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The North Atlantic drift as represented by the Mariano Global Surface Velocity Analysis (MGSVA). The N. Atlantic drift is the broad, northward flow of surface waters that replaces the sinking waters in the N. Atlantic polar seas. | |
Several authors define the region that is occupied by the lethargic NADC as bounded by the cool Irminger current to its west, while its southern and eastern border is weakly constrained by the extension of the North Atlantic Current that bends northeastward, flanking the western edge of the Rockall Plateau. Eventually, the NADC feeds into the Norwegian Current to the north, past Iceland. The loop of the East Iceland Current acts as a boundary along the northern extent of the NADC (Rossby, 1996). Mostly warm surface water trapped between cooler, faster flowing subpolar currents to its east, and the Rockall Plateau to its west, the NADC in the Iceland Basin can extend to a depth of about 1000 meters (Bower et. al., 2002).
The NADC is generally a slow-moving body of water that transports between 2 - 5 Sv (Krauss, 1986) Although other authors have estimated volumes of 16 Sv (Bacon, 1997), the exact values remain unknown. Unlike the currents that surround it, the flow of the NADC current is obscure, evident mainly through decadal-scale observations of drifter data. Its northward speed averages about 3 cm s-1 (Otto & van Aken, 1996) along the western edge of the Rockall Plateau.
According to Krauss and Käse (1984), the North Atlantic Current, and not interference with the Mid-Atlantic Ridge, is the main source of eddy energy in the North Atlantic. In the upper ocean, eddy kinetic energy decreases from about 1000 cm2s-2 (near Newfoundland) to about 300 cm2s-2 in the NAD near western Scotland. East of the Mid-Atlantic Ridge, kinetic energy tapers off to less than 100 cm2s-2 in the form of a homogenous pool of low-kinetic energy (Krauss and Käse, 1984).
As far as it is known, the NADC exists as a "swath" or region, rather than an actual stream-like current, where the main thermocline shoals to the surface along which a stronger baroclinic transport is sustained than either the north or the south. A quiet, warm pool of water at the surface, its salinity can range between 35.2 and 35.7 ppt (Rossby et. al., 1998). The NADC feeds two well-defined currents, transporting warm, saline water to both the northward-flowing Norwegian Current and to the southward-flowing Canary Current (Rossby, 1996).
The southern-most extent of the NADC is marked by the northernmost boundary of the subtropical circulation system, i.e. the eastward flowing NAC along ~52°N. The zonal flow across the Mid-Atlantic Ridge seem to occur in the form of various branches (Krauss, 1996), some of which turn northwards into the subpolar region, feeding the thermohaline circulation; and others turning south and entering the recirculation of the wind-driven subtropical gyre (Rossby et. al., 1998).
Veron, et. al. (1999) determined that spatial gradients in the ratios of 206Pb/207Pb are consistent with thermohaline circulation of the different water masses in the North Atlantic, each having relatively discrete lead isotope signatures. Based on parallels between the initial isotopic data and temperature and salinity measurements, these authors proposed that stable lead isotope compositions may be employed as complimentary tracers of the mixing of source waters in the Nordic seas, particularly the NADC waters and flanking currents (Veron, 1999). The isotopic ratios and a salinity maximum (>35.2) measured in the Faroe Bank Channel, indicates a core of NADC between 130 and 430 m, and that this water mixes with cooler, fresher deeper water to form the Iceland-Scotland Overflow Waters.
Although the more distinct properties of the NADC waters themselves are poorly defined in the literature, the influences this current has on climate are well-documented. It is the contribution of warm waters from the North Atlantic Drift current that is now understood to be the main moderating force of the climate over western Scandinavia, the UK and western Europe (Bigg, 1996; Johnson, 1997; Little et. al., 1997; Moron et. al., 1998; Giraudeau et. al., 2000). And, based on diatom records, the NADC is thought to have been established as early as 13,400 years ago, although with periods of decadal-scale variations of heating and cooling (Koc et. al., 1993). The path of the NADC is clearly seen in the warming of the air over the western North Atlantic extending eastwards-and intensifying-into the Norwegian Sea (Rossby, 1996). Blindheim et. al. (2000) have shown that a positive link exists between the NADC penetration into the Norwegian Sea and the North Atlantic Ocean (NAO) index, defined as the difference in sea-level pressure between two stations close to the low over Iceland and the high over the Azores. High NAO indices imply that only a narrow flow extends northward of the Faeroe-Iceland Strait, resulting in a sea-surface temperature (SST) cooling at the scale of the Greenland and Norwegian basins, owing to the spread of polar waters eastward. During these conditions, flow is simultaneously intensified in the narrow band along the Norwegian shelf, northwards towards Svarlbad. The strong westerlies caused by the high winter/spring indices bring in warm, moist air over the European continent and leads to a rather mild, maritime winter. Conversely, a low winter/spring index reflects weaker mean westerlies over the NAO, which in turn corresponds to colder European winters (Eynaud, et. al., 2002). Movement of the NADC because of anomalous winds, will cause a significant latitudinal alteration in the climate zones of western Europe (Bigg 1996).
So important, in fact, is the transport of warm water to this region, that a decadal-scale shift in the flow of the NADC can initiate an ice age (Johnson, 1997). A study was conducted by Moron et al. (1998) aimed at giving a global description of climactic phenomena that exhibited some regularity during the twentieth century. They first analyzed multi-channel singular spectrum data, which was then used to extract long-term trends and quasi-regular oscillations of global SST fields since 1901. From 1910 to about 1940, the authors observed a general warming trend with a short cooling before another brief warming trend. Substantial cooling occurred in the North Atlantic, from about 1950-1980, and continues today. Overall, a 13-15 year see-saw pattern oscillation between the Gulf Stream and the NADC was observed, and also found to affect the tropical Atlantic (Moron et. al., 1998). At times of increased trade wind strength, tropical and subtropical waters are forced across the equator, enhancing the pool of warm water to be transferred to the high latitudes of the North Atlantic via the Gulf Stream and North Atlantic Drift, thereby increasing the pull of the thermohaline convective conveyor. The increased supply of warm water to the polar regions of the northern hemisphere increases the ice-ocean moisture gradient and can accelerate ice sheet growth (Little et. al., 1997).