salinity maximum (~34.80„„) at 150-200 m. 

 depth, could not produce the high inshore sur- 

 face salinities off western Lower California. 



In situ evaporation, though at a maximum at 

 the end of the year, is not great enough in this 

 region to produce the observed salinities 

 (Jacobs, 1951; Roden, 1959). 



Reid et al. (1958) state that warm, saline 

 (^35.00%o) water from the eastern central 

 Pacific is advected into the California Cur- 

 rent on its western side, so that it can hardly 

 be the cause of high salinities inshore, on the 

 eastern side (see Reid et al., figs. 5b and 10a). 



The most obvious and tenable view is that 

 these high salinities are due to Gulf water 

 which, with eastern tropical Pacific water, 

 constitutes the autumn countercurrent. By 

 early spring the northward countercurrent off 

 western Lower California has ceased and 

 southward flow resumes, with increased up- 

 welling (compared with winter's) inshore; 

 this strong upwelling continues throughout the 

 summer. 



For this area in spring and summer, the 

 California Current water referred to through- 

 out this paper is, therefore, a mixture of the 

 somewhat more typical California water to the 

 north (fig. 5) and some Gulf and eastern tropi- 

 cal Pacific water from the previous autumn 

 and winter. For present purposes, I call it 

 transitional California Current water. There 

 is then the further complication caused by the 

 upwelling of this water. 



The frontal system shown in figure 2 is 

 probably formed by the three main kinds of 

 water alluded to, with the possibility that far 

 out to sea it could be formed between the 

 central Pacific water and eastern tropical 

 Pacific water. The part of the system extend- 

 ing south from Cape San Lucas (or Cape Falso) 

 is formed by Gulf and California Current 

 water, and the part turning west from there 

 (for example, near station 29, figs. 1 and 2) 

 probably formed by eastern tropical Pacific 

 and California Current water. No marked sys- 

 tem is formed between the equatorial and Gulf 

 waters in the eastern half of the Gulf entrance 

 because the differences between these two kinds 

 of water are small at the time of year being 

 considered (spring). The small sharp frontal 

 system often found at Cape Corrientes (Roden 

 and Groves, 1959) would be formed by Gulf and 

 equatorial water, but probably only when up- 

 welling of the equatorial water is strong at 

 Cape Corrientes (in April-June and, though 

 less, in October-December, according to 

 Cromwell 1958). 



The fronts are not discussed in their chrono- 

 logical order but as follows: (1) front 5, the 

 best studied (in April 1961 ), together withfront 

 3 which was found in the same area but 11 

 months before (May I960); (2) front 1 (May 

 I960); and (3) fronts 2 and 4 which were found 

 in the same place, but 13 days apart, also 

 during May i960. 



Fronts 3 and 5 



Front 5, by virtue of the more detailed and 

 varied study given to it, and because of its 

 marked features, is discussed more fully than 

 the others. Study of these others was more 

 difficult because they were, except front 3, 

 moving faster and were not formed of readily 

 identified waters. Front 5 undoubtedly was 

 formed at a boundary between water from the 

 Gulf of California and from the California Cur- 

 rent System. Even so, the California Current 

 water was apparently partially composed of up- 

 welled water (see below). Front 3, which was 

 very similar to front 5 in its surface tem- 

 perature distribution, was probably formed of 

 the same kinds of water. 



Surface temperature distribution .- - The first 

 phase of study at front 5, the initial thermo- 

 graph survey, enabled us to plot the surface 

 isotherm contours. These are shown, with the 

 ship's track, in figure 9. The tongue of cool 

 water protruding south and eastward round 

 Cape Falso is probably upwelled (see below) 

 inshore between Magdalena Bay and Cape 

 Falso. It forms the front with the warm, 

 saline water coming from the Gulf of Cali- 

 fornia. This intrusion of cool water persisted 

 for the period of the study ( 1 9-24 April), and 

 intruded further eastward, according to the 

 thermograph survey made at the end of the 

 work. The surface isotherm contours from 

 this latter survey are drawn in figure 10; 

 judging from thermograph records made on the 

 voyage from Cape Falso to San Diego, the 

 origin of the cool water, mentioned above, is 

 inshore north of Cape Falso, after upwelling. 

 That upwelling occurs in this area at this 

 time of year is shown by figure 2, as well as 

 by many other CalCOFI data. 



It is quite clear that even after 5 days the 

 surface temperature distribution in the area 

 was basically unchanged, though the obtru- 

 sions of cool water to the south and west, and 

 of the warm water to the north and east, had 

 elongated, causing the front to be aligned 

 roughly parallel to the coast. 



As were the rest, front 3 was found by 

 thermograph. A thermograph survey indicated 

 that the front was not strongly developed, and 

 no further work was done. Plots of the cor- 

 rected thermograph temperatures showed that 

 a sharp front very probably existed directly 

 between, roughly parallel to, two track lines 



(fig. 11). 



There is an obvious similarity between the 

 results of the thermograph surveys of front 3 

 and front 5. The cool eddy (<22.50 C.)at front 

 3 appears to have been cut off from the remain- 

 ing cool water to the north and west by the 

 obtrusion of the warm (Gulf) water west of 

 Cape Falso. Such a situation could be derived 

 from the situations shown in figures 9 and 10. 



15 



