OC103 Lesson #15: Ocean Surface Currents Circulation in the oceans is ruled by two very different things: Global wind patterns drive currents on the ocean surface, while seawater density variations drive deep ocean circulation. This lesson covers the surface ocean currents, and the next one covers the deep circulation. The most prominent surface ocean currents are shown on the map below. Notice how the currents circulate around large areas of the ocean basins. Although these currents are wind-driven, it is not as simple as you might think (or hope). But before getting into that, let’s cover how we know where the currents go.Measuring Surface Ocean Currents There are several ways to monitor and measure the direction and speed of surface ocean currents. • In some areas oceanographers deployed buoys that are anchored to the bottom and have current meters attached that can measure the speed and direction of the passing current. It is not usually necessary to constantly measure the currents in one spot since they do not vary much from year to year, so this approach is only practical in areas where a buoy was going to be deployed anyway (say for weather or wave height monitoring). • The typical way to determine currents in various parts of the ocean is to do temporary experiments for a month or two by deploying neutrally buoyant floats that ride around wherever the currents take them. The location of the buoys can be monitored by satellite, so the paths and speeds of the currents can be determined. • A third method is similar to the neutrally buoyant float experiments, but is unplanned and usually accidental. This is called a floats of opportunity study, and consists of tracking the paths of a bunch of floating objects over the course of months or years. The floating objects are usually something that was accidentally spilled off a ship at some known point in the ocean at some known time (usually during a bad storm). By putting the word out among beachcombers that they should report the location and date that they find any of these objects floating or stranded on a beach, it is possible to plot out the path that the objects took as they floated along on the ocean currents. Two famous examples of floats of opportunity studies occurred when cargo ships transiting across the Pacific from Asia to North America lost containers full of objects that float. In one case a ship lost a container full of 29,000 plastic bathtub toys (including rubber duckies!), and in the other case a ship lost a container full of 60,000 Nike athletic shoes. A tragic example is the floating debris from the March 2011 tsunami in Japan. Debris washed out to sea by that tsunami started arriving at the Oregon coast a little over a year later. o The Story of the Lost Nikes: The map at right shows the path and locations of the lost Nikes, with the numbers on the map matching up to the various beaches and the dates they were found during different stages of their journey. o Location 1, May 1990: 60,000 shoes spilled from a cargo ship o Locations 2–8, 1990–91: Found on beaches from southeast Alaska to northern California o Location 9, 1993: Found on beaches in Hawaii o Location 10, 1994: Found on beaches in the Philippines o In tying the locations together it is obvious that the surface currents circulate clockwise around the North Pacific as shown by the arrows on the map.What Causes Surface Ocean Currents? Surface ocean currents are driven by wind, and winds are driven by the Sun warming Earth's surface. As Earth’s surface is heated, air is warmed, expands, and rises; when it reaches the upper atmosphere, the air cools and eventually sinks. This round-trip is called a “cell”. If Earth did not rotate, these winds would develop into simple cells that rise at the hot equator, sink at the cold poles, and travel along the surface directly back to the equator (see right side of figure at right). But because Earth rotates, the winds do not travel in straight lines. As the winds travel across Earth’s surface, the planet is rotating beneath them, so the winds appear to deviate from a straight line. This phenomenon of moving objects appearing to be deflected from a straight line on a rotating planet is a complicated effect called the Coriolis Deflection or Coriolis Effect (sometimes erroneously called the "Coriolis Force", but it is an effect of the observer's frame of reference, not a physical force). If you think about an airplane that flies due south from Sweden toward Africa (see figure at right), as the airplane flies south, the Earth is rotating underneath it (to the right in this overhead view). So that by the time the plane gets to the place where Africa was when it took off, Africa has moved to the east, and South America has rotated to where Africa used to be. If you look at the final flight path of the plane relative to the ground it looks curved because it is a combination of the plane's planned path plus how far its original destination moved during the flight. However, if you watched the airplane from space, you would have seen that it actually was flying due south as planned, it’s just that as it was flying Earth was rotating underneath it.The Coriolis Deflection Now when the plane from the previous slide was taxiing to the runway in Stockholm it was on the rotating Earth, so it was already moving to the east in the outer-space reference frame, but Africa is rotating to the east faster than Sweden is. How is that possible? Well as Earth rotates on its axis, every point on its surface makes a complete trip around the axis in one day. At points near the poles, the distance around is not very far, so points near the pole have a fairly short trip to make every day (maybe only a few hundreds or thousands of km, depending on how close they are to the pole; a point right next to the pole might only have a few feet to go around with every rotation of Earth). Meanwhile, a point at the equator has to go all the way around the full circumference of Earth (~40,000 km) in a day, so it has to travel much faster to make it around in a day. So the closer a point on Earth's surface is to the equator the faster it is moving (see the figure at right, for a graphic depiction of this). Based on this figure, Stockholm is spinning to the east at ~800 km/hr, while Lagos is spinning east at ~1500 km/hr. So when the airplane took off from Stockholm it was spinning east at the