62 Hydrography of the Stream 



be interpreted as an instantaneous picture at any time. It is possible, of 

 course, to reduce all observations to their approximate positions at any 

 epoch, if the surface velocities are well known and steady, but the practical 

 difficulties are formidable and the final distribution is questionable. Thus 

 the two ships were able to delineate the quasi-stationary thermal and 

 velocity fields fairly well, but were completely unable to do the same for 

 the rapidly changing surface salinity field. It will be very interesting to 

 see what new strategic plan w^ll meet such situations. 



There are a number of possibiHties. For example, more use could be 

 made of drifting and anchored recording and radiotelemetering instru- 

 ments. Instruments left on the bottom for long periods might also be used. 

 Electrical potential gradient and pressure seem to be the best variables to 

 measure on the bottom, where fluctuations in temperature and velocity 

 may be insignificant. The airplane can be used in measuring surface tem- 

 perature rapidly over large areas. It is too early to foresee the new tactics 

 these will demand. 



A leg crossing the Gulf Stream may be estimated as 80-100 nautical 

 miles in length. To take twelve hydrographic stations, each of two casts,^ 

 requires about 70 hr., weather permitting. A leg with a similar number of 

 bathypitotmeter velocity measurements might consume 56 hr. If both 

 hydrographic stations and velocity stations are made, the total time in 

 crossing is 100 hr. These periods may be contrasted to 10 hr. when the 

 bathythermograph alone is used, and 12 hr. for measuring with towed 

 electrodes. An airplane can traverse the same leg in less than 40 min., but 

 the data it can obtain are pretty well hmited to surface temperature and 

 photographs. 



TRANSFER PROCESSES ACROSS THE STREAM 



In 1936 Rossby (1936&) showed that if the Gulf Stream were regarded as 

 a wake stream in the sense used by Tollmien (1926) the observed doAvn- 

 stream increase of mass transport could be accounted for. Rossby also 

 suggested that large lateral exchange processes might be at work carrying 

 Sargasso Sea water across the Stream and into the slope water on the 

 left-hand side. It now appears that the increase of transport in the down- 

 stream direction can be accounted for by other means (see Chapters VII 

 and VIII), and that the turbulent-jet analogy is not necessary; but the 

 question still remains : what kind of transfer occurs across the Stream ? 



' Usually, at any deep-sea hydrographic station not more than twelve reversing 

 bottles and twelve pairs of reversing thermometers are attached to the cable. Three 

 or four separate lowerings must therefore be made, in order to obtain samples at 

 all depths. Each lowering is called a 'cast'. 



