Lab 6 Exercise - Thermohaline Circulation - Week of February 19th

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Name: _______________________________ LAB 6: THERMOHALINE CIRCULATION Week of February 19 th – February 23 rd CONVECTION EXPERIMENT Thermohaline circulation is a model that explains the methods by which water masses move vertically throughout the world’s oceans. This movement of water is driven by global density gradients dur to differences in surfacing heating and salinities. To illustrate how the “thermo” or temperature gradient based circulation cell works, follow the instructions below. Setup for Rotating Tank Experiment Step 1. Fill your rotating tank with 5cms of water. Step 2. Collect your ice-filled tins from your TA and place it in the center of your tank. Step 3: Turn on the Lego motor to begin the rotation. Tank must spin for at least 5 mins before your can begin your OCEAN CONVECTION experiment. Convection Experiment Step 4. Set up a plastic box as illustrated in the image below and make sure that it’s filled with water until the box is about 3/4 full. Step 5. Place one cup full of hot water beneath the tank as shown in the diagram below. Step 6. Place a small drop of red food dye on the bottom of the tank near one end of the box. Position one cup full of hot water beneath the red dye spot. Step 7. Obtain one colored ice cube from your TA. Carefully set the cube into the water at the opposite end of the box from the dye spot. Use a spoon to steady the cube 1

1. Position yourself at eye level with the box. Observe the movement of the water in relation to the ice cube and the drop of red dye. Sketch your observations in the diagram above using red and blue colored pencils. 2. In terms of density, why is the water from the ice cube moving the way that it does? 3. In terms of density, why is the water from the red dye moving the way that it does? OCEAN CONVECTION WITH ROTATION AND TEMPERATURE GRADIENT Now that you’ve determined how changes in temperature can drive vertical water movement, we’re going to represent how that phenomenon works on a rotating sphere that has differential heating between the poles and equators. 4. Observe the setup of your experiment. Where in the tank should the poles be, and where is the equator? 5. How will we create a temperature gradient representative of the equator-to-pole temperature gradients? 2

Rotating Tank Experiment Step 1: Place a few drops of blue or yellow dye onto the surface of the water in the tank near the can. Place a few drops of red or green dye near the outer radius of the tank. 6. Observe what happens to the dye. Then utilize the tank representations below to sketch the flow of the dye from the top and the side of the tank. Side View 7. Compare the movement of the dye between experiment 1 (convection) and 2 (rotating convection). 8. Contrast the movement of the dye between experiment 1 (convection) and 2 (rotating convection). 3 Top View

SALINITY EXPERIMENT It’s easy to imagine that very low-density water masses will layer themselves very close to the surface of the ocean. It’s also easy to imagine that very high-density water masses will layer themselves at the very bottom of the ocean. There can also be intermediate water masses that distribute themselves through the mid-range depths of the ocean. Proceed to the divided water tanks and follow these instructions. Your divided tank should be filled about 3/4 full of water, and the center divider should be inserted. Into the left side of the tank, add 3 drops of red food dye. Into the right side of the tank, add 3 drops of blue food dye and 1/4 teaspoon of salt. Use the stirring spoon to stir both sides of the tank, making sure that the colors are uniform, and the salt is dissolved. 9. Once the two sides, the red side and the blue side, are both independently well mixed, remove the center divider and allow them to interact. Wait for the red water mass and the blue water mass to settle and stabilize into two distinct, flat, stratified layers. Once the layers have stabilized, sketch what you see into the tank below using red and blue colored pencils. 10. Slide the center divider back into the tank. Both sides of the tank should still look the same and still be layered as you have just sketched. Take the stirring spoon and stir only the right side of the tank, essentially destroying the stratification of the red and blue layers and blending them into a single purple water mass. At this moment, the left half of the tank should still preserve a red layer and a blue layer, and the right side should be all purple. You’re now ready. Remove the center divider and observe how the purple water mass on the right interacts with the red and blue layers on the left. Once the layers have stabilized, sketch what you see into the tank below using red and blue colored pencils. Note: red and blue colored pencils make purple. 4

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