Currents 



115 



California by Emery, Gorsline, Uchupi, and 

 Terry (1957) and by Valentine (1955). 



Variations in thickness of the mixed layer 

 produce variations in the depth to a given 

 isotherm in the thermocline. Parts of the 

 shelf having a much greater depth to the iso- 

 thermal surface than elsewhere must usually 

 also be characterized by having a higher sea 

 level. Just as in deep water, the current 

 tends to parallel the contours of depth to the 

 isothermal surface, flowing in a direction 

 such that this surface is deeper on the right- 

 hand side of the current. Precise directions 

 of the current cannot be obtained because 

 the isothermal surface itself is not stationary 

 and because of frictional eff'ects of the bot- 

 tom. Maps showing the topography of iso- 

 thermal surfaces in Santa Monica Bay were 

 prepared for about twenty cruises during 

 1955-1956 by Stevenson, Tibby, and Gors- 

 line (1956). Most of the maps (Fig. 101) 

 indicate a general flow into the center of the 

 bay and outward flow at both ends. The 

 circulation pattern was not constant, how- 

 ever, and diff'erent circulations exist in other 

 bays and shelf areas having diff'erent shapes 

 and relationships to winds and off'shore cur- 

 rents. Currents in shallow water cannot 

 usually be predicted accurately but must be 

 measured for each individual area. 



Current meters can be used as eff"ective 

 indicators of current direction and velocity 

 in some areas of the shelf, particularly off 

 exposed open coasts (Shepard, ReveUe, and 

 Dietz, 1939). Within bays such as Santa 

 Monica, however, the currents are so slow 

 (averaging 10 cm/sec) and changeable as to 

 be below the accurate range of the meters 

 which is set by friction and inertia of their 

 propellors and vanes. For some purposes 

 drift cards or drift bottles are more suitable 

 instruments. These devices are serially num- 

 bered and distributed in quantity at many 

 stations at sea. When they wash ashore their 

 bright markings attract the attention of peo- 

 ple who find within them a note asking that 

 the time and place of finding be indicated on 

 an enclosed form to be mailed to the agency 

 making the study. About 5200 plastic drift 

 cards were set afloat in 14 cruises in Santa 

 Monica Bay during 1955-1956 with a re- 



covery of 38 per cent. Commonly, cards 

 released within the bay came directly ashore 

 or moved laterally away from the center, 

 paralleling the flow inferred from the distri- 

 bution of temperature (Fig. 101). Many 

 cards dropped just outside the bay floated 

 southward and westward to points as distant 

 as San Diego and Hueneme. Others may 

 have reached the California Current to be 

 carried across the Pacific Ocean unless they 

 sank en route. 



Drogues are more elaborate versions of 

 drift cards designed to measure the currents 

 at depth. They consist of a weighted vane- 

 like structure or a weighted parachute (Volk- 

 mann, Knauss, and Vine, 1956) suspended 

 from a buoy. By tracking several of these 

 buoys with a boat, we can obtain the general 

 patterns of currents at various depths more 

 or less simultaneously. At Santa Monica 

 Bay the drogues followed the same paths as 

 the computed currents, that is, parallel to 

 contours of depth of an isothermal surface; 

 these paths were approximately the same as 

 those followed by drift cards. They showed 

 that the surface and subsurface currents 

 moved in approximately the same direction, 

 but this is not necessarily true of all shelf 

 areas. The most complete study of shelf 

 water based on drogues was made by Stewart 

 (1957) for parts of the shelf off San Diego 

 mostly shallower than Santa Monica Bay. 

 A total of 406 observations on drogues were 

 made between June 1955 and March 1957. 

 Comparison with wind direction showed that 

 surface currents moved at the greatest angle 

 (often more than 45°) to the right of the 

 wind when the winds were perpendicular to 

 the shore and at a smaller angle when the 

 winds were parallel to shore. This is doubt- 

 lessly a result of the boundary effect of the 

 shore. The largest ratio of water current to 

 wind speed was about 1.8 per cent, obtained 

 when the wind was perpendicular to the 

 shore. Subsurface currents moved to the 

 right of the surface current, except in shallow 

 water where they tended to follow the con- 

 tours of the bottom rather than move strictly 

 to the right of the surface current, again an 

 effect of boundary conditions. Speeds of 

 subsurface currents were half or more of 



