Abstract
Abstract. The northeast Atlantic is a key horizontal and vertical crossroads region for the meridional overturning circulation, but basic nutrient and oxygen fluxes are still poorly constrained by observations in the region. A surface to bottom northeast Atlantic Ocean budget for mass, nutrients (nitrate and phosphate) and oxygen is determined using an optimization method based on three surveys of the OVIDE transect (from Greenland to Portugal) completed with the World Ocean Atlas 2009. Budgets are derived for two communicating boxes representing the northeastern European basin (NEEB) and the Irminger Sea. For the NEEB (Irminger) box, it is found that 30% of the mass import (export) across the OVIDE section reach (originate from) the Nordic Seas, while 70% are redistributed between both boxes through the Reykjanes Ridge (9.3 ± 0.7 × 109 kg s−1). Net biological source/sink terms of nitrate point to both the Irminger and NEEB boxes as net organic matter production sites (consuming nitrate at a rate of –7.8 ± 6.5 kmol s−1 and –8.4 ± 6.6 kmol s−1, respectively). Using a standard Redfield ratio of C : N = 106 : 16, nitrate consumption rates indicate that about 40 TgC yr−1 of carbon is fixed by organic matter production between the OVIDE transect and the Greenland–Scotland Ridge. Nutrient fluxes also induce a net biological production of oxygen of 73 ± 60 kmol s−1 and 79 ± 62 kmol s−1 in the Irminger and NEEB boxes, which points to the region as being autotrophic. The abiotic air–sea oxygen flux leads to an oceanic oxygen uptake in the two regions (264 ± 66 kmol s−1 in the north and 443 ± 70 kmol s−1 in the south). The abiotic flux is partitioned into a mixing and a thermal component. It is found that the Irminger Sea oceanic oxygen uptake is driven by an air–sea heat flux cooling increasing the ocean surface oxygen solubility. Over the northeastern European basin the mixing component is about half the thermal flux, presumably because of the oxygen minimum in the subtropical thermocline.
Highlights
The northeast Atlantic is a region where subtropical thermocline waters are carried in by the North Atlantic Current (NAC)
In the two sections we describe the domain of analysis, the set of conservation equations used to constrain the circulation, biological source/sink terms and air– sea oxygen fluxes for the northeast Atlantic Ocean and the associated timescale
Top-to-bottom mass transports induce a flow across the OVIDE track of 14.1 ± 0.8 × 109 kg s−1 into the northeastern European basin (NEEB) box. 9.3 ± 0.7 × 109 kg s−1 of this transport are carried to the Irminger box through the RR, while 4.8 ± 0.5 × 109 kg s−1 are exported toward the Nordic Seas through the Iceland– Scotland Ridge
Summary
The northeast Atlantic is a region where subtropical thermocline waters are carried in by the North Atlantic Current (NAC). One noticeable exception is the study by Alvarez et al (2002), who derived nitrate/nitrogen and oxygen budgets for the northeast Atlantic region north of the WOCE A25 4× section between Greenland and Portugal Their mass transport estimates have been improved (Lherminier et al, 2007) so that their tracer transports have to be revisited, which will be done in this study. We propose to combine these transports with transport estimates through the Greenland–Scotland Ridge – quantities well documented from observations in the literature – to compute mass, nutrient and oxygen budgets for the northeast Atlantic, defined here as the area between the OVIDE path and the Greenland–Scotland Ridge (see Fig. 1). 6. In the two sections we describe the domain of analysis, the set of conservation equations (or budgets) used to constrain the circulation, biological source/sink terms and air– sea oxygen fluxes for the northeast Atlantic Ocean and the associated timescale. Error bars are smaller and conservation equations are satisfied within the imposed error bars
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