Ocean Circulation

Ocean Circulation Lecture Notes

Chapter Overview: Ocean Currents

  • Ocean currents are defined as moving loops of water in the oceans.
  • Each ocean basin has a unique circulation pattern.
  • Currents are essential to the redistribution of global heat.
  • Thermohaline circulation influences deep currents in the ocean.
  • Marine life is affected by ocean currents.
  • Ocean currents are explored as a significant source of energy.

Types of Ocean Currents

Surface Currents

  • Nature: Wind-driven and primarily exhibit horizontal motion.
  • Characteristics:
    • Affect only 10% of the ocean water on Earth.
    • Friction exists between wind and ocean surface, leading to water movement. Only about 2% of wind energy transfers to the ocean surface, resulting in surface currents moving more slowly than the corresponding winds.
    • Surface currents generally follow the Earth’s wind belt pattern.

Deep Currents

  • Nature: Driven by density differences caused by variations in temperature and salinity.
  • Motion: Include both vertical and horizontal movements.

Factors Affecting Surface Currents

  • Wind: Accelerates ocean currents through frictional drag, causing horizontal redistribution of water.
  • Gravity: Influences the flow and behavior of currents.
  • Friction: Works against movement, further affecting current dynamics.
  • Coriolis Effect: Causes deviations in current directions due to Earth’s rotation.
  • Distribution of Continents: The layout of continents affects the flow patterns within each ocean basin.

Wind Belts and Surface Current Movement

  • Surface currents are significantly influenced by wind belts, piling up water against continents and creating areas of high and low pressure.

Ocean Dynamic Topography

  • Definition: Ocean dynamic topography refers to the topographical features created on the ocean surface due to currents, resulting in both hills and valleys in sea surface height.
  • Representation: Colored regions indicate higher-than-normal sea levels (red) and lower-than-normal sea levels (purple/magenta).
  • Flow Direction: Indicated by white arrows on the diagrams; longer arrows imply faster flow rates with speeds reported in centimeters per second, typically at about 10 cm/sec.

Gyres

Definition and Structure

  • Gyres: Large, circular loops of moving water, five in total, centered around 30 degrees latitude. Each subtropical gyre is bounded by multiple currents:
    • Equatorial current
    • Western Boundary currents
    • Northern or Southern Boundary currents
    • Eastern Boundary currents

List of Gyres and Their Currents

  1. North Pacific Subtropical Gyre
    • Kuroshio Current, North Pacific Current
  2. South Pacific Subtropical Gyre
    • East Wind Drift, Antarctic Circumpolar Current
  3. North Atlantic Subtropical Gyre
    • Gulf Stream, North Atlantic Current
  4. South Atlantic Subtropical Gyre
    • Brazil Current, Benguela Current
  5. Indian Ocean Subtropical Gyre
    • Agulhas Current, Antarctic Circumpolar Current

Tracking Surface Currents

  • Method: Scientists tracked surface currents using a fleet of 30,000 rubber toys that fell off a cargo ship, aiding in the study of ocean currents.

The Ekman Spiral

  • Developed by V. Walfrid Ekman in 1905, the Ekman Spiral is a circulation model that explains the balance between friction and the Coriolis effect and describes the direction and flow of surface waters at varying depths.

Net Ekman Transport

  • Describes the average movement of surface waters, which is directed 90 degrees to the right in the Northern Hemisphere and 90 degrees to the left in the Southern Hemisphere.

Thermohaline Circulation

  • Definition: Deep ocean circulation driven by temperature and density differences.
  • Occurs below the pycnocline, constituting about 90% of all ocean water and characterized by slower velocities.

Characteristics of Thermohaline Circulation

  • Origin: Starts in high-latitude surface oceans where cooled dense surface water sinks.
  • Identification: Deep water masses can be categorized on temperature-salinity (T-S) diagrams by assessing their temperature, salinity, and resultant density.

Names of Deep Water Masses

  1. Antarctic Bottom Water (ABW)
  2. North Atlantic Deep Water (NADW)
  3. Antarctic Intermediate Water (AIW)
  4. Oceanic Common Water (OCW)
  • Cold surface seawater sinks at polar regions, and these waters move equatorward.

Vertical Mixing and Water Properties

  • High latitude areas lack thermocline and pycnocline, allowing for more vertical mixing. Cold, dense waters from Greenland and Antarctica contribute significantly to the ocean's thermohaline circulation.

Ocean Currents and Regional Climate Effects

  • Different ocean currents affect regional climates, especially those moving warm water, such as the Gulf Stream, which warms the east coast of the United States and influences the climate of northwestern Europe.

Atmospheric-Ocean Connections in the Pacific Ocean

Walker Circulation Cell

  • Normal Conditions: High pressure in eastern Pacific promotes southeast trade winds, pushing warm water to the western side.
  • Upwelling: Occurs on the eastern side off the coast of Peru to replace the displaced water.

El Niño – Southern Oscillation (ENSO)

  • Disruption: The Walker Cell can be disrupted, leading to a weakening of high pressure and trade winds, causing the warm pool to migrate eastward, creating conditions of downwelling and lower biological productivity.

La Niña – ENSO Cool Phase

  • Characterized by stronger pressure differences, stronger trade winds, and increased upwelling in the eastern Pacific, which also leads to a shallower thermocline and cooler seawater with higher biological productivity than normal conditions.

Conclusion on ENSO Events

  • El Niño events can occur every 2-10 years, lasting between 12-18 months and can be studied through sediment records. There is a potential connection between ENSO and the Pacific Decadal Oscillation, which is a long-term natural climate cycle lasting 20-30 years.