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6.26: Deep Ocean Currents

Difficulty Level: Basic / At Grade Created by: CK-12
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Why do small fish live in upwelling zones?

Seawater is always on the move. Water that comes up to the surface from deep is rich with nutrients. Plankton gather where nutrients are plentiful. Small fish come to eat the plankton. Of course, larger fish and birds come to eat the smaller fish. So upwelling is responsible for an abundance of life.

Deep Currents

Currents also flow deep below the surface of the ocean. Deep currents are caused by differences in density at the top and bottom. More dense water takes up less space than less dense water. It has the same mass but less volume. Water that is more dense sinks. Less dense water rises. What can make water more dense?

Downwelling

Water becomes more dense when it is colder and when it has more salt. In the North Atlantic Ocean, cold winds chill the water at the surface. Sea ice grows in this cold water, but ice is created from fresh water. The salt is left behind in the seawater. This cold, salty water is very dense, so it sinks to the bottom of the North Atlantic. Downwelling can take place in other places where surface water becomes very dense (see Figure below).

Deep currents flow because of differences in density of ocean water.

When water sinks it pushes deep water along at the bottom of the ocean. This water circulates through all of the ocean basins in deep currents.

Upwelling

Sometimes deep ocean water rises to the surface. This is called upwelling. Figure below shows why it happens. Strong winds blow surface water away from shore. This allows deeper water to flow to the surface and take its place.

An upwelling occurs when deep ocean water rises to the surface.

When water comes up from the deep, it brings a lot of nutrients with it. Why is deep water so full of nutrients? Over time, dead organisms and other organic matter settle to the bottom water and collect. The nutrient-rich water that comes to the surface by upwelling supports many living things.

An animation of upwelling is seen here: http://oceanservice.noaa.gov/education/kits/currents/03coastal4.html.

Upwelling also takes place along the equator. Winds blow the surface water north and south. This leaves a void that deep water can upwell into. The nutrients rise to the surface and support a great deal of life in the equatorial oceans.

Vocabulary

  • deep currents: Ocean currents that travel vertically up and down. This water also travels deeper in the ocean.
  • downwelling: Ocean water that sinks due to higher density.
  • upwelling: Cold, nutrient-rich water that rises from oceanic depths.

Summary

  • Cooling or evaporation of fresh water from the sea surface makes surface water dense. This causes the surface water to undergo downwelling.
  • Downwelling of cold, dense water pushes deep water along. This drives thermohaline circulation.
  • Upwelling takes place at some coastlines or along the equator. Upwelling brings cool, nutrient-rich water to the surface.

Practice

Use this resource to answer the questions that follow.

Ocean Odyessey-Density Current at http://www.youtube.com/watch?v=FuOX23yXhZ8 (2:39)

  1. How does temperature affect the density of water?
  2. How does salinity affect density?
  3. What does the density difference create?
  4. Where is the NADW?
  5. Where is the AABW?
  6. How long does it take a water molecule to complete the circuit of the global conveyor belt?
  7. What is an upwelling? What does it do?

Review

  1. Why is upwelling important?
  2. What causes downwelling?
  3. How are deep currents created?

Image Attributions

Description

Difficulty Level:

Basic

Grades:

6 , 7 , 8

Date Created:

Jan 04, 2013

Last Modified:

Feb 03, 2014
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\n Front Matter\n

\n

\n Foreword\n

\n

\n The study of Earth Science is the study of Planet Earth. Middle School Earth Science Concepts presents Earth Science as a set of concepts, with each concept centered around a specific category, such as the Earth's crust or plate tectonics. Each concept is comprised of a series of lessons, with each lesson focusing on one specific topic.\n

\n

\n Preface\n

\n

\n Dedication\n

\n

\n Chapters\n

\n

\n \"\"\n

\n
\n \n
\n

\n Back Matter\n

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Original text