The Three-dimensional Temperature Distribution and its Variation in Time 151 



The distance to which each of these streams extend in each meridionally-oriented 

 trough is very largely dependent : 



(1) on the morphological form of the trough, on whether there are deep passages 

 through cross-ridges or whether the stream can flow over any rises, and 



(2) on the kind of water mass spreading above the cold bottom water towards the 

 equator. It combines and interchanges with this and shows much stronger conserva- 

 tism in its character of Antarctic water the lesser the influence of the water above. 

 The most extended is the central Pacific cold stream which, due to the favourable 

 topography and partly also because of the absence of deep warm currents in the North 

 Pacific, reaches as far as 25° N. Also, in the Indian Ocean, the cold-water currents 

 on both sides of the central ridge extend almost to the northern limit of the ocean. 

 The most impressive one of these streams is, however, the west Atlantic cold water 

 spreading where the Antarctic water penetrates through gaps from deep-sea basin to 

 deep-sea basin as far as the Para Rise at 8° N., and finally warms up by mixing with 

 the relatively warm North Atlantic deep water and flows into the North American 

 Basin. In the East Atlantic Trough the Whalefish Ridge completely prevents further 

 extension north and there is therefore a large difference in the temperature of the bot- 

 tom water on the north and south sides of this cross-barrier. The bottom layers of 

 the Atlantic Eastern Trough north of the Whalefish Ridge are formed by colder West 

 Atlantic bottom water flowing in through deep gaps in the central parts of the Middle 

 Atlantic Ridge at 0° latitude (Romanche Deep) and at 10° N. There are cross-rises 

 also in the eastern and western deep-sea troughs of the Pacific that prevent the north- 

 ward extension of Antarctic water beyond 22° and 37° S., respectively. 



There is very little bottom water of Arctic origin. The most productive source is 

 probably the outflow from the Okhotsk Sea which extends southwards as a cold 

 stream, with an initial temperature of less than 0-6 °C about 15° N. In the Atlantic 

 deep-sea troughs there are indications of bottom water at less than 1-8° between 53° 

 and 45° N. which is probably of subarctic origin. 



A detailed investigation of the horizontal spreading of the Antarctic bottom water 

 in the Atlantic has been made by WiJST (1936). Figure 63 shows the potential tempera- 

 ture along a quasi-meridional section through the Western and Eastern Troughs below 

 3000 m. In the western section the bottom water is separated from the water mass 

 above by a marked discontinuity in the vertical temperature (and salinity) distribution. 

 It descends from south to north with a gradient of about 20 m in 100 km and follows 

 the bottom topography closely. Such influences on the temperature (and salinity) 

 are recognized as far north as 40° N., 16,500 km away from the origin of the stream 

 at the rim of the South Polar Basin. 



The eastern quasi-meridional section is rather different. The barrier due to the 

 Whalefish Ridge shows even more prominently here and the eff"ect of the local inflow 

 of Antarctic- West Atlantic water through the Romanche Trench is also clearly visible. 

 From here and from the saddle at about 10° N. the bottom water spreads north and 

 south in the eastern Atlantic Basin. The increase in temperature and salinity along 

 the core of spreading of the relatively shallow bottom water is due to mixing processes 

 with the warmer North Atlantic deep water above, comparatively of larger vertical 

 extent. The distribution of temperature and salinity in the bottom water can be re- 

 garded as stationary and this can only happen when advection and mixing are in 



