HIROTA: NATURAL HISTORY OF PLEUROBRACHIA BACHEI IN LA JOLLA BIGHT 



II7»20' 



1 32" 45' 



Figure 6. — Trajectories of drogues during studies on 12-15 March and 4-5 April 1970. Observed 

 positions of the drogues are indicated by open circles, triangles and squares, and the date and time of 

 the triangulation are indicated by the one-four or two-four digit sequence of numbers near the 

 symbols. The respective dates and predicted times (Pacific standard time) and heights of tides in 

 centimeters are given in each inset. 



or near La Jolla Canyon. The surface tides as- 

 sociated with the commencement of the rotational 

 motion were slack ebb tides, and completion of the 

 loops during rotation occurred during flood tides. 

 From 1800 h 3 November until 2300 h 4 November 

 wind speeds were less than 3 m/s with variable 

 direction. From 0240 h 5 November until 0600 h 6 

 November the winds increased to a steady 3-5 m/s 

 from the south southeast to south southwest. The 

 northward drift of the drogues after 0400 h 5 

 November may have been a response to changes in 

 the wind velocity. Some changes in direction oc- 

 curred on the northward drift of the drogues once 

 they were beyond the submarine canyon complex, 

 but these were relatively slight. While the 

 drogues were over shallow water, the mean drifts 

 were slightly onshore during flood tides and 

 slightly ofFshore during ebbs. The net eastward 

 drift was about 0.3 km and the westward drift 

 about 1 km relative to a line true north at 0400 h 5 

 November. 



During two other drogue studies on 12-15 

 March and 4—5 April 1970 drogues were tracked 

 for 1 to 2 days. The study of March 1970 provided 

 the best information associating the surface tides 

 with changes in direction (Figure 6). A drogue set 

 in the axis of La Jolla Canyon drifted slowly to- 

 ward the southeast along the canyon axis until 

 flood tides changed its direction to northeasterly. 

 On the following slack flood tide the drogue slowed 

 and then moved off'shore toward the west on the 

 next ebb and smaller flood. The onshore- offshore 

 motions occurred during the following 

 flood-slack-ebb sequences but are not as well as- 

 sociated with the surface tide as in the first cycle. 

 During this drogue study the weather was foggy, 

 especially in the early morning hours, and the 

 winds were less than 3 m/s during the day from the 

 northwest. At night and in the early morning 

 hours offshore winds were about 2-4 m/s. Note 

 that the east- west horizontal translation during a 

 tidal cycle is on the order of 1-2 km. This effect will 



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