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Hi, I'm Dr. Martins. Glad you found me.

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Hi, Dr. Martins. Dr. D told us that the density of water depends upon its temperature and salinity.

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But how does temperature make the water more dense?

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Cold water molecules are packed together more tightly than warm ones, making cold water denser.

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So do the density currents form near the north and south poles?

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Exactly. The cold air at the poles cools the surface water, making it denser and causing it to sink.

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What about salinity?

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When ice forms in polar waters, it leaves behind salt, which increases the salinity of surrounding water.

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The higher salt content makes the polar water denser.

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I learned that evaporation also makes water saltier.

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That's right. In areas such as the Mediterranean Sea, where there's little rainfall and lots of heat,

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evaporation removes fresh water and leaves extremely salty water.

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Where does the denser water go?

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The denser polar water sinks and creates the thermohaline circulation.

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Thermohaline?

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Thermal for temperature and haline for salinity.

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The cold, salty water masses that create this circulation form in only two places.

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The water mass sinking off Greenland is called the North Atlantic Deep Water, or NADW.

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The water mass sinking off Antarctica is called the Antarctic Bottom Water, or AABW.

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Where do these currents end up?

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They both sink down several kilometers while spreading out horizontally and moving along the bottom of the Atlantic Ocean.

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They eventually mix while circulating around Antarctica.

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Then they move into the deep Indian Ocean and finally into the Pacific.

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So the water from the poles may one day end up on a beach in Hawaii?

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I guess you can look at it like that.

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What goes down must come up.

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It takes the average water molecule a thousand years to make the complete cycle.

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Wow, that's a long time.

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Yes, we call the cycle the Global Conveyor Belt, and it has a large impact on climate.

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I went swimming once, and the water was really cold.

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My mom said it was an upwelling, but what's that?

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The coastal upwelling that you experienced is different from thermohaline circulation.

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It only brings up cold water from a few hundred meters deep.

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It's also not as cold as AABW or NADW.

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Coastal upwelling is generally caused by wind.

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Is upwelling important?

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Good question.

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Upwelling brings lots of nutrients up from deep water.

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We are researching the impact of nitrogen, a crucial nutrient, on Florida Keys reefs.

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How does upwelling affect the reef?

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It appears that there have been major changes in the health and community structure of coral reefs here in the Keys and throughout the world.

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By observing the nitrogen composition of sponges, we can determine the importance of different nitrogen sources.

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Wow, you've given us a lot to think about.

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Thanks, Dr. Martens.

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You're welcome. Good luck on your project.