Figure H. — Distribution of surface isotherms (contour in- 

 terval: 0.5° C.) at front 3, off Cape San Lucas, Lower 

 California, from a thermograph survey (0105 hours till 

 1600 + hours 22 May 1960). -o- timecbeck; -•-tem- 

 perature check; speculative contour. 



the true thermocline underlying the frontal 

 water of both sides. Their results show that 

 the frontal layer extended down from the sea 

 surface first vertically then laterally. Such a 

 layer is not obvious in the profiles we obtained. 

 If the frontal layer is taken to be demarked by 

 the 16°, 17°, 18°, 19°, and Z0° C. isotherms, it 

 assumes the general form of a letter Z, but it 

 is least well-defined between about 25 and 60 

 m., the zone of the large inversion, and reaches 

 the surface well to west of the front observed 

 at the surface (fig. 9). 



The five main cross-hatched areas (1 - V) 

 in figure 15 show water that is practically 

 isothermal vertically; this follows the presen- 

 tation of Cromwell and Reid (1956), In the 

 fronts they reported on, however, there was 

 presumed to be little salinity gradient, so 

 that density was almost wholly determined by 

 temperature; hence, they could hypothesize that 

 vertical mixing would require least energy 

 where water was vertically isothermal, and 

 would be most likely to occur there. In front 5 

 there was also a marked salinity gradient (figs. 

 19, 20, and 25), so that such a hypothesis may 

 not hold true. 



The five main'parcels of vertically iso- 

 thermal water are: the surface water (1) and 

 the deep waters (V); one corresponding to the 

 warm water (III); and the other two (XI and IV) 

 corresponding to the cool water above and 

 below the warm (III). 



The starting positions of subsequent obser- 

 vations (hydrocasts, net tows) are given in 

 figure I6. The temperature profiles derived 



from two hydrographic sections (B and E 

 triplets of table 1; figs. 17 and 18) do not have 

 the details of those from BT passes. The B 

 series profile clearly shows the temperature 

 inversion, but only in the 17° and 17.5° C. 

 isotherms (fig. 17). 



The absence of inversions in the 16°, 18°, 

 and 19° C. isotherms in the B series profile, 

 as compared with the BT section (fig. 15), may 

 be due to two possible causes: ( 1 ) an evolution, 

 by mixing, of the pre-existing situation, as 

 shown by the BT section two days earlier; or 

 (2) the middle cast (station 5B2) was so located 

 in respect to the temperature structure shown 

 in figure 15 (say at the position of BT no. 4), 

 that only the inversion in the 17° and 17.5° C. 

 isotherms was detected. 



The temperature profile from the E series of 

 casts (fig. 18) shows an inversion only in the 

 16.5° C. isotherm, which suggests a develop- 

 ment, such as mixing, from the conditions of 

 the previous day when the B series of casts 

 was made. 



Vertical salinity structure .-- The salinity 

 profiles (figs. 19 and 20) from the hydrographic 

 sections B and E give more definite information 

 about the front below the surface. From salinity 

 data of cruises TO-60-1 (May I960), TO-61-1 

 (April 1961), and 6004-B (May 1 960), and from 

 eight CalCOFI cruises to this area in spring, 

 it is safe to assume that the maximum salinity 

 of California Current surface water in spring 

 is about 34.60°oo. The minimum salinity of Gulf 

 water in the upper 100 m, at this time of year 

 is about 34.70'oo, the surface salinity being 

 > 35.00%„ (Roden and Groves, 1959). It there- 

 fore seems reasonable to consider, taking into 

 account the salinity structure determined from 

 the cast data, that the water between the 34.60/'oo 

 and 34.70°'oo isohalines represents theboundary 

 between the two kinds of water. The boundary, 

 or front, is sharper at some depths than at 

 others, being, perhaps, least definite near the 

 surface and below 50 m. The "salinity front" 

 at the surface roughly corresponds to the sur- 

 face "temperature front". The isohalines be- 

 tween 34.60°„ (maximum surface salinity of 

 California Current water) and 35.00 oointersect 

 the sea surface between the "middle" and the 

 "warm" stations in both B and E series salinity 

 profiles, as do two of three isotherms (figs. 17 

 and 18). 



There is good agreement between the tem- 

 perature inversion in the 17 C. isotherm (B 

 series) and the salinity structure; the cool 

 water in the inversion has a salinity of < 34.70°oo, 

 whereas the deeper, warm water has a salinity 

 of >34.70°'io. The inverted isotherm from the B 

 series is drawn as a dotted line on the cor- 

 responding salinity profile (fig. 19). The rela- 

 tion is not as good for the E series (fig. 20) 

 because some mixing, naarked by the presence 

 of an inversion in the 16.5° C. isotherm only 

 and by the reduced extent of the water of 



18 



