Atlantic Subpolar Research Articles

Subpolar Mode Water formation traced by neodymium isotopic composition

The formation of Subpolar Mode Water (SPMW) is documented by combining neodymium isotopic composition (Nd IC) as a conservative tracer with salinity, temperature and O2 concentration. Nd IC reveals a more pronounced eastward extension of the Labrador Current, particularly its Subarctic Intermediate Water component, than the other parameters. Nd IC also suggests that waters flowing through the Denmark Strait do not significantly contribute to the SPMW formation. A two end‐member mixing, involving Subarctic Intermediate Water (and maybe ultimately Labrador Current Water) and North Atlantic Central Water, is proposed to explain SPMW formation. These results provide important insight into the processes controlling the Nd IC in the North Atlantic Subpolar Gyre, which could provide a means for reconstructing the northward extension of the Gulf Stream in the past.

Physical and biogeochemical fluxes and net budgets in the subpolar and temperate North Atlantic

Atransoceanic hydrographic sectionacrossthe North Atlantic Subpolar gyre from Vigo (northwestern Iberian Peninsula) to Cape Farewell (south of Greenland) was sampled in summer 1997 as part of the World Ocean Circulation Experiment program (WOCE A25, 4x cruise). The circulation pattern across the 4x section is diagnosed using inverse methods. The e ow is constrained with measured mass transports at specie c sites, while conserving mass and salt for the region north of the section and forcing the silicate e ux to a reasonable value. The e uxes of physical (heat and freshwater) and chemical (nutrients and oxygen) properties are estimated and decomposed into barotropic, baroclinic and horizontal components. The heat transport amounts to 0.65 6 0.1 PW poleward, with 54% and 45% of the e ux due to the baroclinic (or overturning) and horizontal circulation, respectively. From the salt conservation, an equatorward freshwater transport of 20.4 6 1.5 Sv is estimated, resulting from net precipitation plus runoff over the North Atlantic Ocean north of the section. The Subpolar gyre exports nutrients and oxygen southward toward the Subtropical ocean at rates of 250 6 19, 26 6 2, 226 6 15 and 21992 6 440 kmol s 2 1 for nitrate, phosphate, silicate and oxygen, respectively. The main mechanism responsible for the nutrient transport is the overturning cell, whereas oxygen is mainly transported southward due to the large-scale horizontal circulation. Combining our e uxes with those from the 36N section (Rintoul and Wunsch, 1991) allows us to examine budgets of physical (heat and freshwater) and chemical (nitrogen and oxygen) properties for an enclosed area of the Subpolar and Temperate North Atlantic. The tentative nitrogen budget for the box between the 4x and the 36N sections suggests that the Temperate North Atlantic is exporting

Recruitment of Atlantic cod stocks in relation to temperature and advectlon of copepod populations

Abstract The recruitment of Atlantic cod stocks shows different responses to temperature changes. Cod stocks inhabiting the lower temperature range show generally an increase in recruitment with increasing temperature, while cod stocks inhabiting the uppermost part of the temperature range show a decrease in recruitment with increasing temperature. In the present paper possible functional relationships between temperature and cod recruitment mechanisms are analysed. Temperature influences the recruitment processes in a large number of ways; partly directly on vital rates in cod, and partly indirectly through trophic transfer. The copepod Caianus fin marchions is the dominant prey species for the early stages of cod. It proposed that the recruitment-temperature relation of Atlantic cod is a proxy for the food abundance during the early stages, and that advection of C.finmarchicus from the core production regions in the central parts of the North Atlantic Subpolar Gyre to the fringes where the cod stocks ha...

Micro-algal carbon and nitrogen uptake in post-coccolithophore bloom conditions in the northeast Atlantic, July 1991

A study of carbon and nitrogen uptake by phytoplankton was conducted at two sites in the subpolar northeast Atlantic during July 1991. The main study site at 61°N, 20°W was located within a cyclonic warm core eddy, and the other site was at 62°30′N, 20°W. Based on evidence of coccolithophore sedimentation and the relatively slow movement of the eddy, this structure was probably within the bounds of a mesoscale coccolithophore bloom that was in decline 10–20 days before the present study. As little is known about the conditions leading to the onset or collapse of such a bloom, the data presented here are, to our knowledge, the first to quantify phytoplankton dynamics following such an event. In addition, data from the main site provide one of the first assessments of the carbon and nitrogen dynamics within a cyclonic eddy that is typically observed in the northeast Atlantic. The findings at both sites revealed relatively high rates of primary production and concomitant uptake of inorganic nitrogen, suggesting that production rates were not enhanced within the eddy relative to the surrounding waters. These rates were comparable with those noted in late spring in the northeast Atlantic at lower latitudes (47°N, 20°W; 0.7 g C m −2 day −1) and are slightly in excess of modelled estimates of production for the subpolar North Atlantic in midsummer (0.5–0.6 g C m −2 day −1), but less than estimates based on functional photosynthetically active radiation (PAR)-productivity relationships derived from the North Atlantic Bloom Experiment (NABE) (greater than 1.0 g C m −2 day −1). Such functional relationships, derived at lower latitudes, may not be applicable to the subpolar northeast Atlantic. The findings of this study concur with the previous suggestion that the subpolar Atlantic is indeed productive at this time and may deserve further attention from modellers. A comparison of physical, chemical and biological conditions during the coccolithophore bloom and post-bloom periods indicates little change in environment and suggests the role of some self-regulatory mechanism, such as cell self-shading, as the cause of the collapse of the bloom. Invoking such a non-environmental factor may also explain the observed high rates of post-bloom production.

Deep water formation and Quaternary glaciations

A large fraction of climatic varability on the Quaternary time scale can be explained by nonlinear interactions between the radiation balance of the global atmosphere-ocean system and the mass balance of Northern Hemisphere ice sheets. Recent analyses of paleoclimatic proxy data indicate a further important contribution to this variability from changes in deep-water formation occurring in the North Atlantic Subpolar Sea. We study the effects of these changes on variations in global temperature and ice volume characteristic of the late Quaternary. The novel framework of Boolean delay equations (BDEs) is used to formulate a conceptual model of the climatic system under study, and to analyze this formal model. Selfsustained oscillations in the intensity of the Atlantic Ocean's thermohaline circulation result from the interaction of sea-ice formation with the waxing and waning of continental ice sheets. The comparison of model results with paleoclimatic records suggests a considerable slowing down of the abyssal circulation during glacial episodes.

A Diagnostic Study of the Northern Atlantic Subpolar Gyre

Abstract A three-layer vorticity diagnostic calculation is made for an area of the northern North Atlantic centered at 55°N, 22.5°W. It suggests the circulation there is not driven by vertical Ekman suction due to the wind stress, but due to nearly horizontal due nearly the sloping mean density surface in that portion of the upper ocean mixed by deep wintertime convection. The computed act annual heat gain is −44 ± 8 W m−2. Such diagnostics define a linear relation between buoyancy gain and Ekman pumping and hence serve as a link; through the ocean, between meteorologist's charts of wind stress and heat gain. The meridional transport is strongly influenced by bottom topography; the upward slope of the bottom toward the north permits a large net northward transport.