Wind & Ocean Circulation

Wind & Ocean Circulation Overview

  • Chapters Covered:

    • Atmospheric processes

    • Wind-driven surface currents

    • Convergence and divergence

    • Thermohaline circulation

    • Global circulation patterns

    • Circulation in semi-enclosed bays

    • Measuring currents

Interdependence of Atmosphere and Oceans

  • Intrinsic Linkage:

    • The atmosphere and oceans influence each other.

  • Density Factors of Air:

    • Influenced by:

    • Temperature

    • Pressure

    • Moisture content (H2O has lower molecular weight than O2 and N2)

    • Warm air is less dense than cold air.

    • Moist air is less dense than dry air.

  • Air Pressure:

    • Defined as the weight of air from Earth’s surface to the top of the atmosphere.

    • Standard air pressure at sea level: 1.04 ext{ kg/cm}^2 (one atmosphere).

Air Pressure Dynamics

  • Low Pressure Zone:

    • Occurs when air density is lower due to:

    • Warmer air

    • Higher moisture content (one molecule of H2O is lighter than one molecule of O2 or N2).

    • Solar heating causes expansion, lowering air density and creating rising air at the surface.

  • High Pressure Zone:

    • Occurs when air pressure is higher due to:

    • Cooler air

    • Lower moisture content.

    • Cooling leads to contraction, increasing density and causing sinking air at the surface.

Wind Generation and Pressure Gradients

  • Wind Mechanism:

    • Fluids (air and water) flow from high pressure areas to low pressure areas, resulting in WINDS.

    • The change in pressure across a horizontal distance creates a pressure gradient.

    • A greater pressure difference and shorter distance define a steep pressure gradient, indicating stronger winds.

  • Wind Naming Convention:

    • Winds are named from the direction they come from.

    • Ocean currents are named for the direction they travel towards.

Global Wind Patterns

  • Atmospheric Pressure Variations:

    • Distributed unevenly due to solar radiation.

    • High solar radiation at the equator leads to low pressure.

    • Low solar radiation at the poles results in high pressure.

    • Simple model suggests airflow from high latitudes to low latitudes.

Coriolis Effect

  • Definition: The apparent deviation of objects moving across Earth’s surface due to Earth's rotation.

    • Northern Hemisphere: Objects are deflected to the right.

    • Southern Hemisphere: Objects are deflected to the left.

  • Visual Example: Illustrated with a ball being thrown between two people riding horses on a merry-go-round.

Coriolis Deflection Details

  • Deflection Mechanism:

    • Moving objects experience an apparent deflection:

    • Right in the Northern Hemisphere.

    • Left in the Southern Hemisphere.

    • Amount of deflection correlates with speed of the object and latitude:

    • Increases with speed and distance from the equator.

    • Polar winds experience more deviation relative to the Earth’s surface than tropical winds.

    • Fast winds veer more sharply than slower winds.

    • No Coriolis effect at the equator.

Wind Circulation Dynamics

  • Heating and Pressure Formation:

    • Heating at the equator leads to low air pressure at the surface.

    • Warm air rises, cools, loses moisture, increasing density.

    • High pressure zone develops at approximately 30° N/S, with air sinking and moving back toward the equator.

    • Air is deflected by Coriolis:

    • Northern Hemisphere: Right, creates northeast trade winds.

    • Southern Hemisphere: Left, creates southeast trade winds.

Major Convection Cells

  • Convection Cells Present in Each Hemisphere:

    • Hadley Cell: Extends from the Equator to about 30° latitude.

    • Ferrel Cell: Extends from 30° to about 50° latitude.

    • Polar Cell: Extends from 90° to about 50° latitude.

Global Wind Circulation Patterns

  • Zonal Wind System Development:

    • Result of unequal heating of Earth's surface and Coriolis deflection.

Interaction Between Wind and Ocean

  • Wind-Driven Ocean Currents:

    • Interaction occurs as wind moves across water, leading to air molecule and water molecule collision.

    • Energy transfer from air to water is inefficient:

    • Water movement is typically around 3-4% of wind speed.

Ocean Current Mechanics

  • Zonal Wind Flow:

    • Wind moving nearly parallel to latitude results from Coriolis deflection.

  • Gyre Definition: A circular current caused by:

    • Westerly-driven ocean currents in trade winds.

    • Easterly-driven ocean currents in westerly wind belts.

    • Coriolis effect (referred to as Ekman Flow) impacting moving water.

    • Deflection of ocean currents by continents.

Surface Ocean Currents Configuration

  • Global Wind-Driven Ocean Circulation:

    • Consists of gyres that rotate:

    • Clockwise in the Northern Hemisphere.

