Upwelling Explained

Larry De RidderWeather

Upwelling. In general terms upwelling means that deep water, usually cold and nutrient rich but oxygen deficient, is rising to the surface. Oceanographers generally recognize five types of upwelling, but this article will describe what we salmon fishermen most commonly experience, which is coastal upwelling.
Northern California is blessed (and sometimes seemingly cursed) by the generally northwest winds which affect our coast much of the year, particularly summer months. These winds drive the water masses which can make or break our salmon fishing seasons. The primary means by which winds affect our salmon fishing, aside from establishing whether the ocean is even safe for boating, is its influence on cold water upwelling which we often observe off Northern California. Understanding upwelling requires a background in a couple of causative physical phenomena.
 
Coriolis effect. The Coriolis Effect is an apparent deflection of moving objects when they are viewed from a rotating frame of reference. Take a moment and visualize yourself at the equator with a really large cannon, aimed north. Fire your imaginary cannon. Assuming you had really terrific eyesight and could watch your cannonball for a very long distance, the ball would appear to veer eastward (right) as it traveled north. The reason is that in addition to the northward movement of the ball, it also has a built-in motion to the east, imparted by the fact that it was fired from the equator, which is rotating eastward. As the ball travels north, it is passing over parts of the earth which are moving eastward progressively more slowly than the equator, and so your cannon ball is traveling east faster than the ground beneath it. The further north your cannon ball flies, the faster its eastward motion relative to the ground below. By the same token, a cannon ball fired south from Eureka would gradually appear to veer west (right), because it would be flying eastward based on the speed Eureka is moving, while the ground below its trajectory would be moving faster as the ball approached the equator, thus out-pacing your cannonball. Make sense? The bottom line is that anything moved in a north-south line carries with it the eastward momentum it acquired from its origination point, and in the northern hemisphere that means it will curve to the right. Thus the Coriolis Effect is caused by the rotation of the earth and the inertia of the mass experiencing the effect (our imaginary cannonball). Now let’s apply that to water masses.
 
Ekman Spiral (aka Ekman Transport). In the late 1800s and early 1900s it was noted that ice burgs travel at an angle to the direction of the wind, not straight downwind. Why would that be? Visualize a large body of water as a number of thin slices of water stacked atop one another, rather than as a single mass. Now keep in mind the Coriolis Effect. As winds pass over the water they transfer some of their energy and momentum to that surface layer of water – and that is all the wind directly affects. In the northern hemisphere, if the wind moves the water surface layer south, it carries it’s eastward momentum with it, and the water will veer very slightly to the right (westward). However, the surface layer of water affects the next “slice” of water below it, imparting energy and motion to that next layer, and inducing a further slight right-hand direction change. In fact, each horizontal “slice” in our mass of water will be given energy and motion by the water layer above, and impart it to the layer below, and at each level of this water mass the water will incur a slight tendency to move further to the right. The deeper in the water column you go, the less energy is transferred from above, but the greater the ultimate rightward motion. If you average out all the energy and motion transfers, the ultimate affect is that the overall mass of water affected by the wind will move 90 degrees to the right of the wind direction (again, northern hemisphere). Since 90% of an iceberg is underwater, it is carried in the direction the water mass is moving, not necessarily the direction the wind is blowing. You have just discovered the Ekman Spiral, or Ekman Transport, in a “perfect” body of water, with no confounding land masses.
 
Coastal Upwelling. Now let’s take what we’ve learned and apply it locally. As noted, our typical summer weather pattern is for winds from the north or northwest. As wind energy passes over the ocean and energy is transferred to the water, the Coriolis Effect will impart a right hand (westward) movement to the surface waters. The Ekman Spiral will transfer that energy further down and ultimately the upper layers of our local ocean will move west, offshore. Since the ocean is a fluid, something must fill the “vacancy” left by the westward flowing water mass. Since the coastal land mass prevents other surface water from filling the void, cold lower level waters rise to the surface to replace the water mass headed offshore. 
 
The next time you are offshore and find a “river” of surging water extending north and south, check your boat’s temperature gauge. It probably indicates the water temperature just dropped a degree or two. And by the way, get that line in the water, there are likely to be plankton, bait fish and salmon in the area!