104 



Water 



Figure 90. Temperature-salinity ( T-S) diagram showing 

 percentage mixtures of waters characteristic of northern 

 California and those of Mexico. Two examples of mixed 

 waters (stations occupied during February 1950) show 

 the method of computing percentage of Southern Water 

 (20 and 65 per cent, respectively) and depth in meters to 

 water consisting of 50 per cent mixture of southern and 

 northern types (more than 500 meters and 200 meters, 

 respectively). 



and below the depth of 50 per cent Southern 

 Water, taken for simplicity as between 200 

 and 500 meters. Incomplete mixing of the 

 waters is sometimes revealed by small tem- 

 perature inversions measured with a 300- 

 meter bathythermograph over San Pedro 

 and Santa Monica Basins (Fig. 92) and with 

 an electronic method (Snodgrass and Cawley, 

 1957) over San Diego Trough. The percent- 

 age of Southern Water between 200 and 500 

 meters at each oceanographic station of a 

 cruise can be determined from the T-S 

 diagram. Resulting maps (Fig. 91) com- 

 monly show a tongue of high percentage of 

 Southern Water extending into the conti- 

 nental borderland, where most of it is 

 trapped by the Santa Rosa-Cortes Ridge. 

 Small amounts of Southern Water that are 

 present seaward of the ridge reached that 

 area by moving northward along the out- 



side of the ridge, by leaking through gaps in 

 the ridge, or by passing over it. 



Approximate currents at the depth of 50 

 per cent Southern Water can be computed 

 by mapping the dynamic topography of the 

 300-decibar level with respect to the 800- 

 decibar level. Reference to deeper levels 

 would have provided more accurate current 

 data, but the necessary measurements of 

 temperature and salinity are relatively rare 

 for depths greater than 800 meters. The re- 

 sulting computed currents at 300 meters 

 show a characteristic northwesterly move- 

 ment (Fig. 91). For many of the cruises, 

 such as the one for July 1950, much more 

 water flowed past San Diego at velocities of 

 as much as 12 cm/sec, 0.25 knot, than es- 

 caped from the continental borderland 

 through the gaps at 300 meters in the Santa 

 Rosa-Cortes Ridge. That the water is not 

 merely temporarily trapped is shown by the 

 frequency of this current condition. It is 

 evident, therefore, that the water must es- 

 cape, and the only avenue available to it is 

 that of rising and crossing the top of the 

 Santa Rosa-Cortes Ridge. This flow of in- 

 termediate water replaces part of the surface 

 water entrained by the California Current 

 and drawn off" across the Santa Rosa-Cortes 

 Ridge. Precise computation of transport 

 volumes would make an interesting study. 



Amounts of dissolved oxygen in the water 

 at intermediate depths are much lower than 

 those at the surface, probably chiefly be- 

 cause most of the oxygen has been used in 

 the oxidation of organic matter that sank 

 from the surface through the intermediate 

 water. The same results are achieved 

 whether the oxidation is by respiration of 

 the phytoplankton, feeding and respiration 

 by zooplankton or larger animals, bacterial 

 activities, or chemical reactions. Because of 

 the probable longer time since the Southern 

 Water was at the surface and able to absorb 

 oxygen, its oxygen content is lower than 

 that of the Northern Water for any given 

 temperature (Fig. 93) as shown by Sverdrup 

 and Fleming (1941). If controlled only by 

 the percentage of Northern and Southern 

 Water, the oxygen content at intermediate 

 depths could be computed from the tem- 

 perature-salinity curves. However, observed 



