THE GULF STREAM 



About a century later, in 1860, a map was published by Matthew 

 F. Maury, in his Phxjsical Geography of the Sea, showing the cur- 

 rent system of the North Atlantic. It contains gross errors, 

 which must be attributed to his difficulty in obtaining information 

 from those seafarers who would have it. For instance, the 

 northern limit of the Gulf Stream is shown close to Cape Cod and 

 the Nova Scotia shore and above the Grand Banks, whereas it 

 never reaches George's Bank, which lies to the east and south of 

 Cape Cod, nor does it ever flow onto the Grand Banks, a fact 

 which even then would be known to all fishermen in those regions. 



A more accurate knowledge of the Gulf Stream was not ob- 

 tained until physical oceanography had been established as a 

 science in the present century. The real beginning of oceanog- 

 raphy was the Challenger expedition of 1872-76, but the cost of 

 oceanographic research has made progress very slow. As in 

 other branches of science a great impetus to development has come 

 with each war. After 1920 the technique of sounding was revo- 

 lutionized, with sonic depth finders replacing the old sounding 

 wire. During the second World War the bathythermograph for 

 temperature measurements became standard equipment for de- 

 stroyers and submarines, and submarine warfare in general made 

 it necessary to know what was happening within the ocean waters 

 as well as on their surface. Scientific explorations of the Gulf 

 Stream were initiated by the German and Norwegian oceanog- 

 raphers, but since the founding of the Woods Hole Oceanographic 

 Institution in 1930 by a Rockefeller endowment much more infor- 

 mation has been obtained about the Stream. 



WHAT DRIVES THE GULF STREAM? 



There are two major causes for the ocean currents in a large 

 basin such as the North Atlantic — unequal distribution of density 

 and the friction of the permanent wind systems. With the north- 

 east trade winds blowing across the southern part and the prevail- 

 ing westerlies across the northern part, one would expect that a 

 large clockwise circulation of the water would develop, provided 

 that no internal forces arose to prevent this motion. The friction 

 of the wind seems to be the most important factor in determining 

 the motion-of this gigantic North Atlantic eddy, but the distribu- 

 tion of density complicates the details of the motion. Fresh 

 water is added to the surface by the great rivers, and more heat is 

 absorbed from the sun in the tropics than in higher latitudes. 

 The process of evaporation influences both salinity and tempera- 

 ture. When the sea surface is warmer than the air above, it loses 

 heat rapidly to the air and this air rises with turbulence, some- 

 times producing thunderstorms. But when the water is colder 

 than the air the reverse process does not take place, because the 

 air that is chilled does not rise and mix with that above it, so that 

 heat transfer downward is very slow and inefficient. Evaporation 

 utilizes heat while condensation releases heat. The greatest 

 evaporation occurs, not when the air is hot, but when cold air 

 flows over warm water. It may be surprising that in middle and 

 higher latitudes, where the sea surface in winter is warmer than 

 the air, the evaporation is greatest in the winter, not in the sum- 

 mer. This illustrates the intricate relationship between the phys- 

 ical conditions in the ocean waters and the meteorological condi- 

 tions. Two seasonal factors that influence the oceanic circulation 

 are the influx of cold but fresh water from the melting of ice in 

 the polar regions, and the winter cooling of the surface waters 

 along the coast, which so increases the density that the surface 

 water sinks and the whole column of water is overturned and 

 thoroughly mixed. To understand the structure of the Gulf 

 Stream it is necessary to look briefly at the dynamics of ocean 

 currents in general. Owing to the rotation of the earth, there is 

 an apparent force which tends to deflect a moving mass of water 

 to the right in the northern hemisphere. A steady state of flow 

 can be achieved only if there is a horizontal pressure-gradient to 

 oppose this, and in turn this pressure gradient can be produced 

 only by a distribution of density such that the lighter water is on 

 the right with the isobaric surfaces sloping upward to the right 

 of an observer looking in the direction of the stream. This means 

 that the ocean surface above a current is not level, but for a cur- 

 rent of 2 knots in latitude 45° the slope is only a rise of 1 centi- 



meter in a distance of 1 kilometer, or one in 100,000. 



