Coastal Productivitv and Dynamic Patterns 

 July 1979 



Persistent cloud cover seriously hinders regular monitoring of the 

 Vancouver Island region, but occasional cloud-free images show the 

 variety of features that occur. The CZCS image on 14 July (22-2) shows the 

 waters around Vancouver Island. B.C.. Canada, on a clear day during the 

 summer period when coastal upwelling leads to high productivity on the 

 continental shelf. This image is remarkable in the clarity with which the 

 resulting pigmented water has traced out interesting features of the coastal 

 dynamics. The locator map (22-1) provides ocean features and bottom 

 contours at 200- and 2.000-m depths for comparison. It should be noted 

 that most of the features visible in the image have very low temperature 

 contrast with the surrounding water, so that thermal infrared images will 

 often not record them. 



The waters on the continental shelf and in the straits of Georgia and 

 Juan de Fuca show high pigment levels (>5 mg m"-'), although apparent 

 values in the image will be increased by silt from coastal runoff. This 

 increase is especially evident at the mouth of the Fraser River (see 

 enlargement, 23-1). whose plume can be faintly seen in the image. Isolated 

 brighter areas in Queen Charlotte Sound, north of Vancouver Island, 

 correspond roughly to the positions of shoals S. The fresher surface water 

 from this sound flows southward F down the west coast of Vancouver 

 Island where it is entrained in the cyclonic eddy El. CZCS imagery for the 

 next two days (not shown) indicate that the eddy remained stationary 

 (<3 km translational motion per day) while rotational velocities of 

 20 km day' (20 cm s"') are implied by displacement of surface features. 

 The apparent center of the eddy is located at 49°32'±r N and 

 I27°33'±2' W. The diameter of the eddy is about 20 km. but 

 counterclockwise rotation can be seen over an area at least twice this large. 

 Thomson (1984) has studied an eddy at this same location that was 

 observed in September 1980 in hydrographic data, but which was not 

 present in July of that year. He suggests that the position of the eddy is 

 ultimately related to bottom topography, and that its formation is 

 triggered by baroclinic instability due to the vertical shear between the 

 opposing southward surface flow and the northward subsurface flow in 

 the California Undercurrent along the continental slope. In the present 

 case, the eddy may have formed from the horizontal shear between the 

 temporary northward flow of shelf water, caused by the strong wind 

 reversal that occurred a few days earlier, and the southward flow further 

 offshore. 



The increasing width of the band of pigmented water (>2 mg m^') to the 

 southeast of this eddy corresponds to the widening continental shelf (see 

 200-m contour. 23- 1 ). Presence of a second eddy E2 (23- 1 ). on the shelf off 

 the mouth of the Strait of Juan de Fuca. can be traced by the motion of 

 features between successive days. This eddy appears to be a regular feature 

 of the summer circulation (Freeland and Denman. 1982). At its center, 

 water is brought up to the surface from the California Undercurrent 

 through a branch of the Juan de Fuca Canyon. 



A clearly visible hammerhead feature H is moving to the west at about 

 20 km day" I. The length of the head of this feature is increasing at a rate 

 that, on 14 July, implies an age of about five days. Its starting location, at 

 the edge of the continental shelf near the boundary of the eddy E2, has a 

 similar pigment concentration. The narrow trail of high-pigment water to 

 the east of this feature appears to trace out its path over this time period, 

 ahhough rotation of the eddy E2 has moved this trail to the south and 

 lengthened it. 



The starting date of 9 July deduced for the feature H corresponds to a 



wMmmmm. 

 Legend: 



El, E2: eddies 

 F fresh water 



H hammerhead feature 

 S shoal area 



22-1. Locator Map for 22-2 and 23-1 



ORBIT 03643 14JULT9 195313 TO 195513 GMT OhIH l-t=l THPE-HOLD OFF TILT hIIGLE li^< 

 GRID CEtlTEP DhTh: 195413 liMT 48-e.5H 131 -iZN 'lUtl EL 61 hZ 15Z ROLL -O-Z PITCH O-l VhII iyr 



FlLlEIIT 



ALGORITHMS: 01 00 00 00 00 02 01 00 00 00 



969 ?rAN LIME? PROCESSED ?CAN LIME? MliSIMG 



1.28 2.11 :•■•■: 5.«9 9-35 11-98 25.i5 



CIP VER CRT NO DPS VER SPEC # FRAME # 

 ZEZZ480Z V8Z05O1 FT32040 ZGZZ4804 



22-2. Nimbus-7. Orbit J643. 14 July 1979. CZCS Pigment Image 



22 



