Front 1 



Front 1 (fig. 1) was fovind by thermograph 

 early in the morning on 7 May 1960. As with 

 the other fronts studied, there was no obvious 

 demarcation in the ocean surface, though dif- 

 ferences between the waters of either side 

 could be seen. We started the thermograph 

 survey as a regular pattern, a "rectangular 

 spiral", but this we modified (at 0913 hours) 

 as temperature data were gathered (fig. 31). 

 No front was crossed between 0913 and 0935 

 hours, nor after turning 90° to the right. At 

 0935 the vessel reversed its course and im- 

 mediately passed through the front. To avoid 

 confusion this backtrack is set off from the 

 first track (0928 - 0935 hours) in figure 31. 

 This experience demonstrates the sort of dif- 

 ficulty that can be expected in the study of 

 fronts, particularly of the faster moving and 

 more sinuous kind. 



The surface currents as measured by GEK 

 are shown on figure 31; flow was slightly 

 faster on the cool side. 



The track between 1Z59 and 1 345 hours was in 

 water of about 19.5° C. This isotherm was in 

 the front earlier in the day. Assuming that 

 this is a suitable indicator isotherm and that 

 the drawn track is not markedly in error, we 

 may conclude that the front moved about 2 to 

 3 nniles in 4 to 5 hours - an average speed of 

 about half a knot. 



On turning left at 1345 hours, we expected 

 to recross the front. Although the temperature 

 increased slowly up to about 20° C, no front 

 was encountered, so we returned to the regular 

 station pattern of cruise TO-60-1. 



The frontal isotherms at front 1 fall into two 

 groups (fig. 31). These are not joined because 

 there is evidence that the front apparently was 

 moving roughly westward; hence, the front 

 found between about 0800 and 0930 hours likely 

 would have moved to a position approximately 

 south of the front as found between about 1100 

 and 1300 hours. Therefore, to join the two 

 groups of isotherms would introduce a false 

 curvature. It is possible also that the orienta- 

 tion of the front may have changed during the 

 day and that the southern part diffused. 



Although the tennperature distribution indi- 

 cates a westward movement, the fact that 20° C. 

 water was found (at about 1400 hours) east of 

 20.5° C. water (at about 0950 hours) contra- 

 dicts this idea. A possible solution is that the 

 front was sinuous; and, owing to accumulating 

 navigational error, there can be some doubt of 

 the accuracy of the plotted track later in the day 

 in relation to one plotted earlier in the day. 



The direction of flow according to the GEK 

 observations is as would be expected from 

 geostrophic flow calculations, assuming the 

 surface temperature reflected the density 

 structure over some depth of no motion. It is 

 difficult to reconcile the northerly flow (as 

 shown by GEK with the apparent westward 



movement indicated by the isotherm distribu- 

 tion in time (but see p. 16). 



If a front is in motion, determining both its 

 main orientation (axis) and its change of posi- 

 tion is difficult. Even though the front as a 

 whole may be stationary, parts of it may move 

 sinuously. Front 1 exemplifies this problem, 

 but the problem pertains also to fronts 2 and 4, 

 and, to a much lesser degree, to fronts 3 and 5. 



The temperatures at front 1 were such that 

 when compared with the isotherms of the gen- 

 eral area (fig. 2) there can be little doubt that 

 the warmer water was regular surface water 

 and the cooler water was upwelled water of the 

 California Current system. Station 15, by front 

 1, was, in fact, at the southern end of a tongue 

 of cool, upwelled water originating inshore well 

 to the north of Cape San Lucas (fig.Z). 



Front 2 



At front 2 (fig. 32) we guided the ship so the 

 movement of the front would be detected, 

 though the general form would not. Presumably 

 because of time-lag and sinuosity the move- 

 ment of the front within the narrow area 

 studied was not defined clearly. 



Because no thermograph survey was at- 

 tempted, BT passes (fig. 32) were started first. 

 As at front 5, the temperature profiles drawn 

 from the BT data were similar for all three 

 passes; pass no. 2 is taken as representative 

 (fig. 33). 



The important characteristic is a thin layer 

 of relatively warnn (~20° C.) water spreading 

 across the front between depths of about 35 

 and 60 m. In all three profiles, it weakens or 

 disappears immediately beneath the front 

 proper (i.e., where isotherms cut the sea 

 surface). Presumably, the warm, thin layer on 

 the cool side originated from water on the 

 warm side. This warm layer could be de- 

 rived from an intrusion below the surface of 

 the water found at front 5 (fig. 15), such 

 an intrusion being separated frona its 

 source by a complementary intrusion, above 

 and perhaps below, of cool water, as might 

 have happened eventually at front 5. 



On pass no. 2, we took surface water 

 sannples with each BT and measured the salin- 

 ity of each sample. The warnn water was more 

 saline, as at front 5. With contemporaneous 

 measurements of surface temperature and 

 salinity, we determined the corresponding 

 thermosteric anomaly values. These three 

 properties are plotted in figure 34 in rela- 

 tion to the BT pass. They have a strong 

 qualitative and quantitative similarity to those 

 of front 5 (fig. 21). The distributions, com- 

 puted as before by dividing the maximum dif- 

 ference by the distance between the BT's 

 corresponding to that difference, are tem- 

 perature, 0.33° C.kmT ; salinity, 0.12 %o km7; 

 and thermosteric anomaly 5.1 cl.ton'^kmT . 

 The respective BT's are 3-14 (T), 7-14 (S), 



42 



