THE NORTH ATLANTIC DRIFT
CURRENT
The North Atlantic Drift Current (NADC) is a slow-moving body of water located between about 50°-64°N and 10°-30°W. NADC is also considered to be an extension of the North Atlantic Current. It is recognized as a shallow, widespread and variable wind-driven surface movement of warm water that covers a large part of the eastern subpolar North Atlantic and slowly spills into the Nordic Seas. It is also sometimes included as the Subarctic or Subpolar Front as it is thought of as the boundary between the cold, subpolar region and the warm, subtropical gyre of the Northeastern Atlantic. Generally, the NADC originates from the Gulf Stream-North Atlantic Current system and from the northern Sargasso Sea. These waters then slowly flow northward into the Labrador and European basins, eventually becoming the NADC as it enters the Iceland Basin. The current is unique in that it transports warm waters to latitudes higher than in any other ocean, thereby producing the moderate climate of Europe and western Scandinavia. Because of the rapid advection of the North Atlantic gyre, the temperature of the surface waters of the NADC almost always exceeds that of both surrounding waters and the overlying atmosphere. Water temperatures in March are at around 8degC in the NADC while ranging from 2°C to 6°C in surrounding waters.
Gulf Stream, North Atlantic Drift ocean currents
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. 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.
The NADC is generally a slow-moving body of water that transports between 2 - 5
Sv, although other authors have estimated volumes of 16 Sv, 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 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 cm
2s-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.
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. 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.
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, 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.
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. 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. 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. 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. 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. Movement of the NADC because of anomalous
winds, will cause a significant latitudinal alteration in the climate zones of
western Europe.
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. 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. 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.
Note: The symbol Sv is a unit of volume transport used in describing ocean currents. One sverdrup is 1,000,000 cubic meters per second. It is used almost exclusively in oceanography.
