102 research outputs found
The present and future system for measuring the Atlantic meridional overturning circulation and heat transport
of the global combined atmosphere-ocean heat flux and
so is important for the mean climate of the Atlantic
sector of the Northern Hemisphere. This meridional heat
flux is accomplished by both the Atlantic Meridional
Overturning Circulation (AMOC) and by basin-wide
horizontal gyre circulations. In the North Atlantic
subtropical latitudes the AMOC dominates the meridional heat flux, while in subpolar latitudes and in the subtropical South Atlantic the gyre circulations are
also important. Climate models suggest the AMOC will
slow over the coming decades as the earth warms, causing widespread cooling in the Northern hemisphere and additional sea-level rise. Monitoring systems for selected components of the AMOC have been in place in some areas for decades, nevertheless the present observational network provides only a partial view of the AMOC, and does not unambiguously resolve the full variability of the circulation. Additional observations, building on existing measurements, are required to more completely quantify the Atlantic meridional heat transport. A basin-wide monitoring
array along 26.5°N has been continuously measuring the strength and vertical structure of the AMOC and meridional heat transport since March 31, 2004. The array has demonstrated its ability to observe the AMOC variability at that latitude and also a variety of surprising variability that will require substantially longer time series to understand fully. Here we propose monitoring the Atlantic meridional heat transport throughout the Atlantic at selected critical latitudes that have already been identified as regions of interest for the study of deep water formation and the strength of the subpolar gyre, transport variability of the Deep Western Boundary Current (DWBC) as well as the upper limb of the AMOC, and inter-ocean and intrabasin exchanges with the ultimate goal of determining regional and global controls for the AMOC in the North and South Atlantic Oceans. These new arrays will
continuously measure the full depth, basin-wide or choke-point circulation and heat transport at a number
of latitudes, to establish the dynamics and variability at
each latitude and then their meridional connectivity.
Modeling studies indicate that adaptations of the 26.5°N
type of array may provide successful AMOC monitoring at other latitudes. However, further analysis and the development of new technologies will be needed to optimize cost effective systems for providing long term monitoring and data recovery at climate time scales. These arrays will provide benchmark observations of the AMOC that are fundamental for assimilation, initialization, and the verification of coupled hindcast/forecast climate models
Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation
Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAPâOverturning in the Subpolar North Atlantic Program) to show that winter convection during 2014â2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region
Dissolved Organic Carbon in the North Atlantic Meridional Overturning Circulation
The quantitative role of the Atlantic Meridional Overturning Circulation (AMOC) in dissolved organic carbon (DOC) export is evaluated by combining DOC measurements with observed water mass transports. In the eastern subpolar North Atlantic, both upper and lower limbs of the AMOC transport high-DOC waters. Deep water formation that connects the two limbs of the AMOC results in a high downward export of non-refractory DOC (197 Tg-C·yr-1). Subsequent remineralization in the lower limb of the AMOC, between subpolar and subtropical latitudes, consumes 72% of the DOC exported by the whole Atlantic Ocean. The contribution of DOC to the carbon sequestration in the North Atlantic Ocean (62 Tg-C·yr-1) is considerable and represents almost a third of the atmospheric CO 2 uptake in the region
Inputs and processes affecting the distribution of particulate iron in the North Atlantic along the GEOVIDE (GEOTRACES GA01) section
The aim of the GEOVIDE cruise (MayâJune 2014, R/V Pourquoi Pas?)
was to provide a better understanding of trace metal biogeochemical cycles in
the North Atlantic Ocean. As marine particles play a key role in the global
biogeochemical cycle of trace elements in the ocean, we discuss the
distribution of particulate iron (PFe), in relation to the distribution of
particulate aluminium (PAl), manganese (PMn), and phosphorus (PP). Overall,
32 full vertical profiles were collected for trace metal analyses,
representing more than 500 samples. This resolution provides a solid basis
for assessing concentration distributions, elemental ratios, size
fractionation, and adsorptive scavenging processes in key areas of the
thermohaline overturning circulation. Total particulate iron concentrations
ranged from as low as 9 pmol Lâ1 in surface waters of the Labrador Sea
to 304 nmol Lâ1 near the Iberian margin, while median PFe
concentrations of 1.15 nmol Lâ1 were measured over the sub-euphotic
ocean interior.
Within the Iberian Abyssal Plain, the ratio of PFe to PAl was identical to
the continental crust molar ratio (0.21 mol molâ1), indicating the
important influence of crustal particles in the water column. Overall, the
lithogenic component explained more than 87% of PFe variance along the
section. Within the Irminger and Labrador basins, the formation of biogenic
particles led to an increase in the PFeâPAl ratio (up to
0.64 mol molâ1) compared to the continental crust ratio. Continental
margins induce high concentrations of particulate trace elements within the
surrounding water masses (up to 10 nmol Lâ1 of PFe). For example,
horizontal advection of PFe was visible more than 250 km away from the
Iberian margin. Additionally, several benthic nepheloid layers were observed
more than 200 m above the seafloor along the transect, especially in the
Icelandic, Irminger, and Labrador basins, suspending particles with high PFe
content of up to 89 nmol Lâ1.</p
Mercury distribution and transport in the North Atlantic Ocean along the GEOTRACES-GA01 transect
We report here the results of total mercury (HgT) determinations along the 2014 Geotraces Geovide cruise (GA01 transect) in the North Atlantic Ocean (NA) from Lisbon (Portugal) to the coast of Labrador (Canada). HgT concentrations in unfiltered samples (HgTUNF) were log-normally distributed and ranged between 0.16 and 1.54âŻpmolâŻLâ1, with a geometric mean of 0.51âŻpmolâŻLâ1 for the 535 samples analysed. The dissolved fraction (<âŻ0.45âŻÂ”m) of HgT (HgTF), determined on 141 samples, averaged 78âŻ% of the HgTUNF for the entire data set, 84âŻ% for open seawaters (below 100âŻm) and 91âŻ% if the Labrador Sea data are excluded, where the primary production was high (with a winter convection down to 1400âŻm). HgTUNF concentrations increased eastwards and with depth from Greenland to Europe and from subsurface to bottom waters. The HgTUNF concentrations were similarly low in the subpolar gyre waters (ââŒââŻ0.45âŻpmolâŻLâ1), whereas they exceeded 0.60âŻpmolâŻLâ1 in the subtropical gyre waters. The HgTUNF distribution mirrored that of dissolved oxygen concentration, with highest concentration levels associated with oxygen-depleted zones. The relationship between HgTF and the apparent oxygen utilization confirms the nutrient-like behaviour of Hg in the NA. An extended optimum multiparameter analysis allowed us to characterize HgTUNF concentrations in the different source water types (SWTs) present along the transect. The distribution pattern of HgTUNF, modelled by the mixing of SWTs, show Hg enrichment in Mediterranean waters and North East Atlantic Deep Water and low concentrations in young waters formed in the subpolar gyre and Nordic seas. The change in anthropogenic Hg concentrations in the Labrador Sea Water during its eastward journey suggests a continuous decrease in Hg content in this water mass over the last decades. Calculation of the water transport driven by the Atlantic Meridional Overturning Circulation across the PortugalâGreenland transect indicates northward Hg transport within the upper limb and southward Hg transport within the lower limb, with resulting net northward transport of about 97.2âŻkmolâŻyrâ1
Introduction to the French GEOTRACES North Atlantic Transect (GA01): GEOVIDE cruise
The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this special issue (https://www.biogeosciences.net/special_issue900.html), results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among 18 articles. Here, the scientific context, project objectives, and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue
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