Large Scale Circulation

Large Scale Circulation

I. Thermohaline Circulation

Thermohaline Circulation (THC): The large-scale movement of ocean water driven by density differences, which depend on temperature (thermo) and salinity (haline).

  • Links surface currents and deep ocean currents globally.

  • Also called the “global conveyor belt.”

Driving Forces

  1. Density Differences

    • Cold water → denser → sinks.

    • Salty water → denser → sinks.

    • Warm or fresh water → less dense → stays near surface.

  2. Surface Cooling

    • Polar regions (North Atlantic, around Greenland, and Antarctic) lose heat → water cools → sinks.

  3. Salinity Changes

    • Evaporation in subtropics → increases salinity → water becomes denser.

    • Freshwater input from rivers or melting ice → decreases salinity → less dense, stays at surface.

  4. Gravity & Pressure

    • Dense water sinks, lighter water rises → maintains circulation.

Key Components

  1. North Atlantic Deep Water (NADW)

    • Forms near Greenland and Labrador Seas.

    • Cold, salty, dense → sinks → flows southward along deep Atlantic.

  2. Antarctic Bottom Water (AABW)

    • Forms near Antarctica.

    • Very cold, dense water → sinks to ocean floor → spreads into Atlantic, Indian, Pacific.

  3. Upwelling Zones

    • Deep water rises at equator and coasts → brings nutrient-rich water to surface → supports phytoplankton.

  4. Surface Currents

    • Warm surface water moves from tropics to poles (e.g., Gulf Stream, Kuroshio).

    • Water cools, sinks in high latitudes, and returns to tropics through deep currents.

Processes in THC

  1. Sinking – Dense water in polar regions drives downwelling.

  2. Flow of deep currents – Moves water across ocean basins at ~1–2 cm/s.

  3. Upwelling – Returns deep water to surface, especially at coasts and equatorial regions.

  4. Mixing – Occurs at boundaries and through eddies, helping redistribute heat and nutrients.

Global Effects

  1. Climate Regulation

    • Transfers heat from equator to poles → moderates global climate.

    • Western Europe is warmer than similar latitudes due to the Gulf Stream.

  2. Nutrient Transport

    • Deep water rising in upwelling zones brings nutrients → high productivity, supports fisheries.

  3. Oxygen Transport

    • Sinking surface water carries oxygen into the deep ocean → supports deep-sea life.

  4. Carbon Cycling

    • Moves dissolved CO₂ from surface to deep ocean → regulates atmospheric CO₂.

Regions of High Importance

  1. North Atlantic – NADW formation → key driver of Atlantic Meridional Overturning Circulation (AMOC).

  2. Southern Ocean – AABW formation → spreads cold water to all ocean basins.

  3. Equatorial and coastal upwelling zones – nutrient-rich surfaces → biological hotspots.

Factors That Can Disrupt THC

  1. Global warming → melts ice → freshwater input → reduces salinity → slows sinking.

  2. Changes in surface temperature → affects density gradients.

  3. Large-scale ocean circulation changes → can alter climate patterns.

II. Global Ocean Energy Transport

  • The movement of heat energy by ocean currents and circulation around the world.

  • Oceans transfer thermal energy from the equator toward the poles, moderating climate.

Mechanisms of Energy Transport

  1. Surface Currents (Wind-Driven)

    • Driven by prevailing winds (trade winds, westerlies).

    • Move warm water from tropics to higher latitudes.

    • Examples:

      • Gulf Stream → North Atlantic → warms Western Europe.

      • Kuroshio Current → North Pacific → warms Japan.

    • Cool surface currents carry cold water toward equator (e.g., California Current).

  2. Thermohaline Circulation (Density-Driven)

    • Driven by temperature and salinity differences.

    • Cold, salty water sinks at poles → flows along ocean floor → redistributes heat globally.

    • Complements surface currents in long-term heat transport.

  3. Upwelling and Downwelling

  4. Eddies and Mesoscale Circulation

    • Swirling currents redistribute heat and momentum locally and regionally.

Regional Energy Transport

  1. Equator → Poles: Net transport of heat from low to high latitudes.

  2. Western Boundary Currents: Strong, fast, and warm → carry large amounts of heat poleward.

  3. Eastern Boundary Currents: Slow, broad, cold → return cooler water toward equator.

  4. Polar Oceans: Sites of heat loss to atmosphere; sinking drives deep circulation.

Effects on Climate

  1. Moderates Coastal Climate

    • Warm currents → milder winters (e.g., Western Europe).

