NATURE ADRIFT 



formed when the ice melts in the summer and such conditions are often 

 helped in winter by snow, or where there is excess cold and ice crystals 

 can form before the water has time to sink. Shallow waters can freeze more 

 easily as there is no mass of deep water below to absorb and warm the cold 

 water as it sinks. 



Really cold weather at the poles forms more and more ice, and there is 

 less circulation than before. This behaviour of salt water on cooling also 

 means that the coldest water in the deepest part of the sea is always nicely 

 above its freezing point with no seasonal variation. 



Both temperature and the amount of salt affect the density of the sea and 

 dense water always tends to sink and to be replaced by inflow of less dense 

 water whether the density is due to coldness, to salt content or to both, so 

 that the current systems are more complex than if caused by temperature 

 alone. Wind also has its effect. It blows surface water in front of it and water 

 from sub-surface or deeper layers must upwell to replace it. This may be 

 on a small local scale or the vast scale of the whole ocean system where 

 the currents are thus due to a complex combination of winds, convection 

 currents caused by temperature changes, and the rotation of the earth itself. 



Relative to the size of the oceans these currents are slow moving, though 

 they transport immense volumes of water. The Gulf Stream, east of Florida, 

 sweeps across the Atlantic spreading out and slowing down in the process, 

 so that although at first it is a narrow stream fast enough to be a nuisance to 

 shipping making headway against it, it is hardly noticeable as a current a 

 hundred miles across by the time it reaches Europe. The crossing takes about 

 three years, and at such a speed it seems very slow compared, for example, 

 with the mighty roar of Niagara, which copes with 200,000 cubic feet of 

 water a second. Nevertheless it would take Niagara three years to deal with 

 the amount of water carried by the Gulf Stream in a single day. Only part 

 of this Gulf Stream reaches the northern part of Europe as most of it con- 

 tinues in its circular motion towards the African coast and south to the 

 Equator again. The northern part, called the North Atlantic Drift sends its 

 main branch to Europe affecting the area from the Bay of Biscay northwards 

 west of the British Isles, round the north of Scotland, between the Faroe 

 Islands and Shetland, and so towards Spitsbergen (Fig. 28). A minor branch 

 of this turns into the North Sea. Another continues northwards well to the 

 west of the British Isles towards the south and west of Iceland. The topo- 

 graphy of the bottom has its effect, submarine ridges in particular causing 

 big changes in what might otherwise be a fairly simple system. Of these 

 the most important are the Iceland-Faroe Ridge and the Wyville Thomson 

 Ridge from Faroe Bank to the Hebrides, which are approximately only 

 500 metres in depth. The North Atlantic Drift crosses the Wyville Thomson 



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