    • Counterclockwise in the Southern Hemisphere.

Ocean Topography and Influence on Currents

  • Surface Irregularities:

    • Sea surface exhibits mounds and depressions causing pressure gradients.

    • Mounds arise from converging currents.

    • Depressions arise from diverging currents.

  • Water Flow Analysis:

    • Water flows down pressure gradients from mounds to depressions.

    • Deflection caused by Coriolis is dependent on:

    • Latitude

    • Current speed

    • Direction of flow (whether poleward or equatorward).

Ekman Spiral Dynamics

  • Wind-Driven Surface Water Motion:

    • Extends downward into the water column over time:

    • Speed decreases with depth.

    • Direction changes due to Coriolis deflection, resulting in the Ekman Spiral.

  • Ekman Transport: Defined as net transport of water through wind-induced motion:

    • In an Ekman spiral, water is deflected by 90° relative to the wind's direction.

    • Along coasts, it induces:

    • Downwelling (water moving toward coast)

    • Upwelling (water moving away from coast).

Examples of Wind-Induced Motion and Water Interaction

  • Vertical Currents in Open Ocean:

    • Occurs at locations such as 30° N or S.

    • Surface currents of gyres converge due to Coriolis leading to downwelling.

  • Langmuir Circulation:

    • A complex horizontal helical motion that is parallel to the wind, with adjacent cells rotating in opposite directions.

    • Creates alternating zones of convergence and divergence, concentrating surface material into stripes parallel to the wind direction.

Geostrophic Currents

  • Geostrophic Flow Mechanics:

    • Allows currents to travel long distances without Coriolis deflection apparent.

    • As the height of the mound of water increases, the pressure gradient steepens, causing water to flow outward until equilibrium is achieved.

  • Concept Explanation:

    • Geostrophic flow balances Coriolis deflection with the pressure gradient, resulting in currents flowing parallel to the wind around the mound.

Currents in Ocean Gyres

  • Asymmetrical Flow Pattern:

    • Gyres exhibit narrow, deep, and swift currents along western edges e.g., the Gulf Stream.

    • Exhibit broad, shallow, and slower currents along eastern edges e.g., the Canary Current.

    • The mound of water, which the geostrophic currents flow around, is slightly displaced to the west, owing to a stronger Coriolis deflection on this side.

Sargasso Sea Characteristics

  • Definition and Location:

    • A large warm water lens encircled by the North Atlantic gyre, isolated from cold waters.

  • Thermal Properties:

    • Isotherms illustrate the warm water is separated from colder water by the Gulf Stream.

Boundary Currents Overview

  • Meandering Patterns:

    • Western boundary currents such as the Gulf Stream exhibit a meandering pattern separating warmer waters in gyre centers from cooler coastal waters.

  • Eddy Formation:

    • Meanders can form warm-core and cold-core rings when cut-off, creating EDDIES.

Deep Ocean Circulation Overview

  • Thermohaline Circulation:

    • Defines the density-driven flow of water, influenced by temperature and salinity.

  • Density Mechanism:

    • Denser water masses displace less dense ones upon contact, contributing to sinking behavior.

  • Surface Drivers:

    • Surface water factors: evaporation, precipitation, cooling, heating, and sea ice formation affect density, and therefore, thermohaline dynamics.

Classification of Water Masses

  • Depth-Related Categories:

    • Central waters (0 to 1 km)

    • Intermediate waters (1 to 2 km)

    • Deep/bottom waters (>2 km)

Origin of Deep/Bottom Waters

  • Events Leading to Density Increase:

    • Surface conditions like evaporation and cooling raise salinity and lower temperature leading to increased density; this causes sinking.

  • Continental Connections:

    • Ocean basins interconnect, allowing for water exchange driven primarily by North Atlantic waters.

Summary of Wind and Ocean Circulation Principles

  • Global winds driven by pressure gradients and Coriolis deflection create circulation cells leading to zonal winds.

  • Wind movement induces surface ocean currents affected by Coriolis, resulting in the Ekman spiral phenomenon.

  • Geostrophic currents arise from the balance between Coriolis deflection and pressure gradients, leading to large gyre circulations with distinct clockwise and counterclockwise patterns in respective hemispheres.

  • Gyre circulation exhibits asymmetry attributed to western boundary current dynamics, with significant implications for thermal and salt transport in oceanic systems.

  • Thermohaline circulation forms a subsurface flow driven by water density differences, detailing exchanges across ocean basins via the 'ocean conveyor belt.'