With this background we may examine the Gulf Stream system, 

 first as regards its horizontal distribution and then its internal 

 structure. The North Equatorial Current is driven by the north- 

 east trades towards the Caribbean and joins a branch of the South 

 Equatorial Current which has crossed the Equator. Thus the 

 water entering the Caribbean contains some water of South At- 

 lantic origin, while the water flowing north of the Greater Antilles 

 is the same as that of the Sargasso Sea. The current driven 

 through the Yucatan channel has only one outlet, between Florida 

 and Cuba. This outflow is the genesis of the Gulf Stream system. 

 Only a negligible portion originates in the Gulf itself. The term 

 "Gulf Stream system" is now used to include the whole northward 

 and eastward flow beginning at the Straits of Florida and includ- 

 ing the various branches moving across the North Atlantic from 

 the region south of the Newfoundland Banks. The system is 

 subdivided into the following parts: (1) The Florida Current, 

 from the Straits of Florida to Cape Hatteras, where the current 

 swings outward from the continental slope. Beyond the straits 

 the water from the Yucatan channel is joined by the Antilles Cur- 

 rent, but the name Florida Current is retained up to the cape. 

 (2) The Gulf Stream from Cape Hatteras to a point east of the 

 Grand Banks at about longitude 45°, where the stream begins to 

 fork. This terminology is a restricted use of the popular term, 

 but is essential for clarity. (3) The North Atlantic Current, 

 from this point east and north, including the several branches. 

 In this region the main current is often masked by erratic wind- 

 drift surface-movements, commonly known as the North Atlantic 

 Drift. The major branches ai-e the Irminger Current, flowing 

 west below Iceland; the Norwegian Current flowing north 

 through the Norwegian Sea into the Polar Sea; and an irregular 

 branch which flows towards the Bay of Biscay. The Irminger 

 Current joins the East Greenland Current and can be detected by 

 its salinity (35 per mille-') as far south as Cape Farewell. The 

 Norwegian Current is warm enough to keep the port of Hammer- 

 fest, though well within the Arctic Circle, from ever being closed 

 by ice. 



To return to the Florida Current: The energy of the flow is 

 derived from the diflference in sea level between the Gulf and the 

 Atlantic coast, the diff'erence being 19 centimeters between Cedar 

 Keys and St. Augustine (on opposite sides of Florida). This 

 difference of level is probably maintained by the trade winds. 

 Because of the flow through the straits the lighter water must be 

 found on the Cuban side and the sea surface there should be 

 higher. It is in fact about 45 centimeters higher, and the possi- 

 bility of direct measurement here corroborates our indirect meth- 

 ods of calculating velocities in the ocean from the distribution of 

 density. The direct current measurements of Pillsbury, made 

 from the survey vessel Blake in the years 1885-89, are among the 

 classical data of physical oceanography, since they provide a test 

 for indirect methods which can be applied elsewhere in the ocean. 

 The transport of water through the Straits of Florida is about 26 

 million cubic meters per second. On its left the shallow coastal 

 water is at rest and the transition to the blue water of the stream 

 is so sharp that it appears as a line from horizon to horizon. The 

 Antilles Current brings in about 12 million cubic meters per 

 second, and as the current moves toward Hatteras more water is 

 drawn in from the Sargasso on the right. 



Beyond Hatteras the Gulf Stream swings away from the coast, 

 and on its left are the coastal waters over the shelf and a slope 

 water with temperatures of 4°-10° C. Here great seasonal vari- 

 ations occur, and eddies from the Gulf Stream intrude. Off' Ches- 

 apeake Bay the transport of the stream has increased to about 

 7.5-90 million cubic meters per second, so that a great deal of 

 Sargasso water and deep water has been added, but at the tail of 

 the Grand Banks the transport has fallen off to less than 40 mil- 

 lion cubic meters per second, showing that water has been dis- 

 charged southward from the stream. The dynamics of this flow 

 are not properly understood, though several theories have been 

 propounded. An interesting point is that precise leveling along 

 the eastern coast shows that sea-level increases from Florida to 



" 35 parts of salt in a thousand by weight. 



79 



