We're now going to discuss what creates surface currents and how they're organized. It's going to sound obvious, but surface currents are those currents which occur within and above the peak decline at the surface, to a depth of about 1 kilometer. Because of this, surface currents affect only about 10% of the ocean's volume. These currents form as a result of the wind blowing across the water surface. The friction from the air molecules moving over the water molecules creates waves and subsequently a surface current.
As the water molecules complete their orbital motion in a wave, they move a small distance forward in the direction of the wave. It's not an extremely efficient transfer of energy. Only about 2% of the energy in the wind is transferred into the water.
So a 50 knot wind would only create a 1 knot surface current. Knots. Speaking of knots, what am I speaking about?
When we use the term knot, we're referring to 1 nautical mile per hour. A nautical mile is 1.85 kilometers or 1.15 miles. So back to currents. What's creating the currents?
And they should follow the wind belts of the world, right? Yes, except that there are other factors that come into play here, mainly the continents. If there were no continental land masses, the ocean currents would follow the major wind belts of the world.
But it's not just the currents. It's not just the continents. Things like friction, gravity, and the Coriolis effect come and play a role. The Coriolis effect has a huge role in defining the surface currents of the ocean. In the green arrows on this slide, you'll see the major wind belts of the earth.
The trade winds blowing toward the west in light green and the prevailing westerlies blowing toward the east in dark green. As a result of those winds and other factors, surface currents, indicated by the light blue arrows, form large circular tracks in the ocean called gyres. And we're going to discuss these circular forms in the upcoming slides.
So these large circular or oval patterns are called gyres. A gyre is a large, horizontal, circular, moving loop of water. Because these currents and gyres exist in different parts of the ocean, we can classify them accordingly. Subtropical gyres, equatorial currents, and subpolar gyres are found out in the ocean. There are five subtropical gyres in the world's ocean.
The North Atlantic, the South Atlantic, the North Pacific, the South Pacific, and the Indian Ocean. The center of each of these subtropical gyres is located at a latitude of 30 degrees north or south, hence the name subtropical. The subtropical gyres rotate in a clockwise pattern in the northern hemisphere and a counterclockwise pattern in the southern hemisphere.
But we'll now break it down a bit further and talk about the different currents that comprise a single gyre. Each subtropical gyre has four main currents that flow into each other. These are the equatorial currents, the western boundary currents, the northern or southern boundary currents, and the eastern boundary currents.
We'll start with the equatorial currents. The trade winds, blowing from east to west, set the water in motion toward the west, parallel to the equator. Depending upon which hemisphere the current exists in, the current is either called a northern or southern equatorial current. The next current in the subtropical gyre is the western boundary current.
These currents occur when the equatorial current meets a landmass on the western side of an ocean basin. Due to Coriolis, the current deflects away from the equator and to the right in the northern hemisphere and to the left in the southern hemisphere. Western boundary currents carry warm water from the equatorial region up to higher latitudes and can often impact the climate of these higher latitudes.
I know it sounds obvious, but I'll say it anyway. Western boundary currents are named because of where they occur relative to the center of the gyre. We now move to the northern or southern boundary current. The name again depends upon which hemisphere the gyre occurs in. It's a northern boundary current if it occurs in the northern hemisphere and a southern if it occurs in the southern hemisphere.
Because of the prevailing westerlies blowing from the west toward the east, the water is directed toward the east along the boundary current. Our last current is the eastern boundary current. As the currents make their way east in the northern or southern boundary current, they run into continental land masses and predominantly deflect toward the equator due to the Coriolis effect. The eastern boundary current helps carry cooler water from high latitudes toward the lower latitudes at the equator.
If we take another look at our global ocean gyres with the different currents all labeled, You can see the basic schematic of the four currents we just described. Because the ocean basins are all different sizes and shapes though, the actual currents in them look a little different than our idealized view. The North Pacific subtropical gyre is most representative of the idealized gyre that we just described. But you can see the four currents in all of the subtropical gyres.
the northern or southern boundary currents aren't necessarily called the northern boundary current, just as the western and eastern boundary currents aren't also labeled as such. You don't need to memorize the names of all the currents on this slide. Just know that if you saw this picture later, could you pick out which currents are western boundary currents?
or eastern, or northern, or southern? You may have noticed a current at the equator that appears to be flowing in the opposite direction of the equatorial currents. How can this be? It turns out that with the large volume of water being moved westward, the water tends to pile up on the western side of the ocean basin. Because it's located at the equator, Coriolis has no effect on this water.
What happens is this water then tends to flow downhill due to gravity and make its way back toward the east. This creates a narrow current flowing in the opposite direction of the equatorial currents, called the equatorial countercurrent. It's most pronounced in the Pacific Ocean due to the arrangement of land in the western Pacific.
The land ends up trapping the water better and creating a larger difference in sea level, therefore creating the countercurrent. You may have also noticed smaller gyres toward the poles. These are called subpolar gyres.
The surface currents moving eastward as a result of the prevailing westerlies are driven west by the polar easterlies once they reach subpolar latitudes. These gyres rotate in an opposite direction from subtropical gyres in the same hemisphere, kind of like interlocking gears, and similar to the atmospheric circulation cells we saw earlier. There are fewer subpolar gyres than subtropical gyres, and they are smaller in size. These are located in the Atlantic Ocean between Europe and Greenland, south of Alaska and the Pacific, and two located off Antarctica.