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Meridional transports in the North Atlantic

   
 
 

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 to 1998.

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.

pdf Poster of the North Atlantic Workshop, August 1999: Interannual and Climate Variability of Volume, Heat and Freshwater Transport Estimates at "48°N" in the North Atlantic

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