1 00:00:00,560 --> 00:00:11,720 In 1992, a cargo ship carrying bath toys got caught in a storm. 2 00:00:12,339 --> 00:00:14,300 Shipping containers washed overboard, 3 00:00:14,759 --> 00:00:20,339 and the waves swept 28,000 rubber ducks and other toys into the North Pacific. 4 00:00:20,820 --> 00:00:22,239 But they didn't stick together. 5 00:00:22,739 --> 00:00:23,579 Quite the opposite. 6 00:00:24,059 --> 00:00:26,940 The ducks have since washed up all over the world, 7 00:00:26,940 --> 00:00:32,759 and researchers have used their paths to chart a better understanding of ocean currents. 8 00:00:32,759 --> 00:00:40,399 Ocean currents are driven by a range of sources—the wind, tides, changes in water density, and 9 00:00:40,399 --> 00:00:43,259 the rotation of the Earth. 10 00:00:43,259 --> 00:00:48,119 The topography of the ocean floor and the shoreline modifies those motions, causing 11 00:00:48,119 --> 00:00:52,520 currents to speed up, slow down, or change direction. 12 00:00:52,520 --> 00:00:58,659 Ocean currents fall into two main categories—surface currents and deep ocean currents. 13 00:00:58,659 --> 00:01:03,159 Surface currents control the motion of the top 10% of the ocean's water, while deep 14 00:01:03,159 --> 00:01:06,319 ocean currents mobilize the other 90%. 15 00:01:06,319 --> 00:01:11,040 Though they have different causes, surface and deep ocean currents influence each other 16 00:01:11,040 --> 00:01:15,420 in an intricate dance that keeps the entire ocean moving. 17 00:01:15,420 --> 00:01:20,579 Near the shore, surface currents are driven by both the wind and tides, which draw water 18 00:01:20,579 --> 00:01:24,420 back and forth as the water level falls and rises. 19 00:01:24,420 --> 00:01:29,980 Meanwhile, in the open ocean, wind is the major force behind surface currents. 20 00:01:29,980 --> 00:01:34,739 As wind blows over the ocean, it drags the top layers of water along with it. 21 00:01:34,739 --> 00:01:39,900 That moving water pulls on the layers underneath, and those pull on the ones beneath them. 22 00:01:39,900 --> 00:01:47,219 In fact, water as deep as 400 meters is still affected by the wind at the ocean's surface. 23 00:01:47,219 --> 00:01:51,540 If you zoom out to look at the patterns of surface currents all over the Earth, you'll 24 00:01:51,540 --> 00:01:55,040 we'll see that they form big loops called gyres, 25 00:01:55,040 --> 00:01:57,819 which travel clockwise in the northern hemisphere 26 00:01:57,819 --> 00:02:00,640 and counterclockwise in the southern hemisphere. 27 00:02:00,640 --> 00:02:04,420 That's because of the way the Earth's rotation affects the wind patterns 28 00:02:04,420 --> 00:02:06,920 that give rise to these currents. 29 00:02:06,920 --> 00:02:08,500 If the Earth didn't rotate, 30 00:02:08,500 --> 00:02:10,919 air and water would simply move back and forth 31 00:02:10,919 --> 00:02:14,860 between low pressure at the equator and high pressure at the poles. 32 00:02:14,860 --> 00:02:16,539 But as the Earth spins, 33 00:02:16,539 --> 00:02:21,039 air moving from the equator to the north pole is deflected eastward, 34 00:02:21,039 --> 00:02:24,719 and air moving back down is deflected westward. 35 00:02:24,719 --> 00:02:27,520 The mirror image happens in the southern hemisphere 36 00:02:27,520 --> 00:02:32,979 so that the major streams of wind form loop-like patterns around the ocean basins. 37 00:02:32,979 --> 00:02:35,860 This is called the Coriolis effect. 38 00:02:35,860 --> 00:02:40,319 The winds push the ocean beneath them into the same rotating gyres, 39 00:02:40,319 --> 00:02:43,960 and because water holds onto heat more effectively than air, 40 00:02:43,960 --> 00:02:48,500 these currents help redistribute warmth around the globe. 41 00:02:48,500 --> 00:02:53,740 Unlike surface currents, deep ocean currents are driven primarily by changes in the density 42 00:02:53,740 --> 00:02:55,240 of seawater. 43 00:02:55,240 --> 00:02:58,680 As water moves towards the North Pole, it gets colder. 44 00:02:58,680 --> 00:03:04,080 It also has a higher concentration of salt, because the ice crystals that form trap water 45 00:03:04,080 --> 00:03:06,159 while leaving salt behind. 46 00:03:06,159 --> 00:03:11,819 This cold, salty water is more dense, so it sinks, and warmer surface water takes its 47 00:03:11,819 --> 00:03:17,900 place, setting up a vertical current called thermohaline circulation. 48 00:03:17,900 --> 00:03:23,139 circulation of deep water and wind-driven surface currents combine to form a winding 49 00:03:23,139 --> 00:03:26,539 loop called the global conveyor belt. 50 00:03:26,539 --> 00:03:30,819 As water moves from the depths of the ocean to the surface, it carries nutrients that 51 00:03:30,819 --> 00:03:35,879 nourish the microorganisms which form the base of many ocean food chains. 52 00:03:35,879 --> 00:03:41,379 The global conveyor belt is the longest current in the world, snaking all around the globe, 53 00:03:41,379 --> 00:03:45,099 but it only moves a few centimeters per second. 54 00:03:45,099 --> 00:03:49,599 It could take a drop of water a thousand years to make the full trip. 55 00:03:49,599 --> 00:03:55,099 However, rising sea temperatures are causing the conveyor belt to seemingly slow down. 56 00:03:55,099 --> 00:03:59,860 Models show this causing havoc with weather systems on both sides of the Atlantic, 57 00:03:59,860 --> 00:04:05,340 and no one knows what would happen if it continues to slow, or if it stopped altogether. 58 00:04:05,340 --> 00:04:09,340 The only way we'll be able to forecast correctly and prepare accordingly 59 00:04:09,340 --> 00:04:15,539 will be to continue to study currents and the powerful forces that shape them. 60 00:04:15,539 --> 00:04:20,100 What about the creatures that adapted to survive in these swirling seas? 61 00:04:20,100 --> 00:04:23,500 Take a closer look at some truly fascinating ocean dwellers 62 00:04:23,500 --> 00:04:25,800 with these two animations.