Earth's Ocean Currents

The Movement of Ocean Water

SECTION 1: Currents

After Reading Objectives

• After reading this section, you should be able to answer the following questions:

  • What factors affect ocean currents?

  • Why are ocean currents important?

What Are Surface Currents?

• Definition: Surface currents are horizontal, streamlike movements of water located at or near the surface of the ocean.

• Depth & Length: Can extend to several hundred meters in depth and be as long as several thousand kilometers.

• Main Factors Affecting Surface Currents:

  1. Global Winds: Winds push the water across the ocean's surface, creating currents.

  2. Coriolis Effect: A phenomenon resulting from Earth's rotation that affects the direction of currents.

  3. Continental Deflections: Surface currents are deflected when they meet land masses.

• Example: The Gulf Stream is one of the largest surface currents, transporting at least 25 times more water than all the rivers combined annually.

Global Winds

• Mechanism: Surface winds push water, similar to blowing across a bowl of hot soup.

• Wind Directions:

  • Near the equator: Winds primarily blow from east to west.

  • Between latitudes of 30° and 60°: Winds predominantly blow from west to east.

• Implications for Surface Currents: The direction and strength of these winds are crucial for generating surface currents.

Continental Deflections

• Impact of Land: If Earth had only water, surface currents would flow uniformly. However, continents (covering about one-third of Earth's surface) cause currents to deflect.

• Example: The South Equatorial Current flows toward South America and is deflected southward upon reaching the continent.

The Coriolis Effect

• Definition: The Coriolis effect is the deflection of moving objects (e.g., wind, water) from their straight path due to Earth's rotation.

• Mechanism: The equatorial regions rotate faster than polar regions. When moving:

  • From the poles to the equator: Deflects to the west.

  • From the equator to the poles: Deflects to the east.

• Significance: Most noticeable in fast-moving or long-distance objects; negligible effect for short distances or slower objects.

How Surface Currents Distribute Heat

• Heat Transfer: Surface currents move heat from tropical regions to other parts of the ocean and atmosphere.

  • Example: Water near the equator absorbs heat and then carries it elsewhere.

What Are Deep Currents?

• Definition: Movements of ocean water that occur far below the surface, not driven by wind.

• Control Mechanism: Governed by the density of water, which is influenced by:

  1. Temperature: Cold water is denser than warm water.

  2. Salinity: High salinity increases water density.

• Formation: Deep currents form when the density of ocean water increases, causing it to sink.

Factors Increasing Density of Ocean Water

• 1. Decreasing Temperature:

  • Near polar regions, heat transfers from water to the cooler air, making water denser.

• 2. Increasing Salinity Through Freezing:

  • When seawater freezes, the salts remain in the unfrozen water, increasing its salinity and density.

• 3. Increasing Salinity Through Evaporation:

  • Salt remains in the water when it evaporates, increasing salinity and density.

Main Deep Currents

• Antarctic Bottom Water: The densest water in the ocean, forming near Antarctica.

• North Atlantic Deep Water: Less dense than Antarctic Bottom Water and remains above it.

• Relationship Between Currents: Warm surface currents can cool and become denser, sinking to become cold deep currents:

  • Cycle: Warm water moves from the equator to the poles, cools, sinks, and creates deep currents that flow toward the equator.

Section Vocabulary

• Coriolis effect: The curving of the path of a moving object from an otherwise straight path due to Earth's rotation.

• Deep current: A streamlike movement of ocean water located far below the surface.

• Ocean current: A movement of ocean water that follows a regular pattern.

• Surface current: A horizontal movement of ocean water caused by wind, occurring at or near the ocean's surface.

Review Questions

1. Identify what causes surface currents.

2. Identify what causes deep currents.

3. Describe the three factors that control the path of a surface current.

4. List three ways that the density of ocean water can increase.

5. Explain what causes the Coriolis effect.

6. Discuss which type of water is more dense: cold, salty water or warm, less salty water, with an explanation.

7. Predict what paths the ocean's surface currents would take if there were no continents on Earth.