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Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone (2013)

Fischer, Tim ; Banyte, Donata ; Brandt, Peter ; [weitere]

Copernicus Publications (EGU)

In: Biogeosciences (BG), 10 . pp. 5079-5093.

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Publikationsdatum: 2021-04-21

Beschreibung: The replenishment of consumed oxygen in the open ocean oxygen minimum zone (OMZ) off northwest Africa is accomplished by oxygen transport across and along density surfaces, i.e. diapycnal and isopycnal oxygen supply. Here the diapycnal oxygen supply is investigated using a large observational set of oxygen profiles and diapycnal mixing data from years 2008 to 2010. Diapycnal mixing is inferred from different sources: (i) a large-scale tracer release experiment, (ii) microstructure profiles, and (iii) shipboard acoustic current measurements plus density profiles. From these measurements, the average diapycnal diffusivity in the studied depth interval from 150 to 500m is estimated to be 1×10−5 m2 s−1, with lower and upper 95%confidence limits of 0.8×10−5 m2 s−1 and 1.4×10−5 m2 s−1. Diapycnal diffusivity in this depth range is predominantly caused by turbulence, and shows no significant vertical gradient. Diapycnal mixing is found to contribute substantially to the oxygen supply of the OMZ. Within the OMZ core, 1.5 μmol kg−1 yr−1 of oxygen is supplied via diapycnal mixing, contributing about one-third of the total demand. This oxygen which is supplied via diapycnal mixing originates from oxygen that has been laterally supplied within the upper CentralWater layer above the OMZ, and within the Antarctic Intermediate Water layer below the OMZ. Due to the existence of a separate shallow oxygen minimum at about 100m depth throughout most of the study area, there is no net vertical oxygen flux from the surface layer into the Central Water layer. Thus all oxygen supply of the OMZ is associated with remote pathways.

Materialart: Article , PeerReviewed , info:eu-repo/semantics/article

Format: text

DOI: 10.5194/bg-10-5079-2013

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Time response of oxygen optodes on profiling platforms and its dependence on flow speed and temperature (2014)

Bittig, Henry C. ; Fiedler, Björn ; Scholz, Roland ; [weitere]

American Society of Limnology and Oceanography

In: Limnology and Oceanography: Methods, 12 . pp. 617-636.

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Publikationsdatum: 2019-09-23

Beschreibung: The time response behavior of Aanderaa optodes model 3830, 4330, and 4330F, as well as a Sea-Bird SBE63 optode and a JFE Alec Co. Rinko dissolved oxygen sensor was analyzed both in the laboratory and in the field. The main factor for the time response is the dynamic regime, i.e., the water flow around the sensor that influences the boundary layer’s dynamics. Response times can be drastically reduced if the sensors are pumped. Laboratory experiments under different dynamic conditions showed a close to linear relation between response time and temperature. Application of a diffusion model including a stagnant boundary layer revealed that molecular diffusion determines the temperature behavior, and that the boundary layer thickness was temperature independent. Moreover, field experiments matched the laboratory findings, with the profiling speed and mode of attachment being of prime importance. The time response was characterized for typical deployments on shipboard CTDs, gliders, and floats, and tools are presented to predict the response time as well as to quantify the effect on the data for a given water mass profile. Finally, the problem of inverse filtering optode data to recover some of the information lost by their time response is addressed.

Materialart: Article , PeerReviewed

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DOI: 10.4319/lom.2014.12.617

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Summer upwelling at the Boknis Eck time series station (1982 to 2012) - a combined glider and wind data analysis (2014)

Karstensen, Johannes ; Liblik, Taavi ; Fischer, Jürgen ; [weitere]

Copernicus Publications (EGU)

In: Biogeosciences (BG), 11 . pp. 3603-3617.

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Publikationsdatum: 2019-05-23

Beschreibung: Two consecutive summer upwelling events, each lasting for less than 24 h, were surveyed in high temporal and vertical resolution close to the Boknis Eck time-series station (BE) in the western Belt Sea (Baltic Sea) in summer 2010 with an autonomous glider. Driven only by moderate offshore winds both events resulted in more than 5 K cooling of surface waters, while only for the second event were significant irreversible changes in the vertical stratification observed. Generalizing the glider survey observations with hourly wind data from nearby meteorological stations, it is found that upwelling in the BE area occurs for wind directions between 190 to 260° and wind speed exceeding 4 m s−1. Based on these thresholds the wind-induced summer (June to September) upwelling conditions in the BE area for the period 1982 to 2012 are reconstructed. On average about 18 days of upwelling favourable wind conditions are found for the four summer months, with significant interannual variability ranging from 7.7 days (2006) to more than 28 days (1985). By aligning upwelling favourable wind conditions with the monthly BE surveys it is found that extreme anomalies in BE surveys follow extended periods of upwelling favourable winds.

Materialart: Article , PeerReviewed , info:eu-repo/semantics/article

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DOI: 10.5194/bg-11-3603-2014

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The East Greenland Spill Jet as an important component of the Atlantic Meridional Overturning Circulation (2014)

von Appen, Wilken-Jon ; Koszalka, Inga Monika ; Pickart, Robert S. ; [weitere]

Elsevier

In: Deep Sea Research Part I: Oceanographic Research Papers, 92 . pp. 75-84.

