Downwelling conditions off the Oregon coast


Some days in summer, wind direction changes from southward to northward (on these days, if you stand on a beach facing the ocean, the wind will blow into your left ear). In response to this wind reversal, the southward coastal current slows down and then it may turn to the north. As a combined effect of northward wind and the Earth rotation, surface water will also be displaced towards the coast, forcing cold water, previously upwelled near the coast, to retract back to the deeper ocean. This effect is called downwelling. During periods of summer downwelling, waters that are closer to the coast turn to the north faster than waters farther offshore. The one-day movie below shows trajectories of surface particles released in the large shelf area during a downwelling event. As the movie plays, note that particles move northward near the coast and still southward farther offshore. At the same time, all particles move toward the coast, and water near the coast is warmer during downwelling than during upwelling.

Question 1: In these downwelling conditions, what would happen to oil spilled on the ocean surface, midshelf off Newport or Florence? See answer below.




Answer to Question 1: A part of the oil slick that is closer to the coast would be transported to the north and a part of the slick that is farther offshore would be transported south, causing significant alongshore stretching of the contamination area. At the same time, this stretched oil slick would move toward the beaches.

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