Michael S. McCartney

On the North Atlantic circulation

A summary for North Atlantic circulation is proposed to replace the circulation scheme hypothesized by Worthington in 1976. Divergences from the previous model are in thermohaline circulation, cross-equatorical transport and Florida Current sources, flow in the eastern Atlantic, circulation in the Newfoundland Basin, slope water currents, and flow pattern near the Bahamas. The circulation patterns presented here are consistent with the majority of of published accounts of flow components. 77 refs., 14 figs., 3 tabs.

Distribution and Circulation of Labrador Sea Water

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 ...

The Subpolar Mode Water of the North Atlantic Ocean

The invention concerns a method of producing intense beams of polarized free electrons in which a semiconductor with a spin orbit split valence band and negative electron affinity is used as a photocathode and irradiated with circularly polarized light.

Oceanic transport of subpolar climate signals to mid-depth subtropical waters

The spatial distributions of certain sea-surface properties, such as temperature, fluctuate on timescales from months to decades and in synchrony with the main regional atmospheric patterns comprising the global climate system. Although it has long been assumed that the ocean is submissive to the dictates of the atmosphere, recent studies raise the possibility of an assertive, not merely passive, oceanic role in which water-mass circulation controls the timescales of climate fluctuations. Previously held notions of the immutability of the physical and chemical characteristics of deep water masses are changing as longer time series of ocean measurements indicate that the signatures of varying sea-surface conditions are translated to deep waters,. Here we use such time-series measurements to track signals ‘imprinted’ at the sea surface in the North Atlantic Oceans subpolar Labrador Basin into the deep water of the subtropical basins near Bermuda, and infer an approximately 6-year transit time. We establish a geographic and temporal context for a portion of the long-term warming trend reported for mid-depth subtropical waters over the past 40 or so years,, and we predict that waters at these depths will continue to cool well into the next decade.

Ocean Gyre Circulation Changes Associated with the North Atlantic Oscillation

Abstract Observational evidence is presented for interannual to interdecadal variability in the intensity of the North Atlantic gyre circulation related to the atmospheric North Atlantic Oscillation (NAO) patterns. A two-point baroclinic pressure difference between the subtropical and subpolar gyre centers—an oceanic analogue to the much-used sea level pressure (SLP)-based atmospheric NAO indices—is constructed from time series of potential energy anomaly (PEA) derived from hydrographic measurements in the Labrador Basin and at Station S near Bermuda. Representing the upper 2000-db eastward baroclinic mass transport between the two centers, the transport index indicates a Gulf Stream and North Atlantic Current that gradually weakened during the low NAO period of the 1960s and then intensified in the subsequent 25 years of persistently high NAO to a record peak in the 1990s. The peak-to-peak amplitude difference was 15–20 megatons per second (MT s−1) with a 43-yr mean of about 60 MT s−1 a change of 25%–33%...

Recirculating components to the deep boundary current of the northern North Atlantic

Abstract The meridional overturning system of the North Atlantic is conventionally thought of as transporting warm water (≡ ≥4°C) to high latitudes and covd water to low latitudes. The northern cold water sources for this system are dense overflow from the Nordic Seas and less dense water from the labrador Sea (LSW). The overflow components are carried westward by a deep northern boundary current (DNBC) that is shown here actually to begin southeast of the overflow regions. The DNBC lies along the southern side of a system of islands and submarine ridges that divides the Nordic Seas from the subpolar basins. The overflows act to increase the transport of the DNBC, and this transport is further increased through the entrainment of warm waters. Once inside the Labrador Sea, the DNBC is joined by LSW before continuing south to low latitudes as a cold, deep western boundary current (DWBC). However, published transport estimates are larger than can be explained by warm water entrainment alone, thus indicating entrainment also of recirculating cold components, the subject of this paper. Their sources are shown to be the LSW and cold abyssal waters originating from the southern hemisphere. The LSW is entrained downward into the denser part of the DNBC and laterally into the upper part of the DWBC, whereas the cold abyssal waters are supplied by an eastern-intensified northward flow in the eastern Atlantic (that serves as the initial source of the DNBC) and by a similar northward flow in the western Atlantic. The meridional overturning system described includes recirculations and poleward transport of cold water in addition tothe components described by the conventional system. The cold abyssal waters in both the eastern and western basins have relatively low levels of oxygen and high concentrations of silicate reflecting their southern origin, but their influence in the northwestern Atlantic is somewhat obscurred by strong recirculating cyclonic gyres in the Newfoundland and Labrador Basins. This influence is detected by a deep silicate maximum extending poleward from the mid-latitude western Atlantic through the two gyres with eastern concentration and recirculating back (diluted) toward the south in the DWBC within the respective basins. Such a maximum also extends northward through the Irminger Basin into the DNBC, but there is an ambiguity as to whether this is a direct extension of the signal from the south in the western basin, or if it comes from the mid-latitude eastern basin by way of westward flow through the Charlie-Gibbs Fracture Zone; it is perhaps consequence of both. Estimates of the volume transports of the various recirculating cold components indicate that they supply about as much water to the deep boundary currents as do the combined cold water sources in the north.

