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Satellite AVHRR observations of the intensification of the shelf break current during an upwelling event off Vancouver Island Staples, Gordon C.


AVHRR satellite imagery during July 1984 was used to investigate the spatial and temporal variability of the shelf break current during an upwelling event off Vancouver Island. The onset of the order 10-day upwelling event initially appears as a plume of cold water at the tip of Brooks Peninsula. As the winds increase in strength, the plume of cold water migrates equatorward and takes on a jet-like structure that remains centred just seaward along the shelf break. As the winds dramatically weaken near the end of the upwelling event, there is intense warming of the surface waters along the shelf, but a band of cold water persists along the shelf break. Horizontal scales calculated from thermal fronts in the AVHRR imagery show that along the shelf break, the cross-shore scale is about 20 km, which is consistent with the internal Rossby radius. Approximately 70 km offshore, another thermal front is present, and this front is thought to represent the offshore extent of the shelf break current. AVHRR measurements of the SST were correlated with in situ temperature measurements, and a r² value of around 0.85 was obtained for the upper 3 m of the water column. The temporal variations in the AVHRR SST structure were related to the temporal variations of the coastal wind field, and to temporal variations in temperature measurements obtained from coastal light stations and subsurface moorings. The long-shore velocity was estimated from the AVHRR imagery, and speeds that were calculated to be from 10 and 80cm/s, were in agreement with long-shore speeds of 20 to 30 cm/s measured by a subsurface current mooring, and long-shore speeds between 10 and 100 cm/s calculated using a time-independent barotropic model. Results indicate that wind and topography play important roles in the upwelling response, and under the right conditions, AVHRR observations of SST are representative of dynamical processes within the ocean. The band of cold water along the shelf break is thought to be due to the combined processes of advection and shelf break upwelling.

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