66 Hydrography of the Stream 



SOURCES OF THE GULF STREAM WATER MASSES 



As was explained in Chapter IV, the water which flows out of the Florida 

 Straits originally comes in large part from the southern half of the North 

 Equatorial Current, and in part from a branch of the South Equatorial 

 Current which is apparently spht off from the South Atlantic by the 

 peculiarly wedgelike coastline of Brazil. This water flows through the 

 Caribbean, and then, without mixing with the waters endemic to the Gulf 

 of Mexico, emerges from the Florida Straits in very much the same state 

 as when it entered the Caribbean. Because of the large admixture of 

 Antarctic Intermediate Water at mid-depths which this water has acquired 

 from the South Atlantic, there is a distinct salinity minimum (between 600 

 and 800 m. depth) in the water coming out of the Florida Straits. This 

 salinity minimum is not so marked in the water flowing westward north of 

 the West Indies. Therefore, the difference in the intensity of the salinity 

 minimum serves as an indicator, or tracer, for distinguishing A\'ater in the 

 Stream which has come through the Caribbean from that which has joined 

 the Stream north of the Indies. Iselin (1936) has used the intensity of the 

 saHnity minimum in this way to trace the source regions of various 

 portions of the Gulf Stream in various areas of the North Atlantic. He has 

 plotted charts of salinity anomalies m hich give a rough indication of the 

 sources and degree of mixing of mid-depth waters. 



In a detailed analysis of a Gulf Stream section made in April, 1932, off 

 Chesapeake Bay, Iselin (1936) shows that almost all the water colder than 

 8° C. must have joined the Stream north of the Florida Straits, an amount 

 equal to 15 per cent of the total transport of 82 x 10^ m.^/sec. which he 

 computed for the Stream using a 2000 m. reference level. This is consistent 

 with the fact that most of the water at a temperature below 8° C. is blocked 

 from flowing through the Florida Straits by the shallowness of the chaimel. 

 The water between 8 and 20° C. flowing in the Gulf Stream off Chesapeake 

 Bay is made up of nearly equal parts of water which has come through the 

 Florida Straits and of water from the North Atlantic north of the Indies ; 

 together these components amount to about 71 per cent of the total 

 transport. An additional 13 per cent of the total transport occurs in the 

 warm core of water consisting of surface waters warmer than 20° C, but 

 these waters are so subject to w^nd mixing and to atmospheric cooling 

 that for them the T-S relationship means very little as a tracer. Less than 

 1 per cent of the total Gulf Stream transport off Chesapeake Bay involves 

 entrained masses of coastal, shelf, and slope waters on the inshore edge of 

 the Stream. The reader is reminded that these figures are based on an 

 assumed reference level. 



The T-S method actually gives us a rather broad picture of the possible 



