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M. S. McCartney

Abstract

Property distributions and geostrophic shear from a hydrographic section near 37°W in the Atlantic Ocean show the deep western boundary current (DWBC) in the North Atlantic Deep Water (NADW) established against the western boundary of the Brazil Basin immediately south of the equator (between 2° and 5°S). The DWBC thus has directly crossed the equator to the South Atlantic following the east-southeast trend of the continental slope isobaths. The estimated DWBC transport of NADW is 35 × 106 m3 s−1, similar to other estimates from the tropics discussed here. These large DWBC transports are opposed by flow of deep water to the North Atlantic immediately offshore of the DWBC, with as much as two-thirds of the DWBC transport being returned as these recirculations. One recirculation center is the Guiana Basin north of the equator but extends at least a few hundred kilometers south of the equator; another is visible at 11°S in the Brazil Basin. The degree of connection of these two observed recirculations is not established. These recirculations spread the northern source influences over the width of the recirculation (rather than the DWBC width) and efficiently dilute the northern source concentration with South Atlantic influences, with the self-mixing of the recirculation complicating the interpretation of tracer distributions. A further complication occurs for the uppermost levels of the NADW, for the DWBC flows to the Southern Hemisphere beneath an opposing western boundary current of Antarctic Intermediate Water (AAIW), and downgradient property fluxes mutually erode the upper NADW and the AAIW core characteristics. This causes a displacement of the axis of maximum northern source concentration offshore front the axis of maximum transport of upper NADW in the DWBC, a demonstration that the relationship between a tracer tongue and the flow field can be obscure.

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M. S. McCartney and R. A. Curry

Abstract

In its general northward flow along the western trough of the Atlantic, Antarctic Bottom Water (AABW) must pass over several sills separating the various abyssal basins. At the equator, the western trough is deformed by major east-west offsets of the Mid-Atlantic Ridge and the continental margin of Brazil, forming a newly zonal abyssal channel about 250 km wide, centered at the equator, and extending approximately 1000 km along its axis, in which the AABW is confined. Thus, the general northward flow of AABW is topographically constrained to be westward as it crosses the equator. A hydrographic section across this channel at 37°W shows the AABW isopycnals to be “bowl” shaped within and beneath the level of the channel walls. The equatorial geostrophic relation permits us to compute a zonal velocity from the well-defined parabolic distribution of dynamic height, relative to a reference level at the transition between AARW and the overlying deep water. Here 4.3 × 106 m3 s−1 is estimated for the westward–and ultimately northward–transport of AABW. Although this value exceeds previous estimates of net northward transport in the Brazil and Guiana basins made from International Geophysical Year data of the late 1950s, it fits well into an overall scenario constructed from transport estimates made from section data collected during the 1980s. This scenario includes a flow of approximately 7 × 106 m3 s−1 of AABW into the Brazil Basin from the south. The magnitude of the northward flow diminishes as it moves toward the equator indicated by estimates of 6.7 × 106 m3 s−1 at 23°S and 5.5 × 106 m3 s−1 at 11°S. At the equator, 4.3 × 106 m3 s−1 exits the Brazil Basin to continue northward across the Guiana Basin, and an unquantified amount flows through the Romanche Fracture Zone into the eastern basin. The northward decrease in AABW suggests an upwelling across isotherms. The difference in transports between 11°S and the equator, 1.2 × 106 m3 s−1, is an estimate of the combined amounts of AABW being upwelled and exiting the basin through the Romanche Fracture Zone. In the Guiana Basin at 4°N, AABW transport is estimated at 4.0 × 106 m3 s−1. This flow subsequently splits into two approximately equal flows: continued northward flow through the Guiana Basin, and eastward flow through the Vema Fracture Zone at 11°N to the eastern basin.

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L. D. Talley and M. S. McCartney

Abstract

Labrador Sea Water is the final product of the cyclonic circulation of Subpolar Mode Water in the open northern North Atlantic (McCartney and Talley, 1982). The temperature and salinity of the convectively formed Subpolar Mode Water decrease from 14.7°C, 36.08‰ to 3.4°C, 34.88‰ on account of the cumulative effects of excess precipitation and cooling. The coldest Mode Water is Labrador Sea Water, which spreads at mid-depths and is found throughout the North Atlantic Ocean north of 40°N and along its western boundary to 18°N.

A vertical minimum in potential vorticity is used as the primary tracer for Labrador Sea Water. Labrador Sea Water is advected in three main directions out of the Labrador Sea: 1) northeastward into the Irminger Sea, 2) southeastward across the Atlantic beneath the North Atlantic current, and 3) southward past Newfoundland with the Labrador Current and thence westward into the Slope Water region, crossing under the Gulf Stream off Cape Hatteras.

The Labrador Sea Water core is nearly coincident with an isopycnal which also intersects the lower part of the Mediterranean Water, whose high salinity and high potential vorticity balance the low salinity and low potential vorticity of the Labrador Sea Water. Nearly isopycnal mixing between them produces the upper part of the North Atlantic Deep Water.

A 27-year data set from the Labrador Sea at Ocean Weather Station Bravo shows decade-long changes in the temperature, salinity, density and formation rate of Labrador Sea Water, indicating that Labrador Sea Water property distributions away from the Labrador Sea are in part due to changes in the source.

