this video is going to describe the actions of service currents in the ocean remember when we were talking about what causes seasons on the earth and we talked about how the difference in incoming solar radiation between different latitudes caused an energy imbalance on earth we have said that heat transport had to happen between the equator and the pole in order for the global temperatures to remain balanced it turns out that 30% of global heat transport is accomplished by ocean currents this is one reason that the study of ocean currents is so important to global climate this map should look familiar to you this map shows global surface winds now do we care about these when we're studying ocean currents well it turns out that surface ocean currents are caused by surface winds the winds drag the top layer of the ocean and create our major ocean currents which you see in this map red ocean currents are warm currents blue ocean currents are cold currents take a moment to look at the map you might even want to pause the video a couple of things to notice first of all we notice that in our major ocean basins we have these big circular ocean currents that we have gyres gy r e in the northern hemisphere this gyre circulation moves clockwise and in the southern hemisphere the gyre circulation moves counterclockwise at the equator we have ocean currents that are generally moving straight from east to west and from west to east with three equatorial currents when directly on the equator that's moving from west to east and two that are right off the equator moving east to west a couple of ocean currents that you should be familiar with are the California Current which happens on the west coast of the United States moving north to south offshore from California the Alaska Current which also occurs on our coast line moving north up toward Alaska and the Gulf Stream which happy which exists on the east coast of the United States moving from the Gulf of Mexico along the coast and up out into the northern Atlantic the continuation of the Gulf Stream the North Atlantic Current and the North Atlantic drift go all the way toward Norway and the western part of Europe there are many other currents to notice take a moment to look for your favorites did you notice that there's a circumpolar current that goes all the way around Antarctica without a break so how do these currents form these currents formed by wind stress wind is blowing across the water and just like you do when you blow out a cup of coffee and create a small wave in your cup of coffee the winds blow on the ocean and make large-scale currents in the water the wind stress is accomplishing the same type of force as the pressure gradient force that we discussed in the atmosphere blowing the water from one place to another place however we still have the Coriolis effect in the ocean also which deflects water to the right of the direction the wind is blowing at the surface of the ocean the Coriolis effect actually deflects the water at a 45-degree angle to the right in the northern hemisphere as a side note in the southern hemisphere the Coriolis effect is still deflecting water to the left these two pictures show what happens both at the surface and beneath the surface at the surface again the surface ocean current is going to move at a 45 degree angle to the right of the surface wind current but then as you move down and successively deeper layers of ocean water the currents will turn so the top layer of ocean water is going to exert a drag force on the next layer of ocean water that layer of the ocean will exert a drag force on the next deepest layer and so on and so on until you get down to the bottom of the echo of the layer that is affected by the wind which scientists called the Ekman layer if you add up the direction of all of these different currents you can determine the overall direction that water is moving in this top layer of the ocean which we call the Ekman transport the Ekman transport is 90 degrees to the right of the wind in the northern hemisphere and 90 degrees to the left of the wind in the southern hemisphere the diagram on the Left shows all three dimensions the diagram on the right shows a bird's eye view looking down on the ocean and we can see that if the wind is blowing from south to north the surface ocean current is at a 45 degree angle to the right therefore the surface current is directed from Southwest to Northeast and as you continue to move in deeper and deeper layers of the ocean the current will continue to deflect to the right and also get damped out by friction getting shorter and shorter if the wind is blowing from south to north and like in this example the Ekman transport will move from west to east and a 90 degree angle to the right of the wind let's look at an example so the picture of the earth I have drawn here has two red arrows that show the generalized directions of the Tradewinds on the equator remember at the equator winds tend to move tend to blow from east to west our easterly trade winds because of the because the Ekman transport is 90 degrees to the right of the wind in the northern hemisphere that means Ekman transport is going to move northward in the northern hemisphere in the southern hemisphere Ekman transport moves 90 degrees to the left of the wind and therefore transport will be southward if you're standing on the equator there's going to be Ekman transport to the north in the northern hemisphere and to the south and the southern hemisphere meaning are the Ekman transport is moving away from you on the equator we call this divergence a place where the ocean waters are all moving away from one particular location now I want you to do an example on your own I want you to use art ocean gyres and figure out if we will find convergence or divergence at the center of the gyres to do this exercise I want you to use the wind arrows that I've drawn on the map for you so you'll do the northern hemisphere Atlantic gyre and the southern hemisphere Atlantic gyre separately use the wind direction to figure out which way the water transport is going to go remember transport of water is 90 degrees to the right of the wind in the northern hemisphere and 90 degrees to the left of the wind in the southern hemisphere pause the slide and see if you can figure it out on your own before you continue the answer is there is convergence in the center of the gyres we already said that the trade winds along the equator have transport associated with them that's off the equator so for our northern hemisphere gyre we have transport into the gyre from the south near the equator but we also have transport into the gyre