until MrEu-cn: H ydrofjrajtliic <>I)S( rrations of Scripps I ii.<tl il iilinn "itii 



D. PERIODIC VARIATIONS IN TEMPERATURE AND 

 SALINITY 



1. Seasonal Variations 



Owing to the prevailing variations in temperatiii'c and salinity 

 with respect to position, observations made in a dfc]i-watcr region of 

 small area bnt little affeeted by upwelling and well distributed 

 throughout each year att'ord tlie iiKist satisfactory basis for a study 

 of periodic temperature and salinity variations. Our observations 

 made in sections 40^, 40,, 40,,, 41 ^, 41-,, and 41 „, eight miles off the 

 ('oronado Islands, though not numerous during each month of any 

 one year, meet the requirements better than those made in any other 

 region. Accordingly, these observations, supplemented by those made 

 off the Coronado pier in section 38„ and by "normal" values for the 

 Pacific at this latitude, are employed for this purpose. 



The mean monthly surface temperature of the ocean corresponding 

 to an average year within the period from 1908 to 1915 is shown by 

 plate 25, figure 40. This curve agrees well with that based on sup- 

 plementary observations made off the pier at C'oronatlo Beach (pi. 25, 

 fig. 41 ) , where, owing to the large area of shallow water and the gentle 

 slope of the bottom, the average reduction in temperature due to 

 upwelling is also small.' These monthly temperatures are from one 

 to two degrees lower than the "normal" for the Pacific Ocean at the 

 same latitude, as revealed by comparison with plate 25, figure 42, rep- 

 resenting the temperatures a thousand miles west of San Diego, as 

 shown by the monthly isotherms of the North Pacific ( Thorade, 1909) . 

 Plate 25, figure 42, shows further that the "normal" ainiual range 

 is 7° C, from a mininunu of 15° C in ]\Iarch or April to a maximum 

 of 22° C in August, while oft' the Coronado Islands (pi. 25, fig. 40) the 

 annual range is 6?2 C, from a minimum of 14':3 C in February to a 

 maximum of 20?5 C in August. 



Off the Coronado Islands (pi. 25, fig. 43) the mean monthly salinity 

 has an annual range of 0.25"/,„„ from a minimum of 33.50"/„„ in 

 February to a maximum of 33.75''/„„ in July or August. These values 

 are somewhat lower than those found off Coronado Beach, where 

 (pi. 25, fig. 44) the minimum is 33.55"/„„ in January and the maximum 

 is 33.85''/u,, in July or Augu.st. 



1 On some days the temyeratuies off Coronado Beach are as low as those 

 found at any of the upwelling regions, but such cases are exceptional. 