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M. S. McCartney and L. D. Talley

Abstract

A box Model of warm-to-cold-water conversion in the northern North Atlantic is developed and used to estimate conversion rates, given water mass temperatures, conversion paths and rate of air-sea heat exchange. The northern North Atlantic is modeled by three boxes, each required to satisfy heat and mass balance statements. The boxes represent the Norwegian Sea, and a two-layer representation of the open subpolar North Atlantic. In the Norwegian Sea box, warm water enters from the south, is cooled in the cyclonic gyre of the Norwegian–Greenland Sea, and the colder water returns southwards to the open subpolar North Atlantic. Some exchange with the North Polar Sea also is included. The open subpolar North Atlantic has two boxes. In the abyssal box, the dense overflows from the Norwegian Sea flow south, entraining warm water from the upper-ocean box. In the upper-ocean box, warm water enters from the south, supplying the warm water for an upper ocean cyclonic circulation that culminates in production by convection of Labrador Sea Water, and also the warm water that is entrained into the abyss, and the warm water that continues north into the Norwegian Sea. Our estimates are that 14 × 106 m3 s−1 of warm (11.5°C) water flows north to the west of Ireland, with about a third of this branching into the Norwegian Sea. The production rate for Labrador Sea Water is 8.5 × 106 m3 s−1), and this combines with a flow of dense Norwegian Sea Overflow waters (with entrained warmer waters) at 2.5 × 106 m3 s−1 to give a Deep Western Boundary Current of 11 × 106 m3 s−1. The total southward flow east of Newfoundland is this plus 4 × 106 m3 s−1 of cold less dense Labrador Current waters (there is a net southward flow between Newfoundland and Ireland of about 1 × 106 m3 s−1 supplied by northward flow through the Bering Strait, passing through the North Polar Sea to enter the Norwegian Sea.

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M. S. McCartney, S. L. Bennett, and M. E. Woodgate-Jones

Abstract

The dilute Antarctic Bottom Water of the North Atlantic eastern trough is supplied from the western trough through fractures in the Mid-Atlantic Ridge. In particular, the influence on eastern trough property distributions of flow through the Romanche and Vema fracture zones, near the equator and 11°N, respectively, has been noted previously.

Here, new observations are reported that document the abyssal circulation of the northeastern Atlantic basins (Gambia Abyssal Plain, South Canary Basin, and North Canary Basin) in particular, the dominance of Vema influence, the absence of Romanche influence, and the existence of a system of deep western boundary currents and estimated transport.

Deep isopycnals slope steeply across the Vema's eastern end near 39°W, corresponding to a geostrophic transport through the Vema of 2.1 to 2.3 (×106 m3s−1) colder than 2.0°C. This is half or more of the estimated Bottom Water that flows north across the equator into the subtropical western North Atlantic.

This transport in the Vema debouches into the Gambia Abyssal Plain. A deep western boundary current with 1.3 to 3.0 (×106m3s−1) transport colder than 2.0°C flows eastward. This current subsequently bifurcates into 1) a nearly zonal eastward current (≤1.0 × 106m3s−1 transport) along the plain's southern boundary, the Sierra Leone Rise, and 2) a northward western boundary current along the flank of the Mid-Atlantic Ridge 1.8 to 3.9 × 106m3s−1).

Property and shear fields indicate that, for water colder than 2.0°C, basically none of the eastward flow along the rise passes southward through its deepest passage, the Kane Gap, nor does Romanche-derived water flow north there. The poleward Mid-Atlantic Ridge flank flow in the plain continues northward across the Cape Verde Ridge into the South Canary Basin and from there poleward into the North Canary Basin.

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M. Susan Lozier, Michael S. McCartney, and W. Brechner Owens

Abstract

A comparison of a recently assembled hydrographic database for the North Atlantic with the Lovitus atlas shows striking differences in the vicinity of the Gulf Stream and the North Atlantic Current. On isopycnal surfaces in the main thermocline, isolated pools of warm, saline water are found in the Levitus database but are absent in the new database. Using synoptic data as a proxy for temporally averaged climatological data, it is shown that the anomalous features can be accounted for by the differences in the averaging process. To produce a gridded database from irregularly spaced station data, Levitus averaged the data on pressure surfaces while the new database was prepared with averaging an potential density surfaces. It is shown that averaging on a pressure surface in an area of sharply sloping isopycnals produces a water mass with a θ–S signature uncharacteristic of the local water mass(es). The anomalous potential temperatures and salinities that result are compared to the large-scale water mass anomalies of the North Atlantic and are shown to be of comparable strength. Finally, the consequences of having sizable averaging artifacts are discussed.

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J. W. Hurrell, M. Visbeck, A. Busalacchi, R. A. Clarke, T. L. Delworth, R. R. Dickson, W. E. Johns, K. P. Koltermann, Y. Kushnir, D. Marshall, C. Mauritzen, M. S. McCartney, A. Piola, C. Reason, G. Reverdin, F. Schott, R. Sutton, I. Wainer, and D. Wright

Abstract

Three interrelated climate phenomena are at the center of the Climate Variability and Predictability (CLIVAR) Atlantic research: tropical Atlantic variability (TAV), the North Atlantic Oscillation (NAO), and the Atlantic meridional overturning circulation (MOC). These phenomena produce a myriad of impacts on society and the environment on seasonal, interannual, and longer time scales through variability manifest as coherent fluctuations in ocean and land temperature, rainfall, and extreme events. Improved understanding of this variability is essential for assessing the likely range of future climate fluctuations and the extent to which they may be predictable, as well as understanding the potential impact of human-induced climate change. CLIVAR is addressing these issues through prioritized and integrated plans for short-term and sustained observations, basin-scale reanalysis, and modeling and theoretical investigations of the coupled Atlantic climate system and its links to remote regions. In this paper, a brief review of the state of understanding of Atlantic climate variability and achievements to date is provided. Considerable discussion is given to future challenges related to building and sustaining observing systems, developing synthesis strategies to support understanding and attribution of observed change, understanding sources of predictability, and developing prediction systems in order to meet the scientific objectives of the CLIVAR Atlantic program.

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