120 to 170 m. were g-enerally the same as within 

 the left side of the Gulf Stream at depths of 

 300 to 600 m. This similarity is illustrated in 

 figures 27 to 30, which show temperature- 

 salinity curves for the oceanographic stations 

 made in sections 10, 12, 13, and 19. Coincidence 

 of properties started at about o-t =27.0 and con- 

 tinued to the end of the curves (a little below 

 600 m.) where the approximate value is 

 ff, =27.8. (The properties at 600 to 700 m. in 

 the right side of the Gulf Stream and in the 

 adjacent Sargasso Sea were entirely different- 

 see the T-S curves of stations 0.51-HO19 and 

 052-HO20 in figure 27.) 



The mean upper level of water with coin- 

 ciding properties, computed from all available 

 sections, was 355 m. for the left side of the 

 Gulf Stream and 154 m. for the interaction 

 systems. The horizontal distance between the 

 generation area in the interaction systems and 

 the locations of similar water in the Gulf 

 Stream was 15 to 40 miles. 



Most of the interchanging cold and warm 

 tongues observed in the boundary area and in 

 the interaction system were stable. When un- 

 stable situations do occur in the ocean, they are 

 usually of short duration and it is difiicult to 

 obtain data on their frequency and the volumes. 

 Unstable mass distributions may be created 

 either by caballing or by rapid overflow of 

 water of greater density over water of lesser 

 density in the process of strong, lateral mixing. 

 Oceanogi-aphic stations 031-HOOl, 037-HO07, 

 and 055-HO23 illustrate such unstable situa- 

 tions (figs. 13, 15, and 17). The instability, 

 even if very limited in volume, was evident in 

 3 out of the 10 stations for which salinity data 

 were obtained in the intermediate zone, 

 whereas the total number of oceanographic sta- 

 tions was 42. The proportions may indicate that 

 volumes of water exhibiting unstable mass 

 distribution frequently occurred but lasted for 

 only a brief period. Certainly errors in the 

 data have to be considered; however, the three 

 cases of instability occurred in the zone of in- 

 tense interaction and none was outside of that 

 zone. This fact indicates that the unstable situ- 

 ations were probably real. 



ZONES OF CONVERGENCE AND DIVERGENCE 



The left side of an interaction system is a 

 zone of convergence with a rather permanent 

 downward motion. When an interaction system 

 dissipates, the convergence zone may be ex- 

 pected to move close to the left side of the Gulf 

 Stream boundary. Downward motion caused by 

 convergence is probably locally limited and too 

 weak to transfer significant mass from an in- 

 teraction system to the Gulf Stream. A super- 

 imposed momentum due to transient unstable 

 mass distribution may, however, produce a 

 downward flow strong enough to cover the 

 horizontal distance between the generating and 

 the discharging areas. The descending dense 

 water would be absorbed by the left side of 

 the Gulf Stream at levels about 200 m. lower 

 than its point of origin in the interaction sys- 

 tem. The downward motion, in turn, would 

 intensify the surface convergence and, thus, 

 produce a significant cross-current velocity com- 

 ponent in the left side of the Gulf Stream. 



The mean distance from the left boundary of 

 the Gulf Stream to the location of maximum 

 velocity in the Gulf Stream was 22 miles (40.7 

 km.), derived from four sections of surface 

 velocity distribution across the current (Von 

 Arx, 1952; Worthington, 1954). The distance 

 between the right boundary of the Stream and 

 the location of maximum velocity was 36 miles 

 (66.7 km.). Outcroppings of cool water were 

 present about 25 miles (46.3 km.) from the left 

 boundary of the Stream, as can be seen from 

 sections 3 and 6 (fig. 12) or by comparing sec- 

 tions 4 and 5 (figs. 6 and 7) with sections 6 

 and 7 (figs. 8 and 9). Thus, the cold stripe 

 probably developed in the region of maximum 

 velocity in the Gulf Stream, above the right 

 flank of the steeply sloping isotherms, about 

 where the 18° C. isotherm crossed the 200-m. 

 level in section 5 (fig. 7). 



Because of a strong cyclonic shear at the left 

 of a velocity maximum and a slightly weaker 

 anticyclonic shear at its right, a maximum 

 horizontal gradient of the vertical component 

 of absolute vorticity may be expected in the 

 region of maximum velocity. The strong hori- 

 zontal change in absolute vorticity may cause a 

 tendency to diverge. If convergence in the area 

 of interaction were intensified through the 



416 



U.S. FISH AND WILDLIFE SERVICE 



