Surface Circulation

Surface Circulation

I. Warm and Cool Currents

Surface Current: horizontal flows of ocean water in the upper ~400 m, driven mainly by wind and influenced by Earth’s rotation (Coriolis effect) and continents.

  • transport heat, nutrients, and salinity

Gyres: Any large system of rotating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis Effect; planetary vorticity along with horizontal and vertical friction, which determine the circulation patterns from the wind curl (torque). 

  • Five major gyres: Indian Ocean Gyre (Majid Gyre), North Atlantic Gyre (Columbus Gyre), North Pacific Gyre (Turtle Gyre), South Atlantic Gyre (Navigator Gyre), and South Pacific Gyre (Heyerdahl Gyre).

Warm Surface Currents

  1. Characteristics

    • Originate near the equator.

    • Carry warm water toward higher latitudes.

    • Increase coastal temperatures.

  2. Examples

    • Gulf Stream – North Atlantic, warms Western Europe.

    • Kuroshio Current – North Pacific, warms Japan.

    • Brazil Current – South Atlantic, warms eastern South America.

  3. Effects

    • Moderates climate of nearby coastal regions.

    • Supports coral reefs and tropical ecosystems.

    • Transports heat poleward → influences global climate circulation.

Cool Surface Currents

  1. Characteristics

    • Originate in high-latitude or polar regions.

    • Carry cold water toward the equator.

    • Often flow along western coasts of continents.

  2. Examples

    • California Current – North Pacific, cools western North America.

    • Humboldt (Peru) Current – South Pacific, supports nutrient-rich upwelling.

    • Benguela Current – South Atlantic, cool and nutrient-rich.

  3. Effects

    • Lowers coastal temperatures.

    • Promotes upwelling, increasing nutrients and supporting high fish productivity.

    • Can contribute to arid coastal climates (e.g., deserts near cold currents).

Key Factors Influencing Surface Currents

  1. Wind Patterns – Trade winds, westerlies drive major gyres.

  2. Coriolis Effect – Deflects currents to the right in the Northern Hemisphere, left in the Southern.

  3. Continental Boundaries – Redirect currents along coastlines.

  4. Temperature & Salinity – Affect water density and flow.

II. Coriolis Effect

Coriolis Effect: The apparent deflection of moving objects (air, water, or anything moving over Earth’s surface) due to Earth’s rotation.

Direction of Deflection

  1. Northern Hemisphere → deflected to the right of the motion.

  2. Southern Hemisphere → deflected to the left of the motion.

  3. Equator → effect is essentially zero.

Factors Affecting the Coriolis Effect

  1. Latitude: Stronger near the poles, weaker near the equator.

  2. Speed of the moving object: Faster-moving objects experience more deflection.

  3. Scale of motion: Small-scale motions (e.g., sinks, bat flight) have negligible Coriolis effect; large-scale motions (winds, ocean currents) are strongly affected.

Effects on Ocean and Atmosphere

  1. Ocean Currents

    • Causes surface currents to curve clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

    • Shapes gyres in the oceans.

  2. Wind Patterns

    • Trade winds, westerlies, and polar easterlies are deflected by the Coriolis Effect.

    • Helps create cyclones and anticyclones.

  3. Weather Systems

    • Hurricanes/typhoons rotate counterclockwise in Northern Hemisphere, clockwise in Southern Hemisphere due to Coriolis deflection.

  4. Upwelling and Downwelling

    • Surface winds push water at an angle due to Coriolis → promotes coastal upwelling/downwelling.

III. Surface Convergence and Divergence

Surface Convergence: Occurs when surface waters move toward each other, piling up water in a region.
Surface Divergence: Occurs when surface waters move away from each other, creating a gap in the surface layer.

Causes

  1. Wind Patterns

    • Winds blowing along the coast or across the ocean surface can push water together (convergence) or apart (divergence).

    • Coriolis effect influences the direction of water movement.

  2. Ekman Transport

    • Wind-driven surface water movement is deflected 90° from the wind direction due to the Coriolis effect.

    • Causes spiral of water movement down the water column → leads to convergence or divergence at the surface.

  3. Geostrophic Currents

    • Variations in pressure and Earth's rotation can cause water to pile up (convergence) or spread out (divergence).

