Materials Two transparent cups, exactly the same size An old CD Sandwich bag Scissors Food coloring any color Tap water Microwave oven Ice cubes Thermometer Paper towels for water spills Adult helper Tray Table salt optional Preparation Use the scissors to cut a long strip off the sandwich bag, wide enough to fully cover the hole in the middle of the CD. Place the tray on a work area that can tolerate water spills. Fill both cups with tap water.
Add ice cubes to one of the cups to cool down the water. Once it is cold, remove the ice cubes and fill the cup to the brim with more tap water.
Put the other cup into the microwave for about one minute. If it is too hot, let it cool down until you can pick up the cup. Make sure it is filled to the brim with water. Add a couple of drops of food coloring to the hot water. Set the cup with hot water into the tray. With the thermometer, measure the temperature of both the hot and cold water. Procedure Place the cut plastic strip from the sandwich bag onto the CD so the hole is completely blocked, and part of the plastic strip extends beyond the CD.
Put the CD on top of the cold cup with the plastic strip facing toward the cold water. Then pick up the cup with cold water and the CD on top and slowly turn it upside-down, pressing on the CD to avoid any spillage.
You might need an adult to help you with this step. Place the upside-down cup with cold water on top of the cup with hot water so the CD is separating both cups.
Once you have placed both cups together, what happens? Do you see any water mixing yet? What happens when the hole is open? Do you see any water movement? If yes, in which direction does the water move?
Can you describe your observations? Observe both cups for about 10 minutes. Do you see any changes happening in the top or bottom cup? If yes, how does the water change? After 10 minutes carefully separate both cups again by lifting the CD and the upper cup and inverting both again.
You might want to ask an adult to help you with that. Currents may also be caused by density differences in water masses due to temperature thermo and salinity haline variations via a process known as thermohaline circulation. These currents move water masses through the deep ocean—taking nutrients, oxygen, and heat with them.
Occasional events such as huge storms and underwater earthquakes can also trigger serious ocean currents, moving masses of water inland when they reach shallow water and coastlines. Earthquakes may also trigger rapid downslope movement of water-saturated sediments, creating strong turbidity currents.
The sun heats the Earth unevenly as it turns. These conditions actually affect the air and wind patterns on the planet surface. All of this moving air pushes the water in the ocean around.
Vervoort pulled down an Earth-shaped beach ball from the shelf in his office. He explained that winds blow in different directions. It also makes the winds in the southern hemisphere go to the left.
Ocean currents bend in the same way, caused by the Coriolis effect. The moving water can sometimes also act like a food delivery system. The water starts flowing in the same direction as the wind. But currents do not simply track the wind. Other things, including the shape of the coastline and the seafloor, and most importantly the rotation of the Earth, influence the path of surface currents.
In the Northern Hemisphere, for example, predictable winds called trade winds blow from east to west just above the equator. The winds pull surface water with them, creating currents. As these currents flow westward, the Coriolis effect —a force that results from the rotation of the Earth—deflects them. The currents then bend to the right, heading north.
At about 30 degrees north latitude, a different set of winds, the westerlies, push the currents back to the east, producing a closed clockwise loop.
The same thing happens below the equator, in the Southern Hemisphere, except that here the Coriolis effect bends surface currents to the left, producing a counter-clockwise loop. Large rotating currents that start near the equator are called subtropical gyres. These surface currents play an important role in moderating climate by transferring heat from the equator towards the poles. Subtropical gyres are also responsible for concentrating plastic trash in certain areas of the ocean.
In contrast to wind-driven surface currents, deep-ocean currents are caused by differences in water density. It all starts with surface currents carrying warm water north from the equator. The water cools as it moves into higher northern latitudes, and the more it cools, the denser it becomes.
In the North Atlantic Ocean, near Iceland, the water becomes so cold that sea ice starts to form. The salt naturally present in seawater does not become part of the ice, however. It is left behind in the ocean water that lies just under the ice, making that water extra salty and dense. The denser water sinks, and as it does, more ocean water moves in to fill the space it once occupied.
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