Recirculating components to the deep boundary current of the northern North Atlantic
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The meridional overturning system of the North Atlantic is conventionally thought of as transporting warm water (theta greater-than-or-equal-to 4-degrees-C) to high latitudes and cold water to low latitudes. The northern cold water sources for this system are dense overflows 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 asa 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 transports of cold water in addition to the 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 obscured 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 a 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.