Effects of Surface Convergence

  1. Water piles up, creating a high sea surface level.

  2. Downwelling occurs: surface water sinks to deeper layers.

  3. Transports oxygen-rich surface water into deeper ocean.

  4. Can trap nutrients, often leading to lower surface productivity.

Effects of Surface Divergence

  1. Surface water moves apart, creating a low sea surface level.

  2. Upwelling occurs: cold, nutrient-rich deep water rises to the surface.

  3. Supports high biological productivity (phytoplankton blooms).

  4. Common along coastal regions and equatorial zones.

IV. Western Intensification

Western Intensification: the phenomenon where ocean currents on the western side of ocean basins are stronger, narrower, and deeper than those on the eastern side.

  • Occurs in subtropical gyres of all major oceans.

Causes

  1. Earth’s Rotation (Coriolis Effect)

    • Water moving toward the pole is deflected, causing asymmetry in gyre circulation.

    • Westward-flowing currents are weaker and broader; eastward-flowing western boundary currents are intensified.

  2. Wind Patterns

    • Trade winds and westerlies drive gyres.

    • Ekman transport pushes water toward the center of subtropical gyres.

  3. Pressure Gradients / Geostrophic Balance

    • Piling up of water at the center of gyres creates higher pressure in the east, driving stronger western currents.

Characteristics of Western Boundary Currents

  1. Fast – higher velocities than eastern boundary currents.
    Narrow – confined to a small horizontal width.

  2. Deep – extend further down into the ocean.

  3. Examples:

    • Gulf Stream (North Atlantic)

    • Kuroshio Current (North Pacific)

    • Brazil Current (South Atlantic)

    • East Australian Current (South Pacific)

Effects

  1. Heat transport – moves warm equatorial water toward poles, moderating climate.

  2. Weather influence – warms adjacent coastal regions (e.g., Western Europe by Gulf Stream).

  3. Marine ecosystems – concentrates nutrients and supports diverse marine life in boundary current zones.

  4. Ocean circulation – essential for maintaining subtropical gyres and global thermohaline circulation.

V. Ekman Transport

Ekman Transport: The net movement of surface water at an angle to the wind direction due to the combined effect of wind stress and the Coriolis effect.

Key Features

  1. Direction:

    • Northern Hemisphere → net transport is 90° to the right of the wind.

    • Southern Hemisphere → net transport is 90° to the left of the wind.

  2. Depth Influence:

    • Surface water moves at ~45° to the wind.

    • Each successive layer below moves more slowly and is deflected further → Ekman Spiral.

    • Typically affects the upper ~100–200 m of the ocean.

  3. Magnitude: Depends on wind speed, water density, and Coriolis parameter.

Processes and Effects

  1. Coastal Upwelling

    • Wind parallel to coast + Ekman Transport moves surface water away from the shore.

    • Deep, nutrient-rich water rises → supports high productivity.

  2. Coastal Downwelling

    • Wind-driven Ekman Transport pushes water toward the shore.

    • Surface water sinks → transports oxygen to deeper layers but reduces nutrient supply at surface.

  3. Open-Ocean Convergence and Divergence

    • Ekman Transport toward a central region → convergence → downwelling.

    • Ekman Transport away from a central region → divergence → upwelling.

  4. Gyre Circulation

    • Ekman Transport contributes to water piling up in the center of subtropical gyres, creating geostrophic currents and driving western intensification.


VI. Eddies

Ocean Eddies: Circular or swirling movements of water, usually 10–500 km in diameter, that break off from main ocean currents.

  • Analogous to “whirlpools” but occur on a larger scale in the ocean.

Types of Eddies

  1. Warm-core eddies

    • Form in western boundary currents (e.g., Gulf Stream).

    • Center is warmer than surrounding water.

    • Rotate clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere.

    • Transport heat poleward.

  2. Cold-core eddies

    • Form in eastern boundary currents.

    • Center is colder than surrounding water.

    • Rotate counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere.

    • Transport cold water toward the equator.

Causes of Eddies

  1. Instabilities in strong currents (like the Gulf Stream or Kuroshio Current).

  2. Wind stress and Coriolis effect.

  3. Interaction with coastlines, islands, or bathymetry.

Effects of Eddies

  1. Heat Transport

    • Redistribute heat from equator to poles or between currents.

  2. Nutrient Transport

    • Can trap nutrient-rich deep water in cold-core eddies → enhances phytoplankton productivity.

  3. Mixing and Circulation

    • Stir water, aiding vertical and horizontal mixing.

  4. Impact on Marine Life

    • Concentrates plankton and fish → affects local ecosystems and fisheries.

  5. Ocean Observation and Modeling

    • Important in climate models, satellite oceanography, and predicting ocean circulation patterns.