    • Cold currents → cooler, drier coastal regions (e.g., west coasts of continents).

  2. Supports Global Heat Balance

    • Prevents extreme temperature differences between equator and poles.

  3. Influences Weather Systems

    • Hurricanes/tropical storms fueled by warm surface water.

    • ENSO events redistribute heat and energy across Pacific → global climate impacts.

  4. Ocean–Atmosphere Interaction

    • Transfers energy to atmosphere via latent heat (evaporation) and sensible heat (conduction/convection).

III. Water Masses and Fronts

Formation of Water Masses

  • Occurs primarily in high-latitude regions where water cools and/or becomes salty:

  • Cooling – Cold surface water increases density → sinks (e.g., North Atlantic).

  • Salinity Increase – Evaporation or sea ice formation raises salinity → water sinks.

Major Water Masses

  1. Surface Water Masses

    • Warm, low-density, mixed by wind and waves.

    • Examples: Tropical Surface Water, Subtropical Surface Water.

  2. Intermediate Water Masses

    • Formed at mid-latitudes, moderate temperature and salinity.

    • Example: Antarctic Intermediate Water (AAIW).

  3. Deep Water Masses

    • Cold, salty, dense water formed at high latitudes.

    • Examples: North Atlantic Deep Water (NADW), Antarctic Bottom Water (AABW).

  4. Bottom Water

    • Coldest, densest water near the seafloor.

    • Example: AABW extends to deep ocean basins.

Characteristics

  1. Temperature and salinity are used to identify water masses.

  2. Movement is largely along density surfaces (isopycnals).

  3. Can persist for hundreds of years, transporting heat, nutrients, and oxygen.
    Fronts

Front: A boundary between two water masses with different temperature, salinity, or density.

Types of Ocean Fronts

  1. Thermal Front – Sharp change in temperature.

  2. Haline Front – Sharp change in salinity.

  3. Density (Pycnocline) Front – Sharp change in density, often combining temperature and salinity differences.

Formation

  1. Caused by:

    • Converging currents bringing different water masses together.

    • Upwelling or downwelling zones.

    • Coastal and shelf interactions.

Effects

  1. Enhanced mixing – nutrients and oxygen can be redistributed.

  2. Biological hotspots – front zones often have high productivity (phytoplankton blooms).

  3. Ocean circulation patterns – fronts can steer currents and eddies.

  4. Climate influence – redistribute heat between water masses.

IV: Deep Water Formation

Mechanisms

  1. Cooling of Surface Water

    • Occurs mainly in high-latitude regions (North Atlantic, around Greenland, and Antarctica).

    • Cold temperatures increase water density → sinks.

  2. Increase in Salinity

    • Evaporation or sea ice formation leaves salty, dense water behind.

    • Higher salinity → higher density → contributes to sinking.

  3. Combination of Temperature and Salinity

    • The densest water forms where cold temperatures and high salinity coincide, creating deep water masses.

Major Sites of Deep Water Formation

  1. North Atlantic

    • Forms North Atlantic Deep Water (NADW).

    • Cold, salty water sinks and flows southward along the Atlantic basin.

  2. Southern Ocean / Weddell Sea

    • Forms Antarctic Bottom Water (AABW).

    • Extremely cold, dense water sinks to the deepest parts of the ocean.

Role in Large-Scale Circulation

  1. Thermohaline Circulation (Global Conveyor Belt)

    • Sinking of dense water in polar regions drives deep currents across all ocean basins.

    • Connects surface currents and deep ocean flows globally.

  2. Heat Transport

    • Sinking cold water allows warm surface waters to move poleward → redistributes heat.

    • Moderates climate in coastal and continental regions.

  3. Nutrient & Oxygen Transport

    • Deep water carries oxygen from surface into the deep ocean.

    • Returns to surface via upwelling, supplying nutrients → supports biological productivity.

  4. Carbon Cycling

    • Moves CO₂ absorbed at the surface into deep ocean → helps regulate atmospheric CO₂.

Factors Affecting Deep Water Formation

  1. Climate Change

    • Warming → reduces surface cooling → slows deep water formation.

    • Melting ice → adds freshwater → lowers salinity → less dense water → reduces sinking.

  2. Wind Patterns & Ocean Circulation

    • Influence surface water movement and density distribution.

  3. Seasonal Variations

    • Stronger formation in winter when surface water is coldest.