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Publikationsdatum: 2019-07-10

Beschreibung: Highlights: • Mooring observations show the East Greenland Spill Jet to be ubiquitous. • It is fed by classical DSOW in Denmark Strait, shelf water, and Irminger Sea water. • Its transport is similar to the classical DSOW plume. • It is the origin of a large fraction of the water in the Labrador Sea Water density range. Abstract: The recently discovered East Greenland Spill Jet is a bottom-intensified current on the upper continental slope south of Denmark Strait, transporting intermediate density water equatorward. Until now the Spill Jet has only been observed with limited summertime measurements from ships. Here we present the first year-round mooring observations demonstrating that the current is a ubiquitous feature with a volume transport similar to the well-known plume of Denmark Strait overflow water farther downslope. Using reverse particle tracking in a high-resolution numerical model, we investigate the upstream sources feeding the Spill Jet. Three main pathways are identified: particles flowing directly into the Spill Jet from the Denmark Strait sill; particles progressing southward on the East Greenland shelf that subsequently spill over the shelfbreak into the current; and ambient water from the Irminger Sea that gets entrained into the flow. The two Spill Jet pathways emanating from Denmark Strait are newly resolved, and long-term hydrographic data from the strait verifies that dense water is present far onto the Greenland shelf. Additional measurements near the southern tip of Greenland suggest that the Spill Jet ultimately merges with the deep portion of the shelfbreak current, originally thought to be a lateral circulation associated with the sub-polar gyre. Our study thus reveals a previously unrecognized significant component of the Atlantic Meridional Overturning Circulation that needs to be considered to understand fully the ocean׳s role in climate.

Materialart: Article , PeerReviewed

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DOI: 10.1016/j.dsr.2014.06.002

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Diapycnal diffusivity at the upper boundary of the tropical North Atlantic oxygen minimum zone (2012)

Banyte, Donata ; Tanhua, Toste ; Visbeck, Martin ; [weitere]

AGU (American Geophysical Union)

In: Journal of Geophysical Research: Oceans, 117 (C9). C09016.

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Publikationsdatum: 2019-09-23

Beschreibung: A deliberate tracer release experiment in 2008–2010 was used to study diapycnal mixing in the tropical northeastern Atlantic. The tracer (CF3SF5) was injected on the isopycnal surface σΘ = 26.88 kg m−3, which corresponds to about 330 m depth. Three surveys, performed 7, 20, and 30 months after the release, sampled the vertically and laterally expanding tracer patch. The mean diapycnal mixing estimate over the entire region occupied by the tracer and the period of 30 months was found to be (1.19 ± 0.18) × 10−5 m2 s−1, or, alternatively, (3.07 ± 0.58) × 10−11 (kg m−3)2 s−1 as computed from the advection-diffusion equation in isopycnal coordinates with the thickness-weighted averaging. The latter method is preferable in the regions of different stratification for it yields local diapycnal mixing estimates varying less with stratification than their Cartesian coordinate counterparts. Results of this study are comparable to the results of the North Atlantic tracer release experiment (NATRE). However, the internal wave-wave interaction models predict reduced mixing from the breaking of internal waves at low latitudes. Thus, the diapycnal diffusivity found in this study is higher than parameterized by the low latitude of the site (4°N–12°N).

Materialart: Article , PeerReviewed

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DOI: 10.1029/2011JC007762

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Changes in the Ventilation of the Oxygen Minimum Zone of the Tropical North Atlantic (2010)

Brandt, Peter ; Hormann, Verena ; Körtzinger, Arne ; [weitere]

AMS (American Meteorological Society)

In: Journal of Physical Oceanography, 40 (8). pp. 1784-1801.

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Publikationsdatum: 2020-08-04

Beschreibung: 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.

Materialart: Article , PeerReviewed

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DOI: 10.1175/2010JPO4301.1

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Ocean temperature and salinity inverted from combined hydrographic and seismic data (2010)

Papenberg, Cord ; Klaeschen, Dirk ; Krahmann, Gerd ; [weitere]

AGU (American Geophysical Union)

In: Geophysical Research Letters, 37 (4). L04601.

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Publikationsdatum: 2017-06-20

Beschreibung: Recently seismic reflection methods have been successfully applied to oceanographic issues. Here, we present a new approach, combining XBT and CTD surveys with seismic observations, to visualize long sections with a resolution down to a few meters. The challenge to a full investigation of mixing processes has been the tremendous span of spatial scales ranging from hundreds of kilometers to centimeters. Traditional hydrographic observations could only resolve the large scale effects by measuring temperature and salinity profiles at discrete locations typically several kilometers apart, whereas dedicated localized measurements allowed investigation of the ocean fine structure at the other end of the spatial spectrum. The intermediate scales have in contrast been difficult to observe systematically. Here we present temperature and salinity data inverted from seismic observations that cover the intermediate scales and provide a new approach to image mesoscale processes and allow the investigation of their dynamics at unprecedented resolution.

Materialart: Article , PeerReviewed

Format: text

DOI: 10.1029/2009GL042115

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