions giving rise to the largest diurnal warming events are often associated with a westward-extending ridge of the Bermuda high. In the region studied, 57°–75°W and 29°–43°N, diurnal warming of more than 1°C was found on occasion to cover areas in excess of 300 000 km2, with warming of more than 2°C coveting areas in excess of 130 000 km2.