132 



Water 



MARCH 



16 18 20 



CURRENT 

 AT SURFACE 



FEET/ Ml N. 



WIND 

 (TO) 



M. P. H. 



Figure 1 1 9. Currents relative 

 to tide height in an area 120 

 feet deep off Point Loma, San 

 Diego, on March 1 and 2, 1957. 

 Upper lines (solid for surface 

 measurements, dashed for 80- 

 foot measurements) show tidal 

 current in terms of flood and 

 ebb (to east and west, respec- 

 tively); note that maximum 

 current was usually earlier than 

 midtide and that currents at 

 surface and 80 feet were out of 

 phase. Lower diagrams show 

 current ellipses having clock- 

 wise rotation with time, except 

 when currents were slow and 

 erratic. Times of high, middle, 

 and low tides are indicated by 

 H, M, and L. Note the rotation 

 to right of currents at depth 

 with respect to those at surface. 

 Variation in wind direction 

 shown in bottom polar dia- 

 gram; note that the wind hap- 

 pens to be nearly in phase with 

 tide so that current ellipses may 

 be products of both wind and 

 tide. Based on drogue meas- 

 urements by Geological Diving 

 Consultants. Inc., for city of 

 San Diego. 



Leipper (1955) and Stevenson and Gorsline 

 (1956) to be produced by tidal action. Dur- 

 ing the week between neap and spring tides 

 the increase of tide range and its associated 

 net rise of sea level lead to more intense 

 mixing of the water atop the shelf. When 

 the water is highly stratified, as in summer, 

 the stirring produces a cooling that may 

 reach the water surface and form a dome-hke 

 cool spot of diameter as great as 3 miles. 

 Movement of cool spots by tidal currents 

 may lead to temperature variations that are 

 of approximate tidal period (Arthur, 1954) as 



measured at piers. Such a development of 

 cool spots by tidal origin may take place in- 

 dependently of winds, showing that care 

 must be exerted in ascribing areas of cool 

 water only to upwelling by wind stress. 



Sea Level 



The mean sea level for any period can be 

 computed from the record of hourly tide 

 heights. Such computations show that sea 

 level is not constant for any given locaUty, 

 but that instead it has seasonal and longer- 



