RIVERS IN THE SEA — SMITH 441 



Another consequence of the way in which the Coriolis force causes 

 a redistribution of water density enables changes in the flow of ocean 

 currents to be measured by means of tide gauges. Since the heavier 

 water shifts to the left of the stream, the water surface tilts in order to 

 maintain equilibrium so that the right-hand edge of the stream is 

 higher than the left. For instance, the Atlantic circulation, including 

 the Gulf Stream, flows in a circuit with the Sargasso Sea near the 

 center. On all sides, therefore, there is a downward slope of water 

 from the Sargasso Sea outward. In the Florida Straits this means 

 that the water level at Miami on the coast of Florida is around two 

 feet lower than it is at Gun Cay on the Bahamas side of the Straits. 

 This downhill gradient increases when the current increases, so that 

 comparison of tide-gage records at the two places enables the ocean- 

 ographer to detect changes in the pace of the Gulf Stream, after 

 averaging out the tidal movements, of course. 



AID TO YACHTSMEN 



Yachtsmen in the Newport-Bermuda ocean race go to sea equipped 

 with water thermometers, not through an enduring interest in 

 oceanographic science, but for navigational reasons, since they are able 

 to judge when they enter the Gulf Stream by changes in the surface 

 temperature. Since there is relatively less change in saltness than in 

 temperature in the open ocean it follows that the density distribution 

 is more noticeably reflected in the temperature of the water. For this 

 reason there is a rise of temperature as a ship enters the Gulf Stream 

 from the American side. 



VERTICAL CURRENTS 



Mention has been made of vertical currents and the huge slow move- 

 ments of water deep below the surface. The measurement of these 

 presents a different kind of problem to that of surface currents and 

 so other methods must be used. An obvious approach, of course, is to 

 add up the volume of water flowing into and out of any particular 

 ocean or body of water. When the surface currents are known and 

 hence reliable estimates can be made of the vertical movements, then 

 a balance may be struck and the residual amount of flow must take 

 place below the surface. A water budget, in fact, is set up. Other 

 methods are used, involving the measurement of carbon isotopes in the 

 sea. The ratio of the carbon-12 and carbon-14 atoms in the water 

 varies according to the length of time it has been away from the sur- 

 face so that isotope measurements provide a measurement of the water 

 movement. Other approaches involving changes in oxygen concen- 

 tration, temperature, and salinity have been used and will be de- 

 scribed, it is hoped, in a future article. 



