The Science of Upwelling: How the Ocean Surface “Randomly” Cools Off

Photo: greenseaupwelling.com

When walking onto the beach for a surf check west coast surfers are no strangers to asking each other “How’s the water temp?” It’s an essential question to ask if you haven’t been in the water in awhile, either to mentally prepare for, or altogether avoid, a nasty cold surprise that will lead to ice cream headaches and frozen hands and feet. The answer you aren’t hoping for is “it’s freezing…it’s upwelling out there.”

Now, the term west coast applies to more than just California; it applies to Chile, Peru, to the west coast of Africa and in fact, to the eastern boundaries of the world’s great oceans: the Pacific and the Atlantic oceans. So what exactly is upwelling, what causes it, and why does the water become so cold? You might be surprised at the answers to these questions and the science behind upwelling. The answer to how and why upwelling occurs lies in the action of wind on ocean surface waters, a fascinating phenomenon known as the Coriolis force and some physics.

The Coriolis force is a critical component in understanding the theory of wind-driven ocean currents. The Coriolis force is actually labeled an “apparent” force – a force that describes the apparent deflection of objects moving over the surface of the Earth, objects as diverse as a missile or, for the sake of our discussion, a parcel of water in the ocean. Let’s look at the example of the missile to illustrate the Coriolis force. Consider a missile fired northwards from the equator. As the missile leaves the launch site it flies northward at its firing velocity but also eastward at the same velocity as the surface of the Earth. (Use your mind’s eye to envision as the missile travels north, the Earth is turning eastward beneath the missile.) The eastward velocity of the surface of the Earth is greatest at the Equator and decreases toward the poles, so as the missile travels progressively northwards, the eastwards velocity of the Earth beneath it becomes less and less. As a result, the missile is moving not only northwards but also eastwards at a progressively greater rate. This apparent deflection of objects that are moving over the surface of the Earth, without being frictionally bound to the Earth, is explained in terms of the Coriolis force.

The story of the Coriolis force and the deflection of ocean currents begins in the 19th century. In the 1890’s, a Norwegian scientist named Fridtjof Nansen specially designed a vessel called Fram that would freeze in the arctic ice and drift with the ice for over a year. During this expedition Nansen observed that ice moving in response to the action of wind blowing over its surface was not actually parallel to the wind, but rather at an angle 20 to 40 degrees to the right of it. Subsequently, a Swedish scientist named Vagn Walfrid Ekman developed a theory for wind-driven ocean currents in order to explain Nansen’s observations. As scientists are forced to do in many instances, Ekman simplified the ocean to develop his theory. He considered a steady wind blowing over an ocean that was infinitely deep, infinitely wide and with no variations in water density. This hypothetical ocean consists of an infinite number of horizontal layers. The top layer is subjected to friction by the wind at its upper surface and to friction with the next layer down at its lower surface. The second layer is acted upon at its upper surface by friction with the top layer, and by friction with layer three at its lower surface and so on down through the water column. Because all the layers are moving the Coriolis force acts upon each of them.

By considering the balance of forces, the force of friction and the Coriolis force, on the infinite number of layers making up the hypothetical water column, Ekman deduced that the speed of the wind driven current decreases exponentially with depth. He further found that the direction of the current deviates 45 degrees anticyclonic (clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere) from the wind direction at the surface. Observations of wind-driven currents in the open ocean have shown that surface current direction does in fact deviate in a direction anticyclonic of the wind direction, although generally less that 45 degrees (there are several reasons for the difference between theoretical and observed, but that is more than we need to address here). The significant prediction of Ekman’s theory is the fact that the average motion of the wind-driven layer of the ocean is at right angles to the wind, to the right in the northern hemisphere and to the left in the southern hemisphere. The volume of water that is transported at right angles to the wind direction is known as Ekman transport.

Now, with an understanding of the Coriolis force and Ekman transport, coastal upwelling of subsurface waters in eastern ocean boundaries can be appreciated. Coastal upwelling is the result of a divergence of surface water away from the coastline, and subsurface waters rising to replace the water moved offshore by the wind. This subsurface water can be very cold. Because Ekman transport is in a direction 90 degrees anticyclonic of the wind direction, the most significant offshore transport of surface water – and the most upwelling – occurs in response to longshore equatorward winds, not offshore winds. As an example, California can experience consecutive days of strong northwest winds blowing across coastal waters resulting in upwelling and, subsequently, subjecting surfers to particularly cold water. Again, this northwest wind is predominantly a longshore, equatorward wind. And, since California is in the northern hemisphere the result is an offshore movement of surface water and cold subsurface water rising to replace the surface water moving offshore. Similarly, upwelling off the coast of northwest Africa occurs in response to the northeast Trade Winds. See Figure 1.

Figure 1. Diagram illustrating the principle of coastal upwelling in the northern Hemisphere. Currents, created by winds, are diverted by the Coriolis force, and this results in water being carried away from shore. Deep, cold water rises to replace these waters, hence coastal upwelling. [From: California Space Grant Consortium]

Finally, it is important to note that upwelling does not only occur in response to longshore winds. It may also occur on a local scale as a result of subsurface ocean currents being deflected by features on the seafloor. Who would have ever thought the explanation behind a cold surf after a few days of wind could be so interesting, complete with an effect due to the rotation of the Earth and oceanographic scientific theory developed in the 1800’s? Now when you are standing checking the waves and the subject of upwelling comes up amongst your surfing friends you can step in with a thorough explanation of the subject.

“Do you know what causes upwelling?”

“Well, there is this thing called the Coriolis force, and…”