from the north the fact that there's convergence into these ocean gyres means that anything found in the ocean will tend to collect there if you've ever heard of the Great Pacific Garbage Patch you know that this is exactly what happens garbage actually does collect in the center of our ocean gyres garbage that comes from land also from ships all finds its way into the gyres and we actually have some patches of garbage in the ocean that we need to clean up here's an article about cleaning up the plastic in the ocean that you might find interesting one of the interesting things about the the garbage patch in the Pacific as well as in the other ocean gyres is that most of those pieces of plastic that are found there are very very tiny there are only about 15 milligrams of weight which is the weight of a grass seed these usually come from nurdles which are common plastic debris used in the manufacturing of other plastic items and 5.5 quadrillion per year are put into the ocean which is the equivalent of about 250 billion pounds of plastic these tiny pieces of plastic look like fish eggs to zooplankton fish and birds and in the garbage play patch plastic outweighs plankton every in a ratio of about 6 to 1 if you want to see plankton eating plastic because it thinks it's a fish egg you should watch the YouTube video that I've linked here the important thing to think about is who eats the plankton so getting back to out the physics of our ocean currents let's talk about what happens when we have divergence or convergence in the ocean if we have a place where there's divergence in the ocean meaning I the ocean water is moving away from one particular spot we know that we're not going to form a hole in the ocean it just isn't going to happen some water must move in to fill in the gap to replace the water that's moving away but where does it come from the only place that can come from is below so in the case where you have divergence happening at the surface we also have a phenomenon called upwelling where water moves from the deeper ocean up into the shallower ocean to replace the water moving away the opposite happens in an area of surface convergence where the ocean can't just pile up in one spot and instead you have downwelling where rather than moves downward in the water column let's do another practice problem if we have northerly winds in the summer along the coast of California why do we have upwelling so using the fact that the winds along the coast of California blow from north to south I want you to decide which way Ekman transport is going to go Ekman transport moves water offshore to the west if you're having trouble understanding this try to imagine that you are walking along the wind arrow walking from north to south and you need to make a 90-degree turn to the right which way would you be headed you would be headed offshore out into the Pacific this is the same way that the water travels after it's been affected by the wind in the diagram on the right the purple arrow is showing the wind moving from north to south and the black thin arrow is showing the acumen transport offshore to the west once that transport of water to the west happens we have an area of divergence because the other side is the coastline so there's no water to fill in the gap therefore water must come up from the bottom of the ocean through the process of upwelling this is one reason the ocean on the west coast can get so incredibly cold because that ocean water coming up from the bottom is much colder than the surface water that's been heated by the Sun most recently here is an example of how cold upwelling water can be in Southern Oregon we can see that the sea surface temperature in Northern California and Southern Oregon can be more than 10 degrees colder than the ocean water offshore you can see this in the contour lines in the image on the left or in the shading in the image on the right one last important point about our surface ocean currents is that when we're talking about our oceanic gyres the flow around the gyres is not symmetric so some of the currents are stronger than other currents in order to label which currents are stronger and which are weaker we have to be able to identify where they're located in the ocean basin the Gulf Stream and the canary current are two boundary currents in the Atlantic Ocean the Kuroshio Current and the California Current are two boundary currents in the Pacific Ocean we label ocean currents based on their position with the ocean so the Gulf Stream is considered a western boundary current because it's on the west side of the Atlantic Ocean sometimes students get this confused because the Gulf Stream is found on the eastern coast of the United States but when you're naming ocean currents you have to name them relative to the ocean not to the land so the Gulf Stream is considered a western boundary current and the California Current is considered an eastern boundary current now that we've got that straight we can move on western boundary currents tend to be narrow and fast eastern boundary currents are wide and slow this diagram shows the Atlantic Ocean and it shows the Gulf Stream which is the western boundary current which is narrow and fast and the Calif are the the canary current which is wide and slow the Gulf Stream is about ten times faster than the canary current or any other eastern boundary current here's a picture of the Gulf Stream and you can actually see how narrow it is in the diagram the Gulf Stream is represented by the warmer water represented by the red color whereas the colder water as shown in the blue and purple colors and you can see how this particular ocean current is even visible just by this narrow band of warm waters moving along the coast line from south to north okay to wrap up this unit on surface currents I want you to test yourself number one if the wind is blowing from south to north which way is the movement of surface water in the ocean which way is the Ekman transport number two in an area of convergence would you expect upwelling or downwelling why number three the Gulf Stream is an example of a Western or an eastern boundary current therefore it is fast or slow and wide or narrow pause the video and make your answers if the wind is blowing from south to north the movement of the surface water is Southwest to Northeast 45 degrees to the right of the wind the Ekman transport is directly from west to east 90 degrees to the right of the wind an area of convergence we see downwelling because all the water is moving together and must go down in the ocean and the Gulf Stream is an example of a western boundary current that is fast and narrow