Under the aspect of dynamics, i.e. that of large-scale ocean circulation,
the hydrographic sections are located in the divergence zone of the (wind-driven)
subtropical gyre and of the (thermohaline-driven) subpolar gyre. This area
may also be interpreted as advection path of the North Atlantic current,
the northeastward extension of the Gulf Stream. The current system represents
the upper branch of the Meridional Overturning
Circulation (MOC) in the North Atlantic with its net advection of
warm, subtropical water above 4°C flowing in northeastern direction
into the Norwegian and Greenland seas, and thus toward Northern Europe.
On the water surface along the advection path, heat is emitted continuously
to the atmosphere, a process accounting for the moderate climate in Western
Europe. Climate relevant changes in the ocean are generally associated with
two different mechanisms, which also imply different timescales:
1) Changes depending on the depth of the isopycnals. Such changes may occur
on timescales of several years (which is rather short for the ocean) and
also propagate quickly into deeper ocean areas.
2) Changes in the thermohaline structure along isopycnals, i.e. changes in
the characteristics of water masses. Through advection and mixing, such changes
spread into the ocean depths on timescales of decades.
The biggest temporal changes in the thermohaline structure between the English
Channel and the Grand Banks of Newfoundland have been observed in the deep
western boundary current as well as west and east of the middle Atlantic
ridge. During the investigations carried out so far, it was surprisingly
found that significant changes have taken place in the intermediate and deep
water, which were observed most clearly in the Labrador Sea
Water (LSW) extending at a depth of 1300 to 2200 meters. Between 1957
and 1982, a warming trend and a salinity decrease along Isobars were observed
in the LSW, while in the following decade (1982-1994) cooling took
place while the salinity decreased further. Interrupted only by a short warming
period between 1996 and 1997, the cooling trend has continued since then.
In the period from 1994 to 1997, however, the salinity remained nearly constant.
In 1998, salinity decreased again. During the recent realisation is the eastward
extension and deepening of the cold and fresh Labrador current most
remarkable. In the deep water (water below LSW), cooling and a continuing
salinity decrease along isobars was observed throughout the period from 1957
Changes in ocean dynamics are reflected in changed transport rates of volumes,
heat and freshwater. The observed temporal variability of the meridional
overturning rate - the volume transport of the upper MOC branch - ranges
from 13 to 20 Sv, with minima in 1957 and 1997, and maxima in 1982 and 1996.
Transports of LSW vary between 3 and 8 Sv, those of deep water between 10
and 17 Sv. From 1957 to 1993, transports of the MOC and the deep water showed
a similar pattern while that of LSW transports was the opposite. Between
1996 and 1998, all transports showed a similar behaviour. Maximum values
were reached in 1996, and minimum values just one year later, in 1997.
Investigations into changed transport rates on climate-relevant decadal
timescales have led to the conclusion that the meridional circulation in
the North Atlantic has a bimodal structure, varying between the two modes.
The variations are probably caused by changed forcing fields at the water
surface (temperature and freshwater anomalies) in the areas where the deep
water has its origin. However, analyses of the dataset from the 90s also
show the quick reaction of the oceanic current field to fluctuations of the
wind field above the North Atlantic, the North Atlantic
Oscillation (NAO). To obtain better statistical data on possible physical
forcing mechanisms, also satellite remotely sensed data and XBT
(eXpandable BathyThermograph) data from the BSH's
ship-of-opportunity programme, continued since 1988, are used.
of the North Atlantic Workshop, August 1999: Interannual and Climate Variability
of Volume, Heat and Freshwater Transport Estimates at "48°N" in the
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