Warm-to-Cold Water Conversion in the Northern North Atlantic Ocean

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 productio...

An eastern Atlantic section from Iceland southward across the equator

A long CTD/hydrographic section with closely-spaced stations was occupied In July- August 1988 from Iceland southward to 3°S along a nominal longitude of 20°W The section extends from the surface down to the bottom, and spans the entire mid-ocean circulation regime of the North Atlantic from the subpolar gyre through the subtropical gyre and the equatorial currents Vertical sections of potential temperature, sahnlty and potential density from CTD measurements and of oxygen, silica, phosphate and nitrate, based on d~screte water-sample measurements are presented and discussed in the context of the large-scale circulation of the North Atlantic Ocean The close spacing of hlgh-quahty stations reveals some leatures not described previously The more important findings include ( 1 ) possible reclrculation of the hghtest Subpolar Mode Water into the tropics, (2) a thermostad at temperatures of 8-9°C, lying below that of the Equatorial 13°C Water, (3) the nutrient distribution in the low-sallmty water above the Mediterranean Outflow Water that supports the previous conjecture of northern influence ot the Antarctic Intermedmte Water. 14) a great deal of lateral structure of the Mediterranean Outflow Water, with a number of lobes of high sahnlt), (5) an abrupt southern boundary ot the Labrador Sea Water at the Azores-Biscay Rise and a vertically well-mixed region to its south, (6) a sharp demarcation in the central Iceland Basra between the newest Iceland-Scotland Overflow Water and older bottom water, which has a significant component of southern water, (7) evidence that the Northeast Atlantic Deep Water is a mixture ot the Mediterranean Outflow Water and the Northwest Atlantic Bottom Water with very httle input from the Iceland-Scotland Overflow Water, (8) an usolated core of the high-salinity, low-silica Upper North Atlantic Deep Water at the equator, (9) a core of the high-oxygen, low- nutrient Lower North Atlannc Deep Water pressed against the southern flank of the Mld-Atlanuc Ridge just south of the equator, (10) a weak minimum of salinity, and well-defined maxima of nutrients associated with the oxygen minimum that separates the Middle and Lower North Atlantic Deep Waters south ot the equator, (11) a large body ol nearly homogeneous water beneath the Middle North Atlantic Deep Water between 2(1°N and the Azores-Biscay Rise, and (i 2) a deep westward boundary undercurrent on the southern slope of the Rockall Plateau

Water-mass distributions in the western South Atlantic; A section from South Georgia Island (54s) northward across the equator

A long CTD/hydrographic section with closely spaced stations was made in February-April 1989 in the western Atlantic Ocean between 0”40’N and South Georgia (545) along a nominal longitude of 25W. Vertical sections of various properties from CTD and discrete water-sample measurements are presented and discussed in terms of the large-scale circulation of the South Atlantic Ocean. One of the most important results is the identification of various deep-reaching fronts in relation to the large-scale circulation and the distribution of mode waters. Five major fronts are clearly defined in the thermal and salinity fields. These are the Polar (49SS), Subantarctic (45S), Subtropical (41-425), Brazil Current (355) Fronts, and an additional front at 20-22s. The first three are associated with strong baroclinic shear. The Brazil Current Front is a boundary between the denser and lighter types of the Subantarctic Mode Water (SAMW), and the 20-223 front marks the boundary between the anticyclonic subtropical and cyclonic subequatorial gyres. The latter front coincides with the northern terminus of the high-oxygen tongue of the Antarctic Intermediate Water (AAIW) and also with the abrupt shift in density of the high-silica tongue originating in the Upper Circumpolar Water and extending northward. Two pycnostads with temperatures 20-24°C are observed between 10s and 2% with the denser one in the subtropical and the other lighter one in the subequatorial gyre. A weak thermostad centered at 4°C occurs in the AAIW between the Subtropical Front and the Subantarctic Front and shows characteristics similar to the densest variety of the SAMW. Another significant result is a detailed description of the complex structure of the deep and bottom waters. The North Atlantic Deep Water (NADW) north of 25s contains two vertical maxima of oxygen (at 2000 m and 3700 m near the equator) separated by intervening low-oxygen water with more influence from the Circumpolar Water. Each maximum is associated with a maximum of salinity and minima of nutrients. The deeper salinity maximum is only weakly defined and is limited to north of 18S, appearing more as vertically uniform salinity. South of 25s the NADW shows only a single maximum of salinity, a single maximum of oxygen, and a single minimum of each nutrient, all lying close together. The salinity maximum south of 25s and the deeper oxygen/salinity maximum north of 11s are derived from the same source waters. The less dense NADW containing the shallower extrema of characteristics turns to the east at lower latitudes and does not reach the region south of 25s. The

Eastward Flow through the Mid-Atlantic Ridge at 11°N and Its Influence on the Abyss of the Eastern Basin

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 Vemas 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 transp...