4.3 Deep ocean circulation and thermohaline circulation

Ocean circulation is a complex dance of water movement, driven by temperature and salinity differences. Deep ocean currents, part of the global conveyor belt, transport heat and nutrients across vast distances, shaping our planet's climate and marine ecosystems.

Thermohaline circulation, the engine behind deep ocean movement, operates on long timescales. It connects surface and deep waters, playing a crucial role in regulating global climate and the carbon cycle. Understanding these processes is key to grasping Earth's climate system.

Deep ocean circulation

Global ocean conveyor belt system

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• Deep ocean circulation moves water through deep layers of world's oceans
• Known as the global ocean conveyor belt
• Driven by differences in water density from temperature and salinity variations
• Cold, dense water sinks at high latitudes (North Atlantic, Antarctica)
• Upwelling brings nutrient-rich deep waters to surface
• Wind-driven surface currents interact with deep circulation
• Timescale much longer than surface currents
  • Water takes hundreds to thousands of years to complete full circuit

Formation of deep water masses

• Dense water masses form in specific regions
  • Labrador Sea
  • Weddell Sea
• Surface waters cool and increase in salinity
• Density-driven convection causes sinking
• Displaces less dense water
• Initiates deep ocean currents
• Caballing process contributes to circulation complexity
  • Mixing of water masses results in denser product

Vertical structure and movement

• Ocean's vertical density structure called stratification
• Influences strength and patterns of deep circulation
• Density gradients between water masses create pressure gradients
• Drives horizontal movement in deep ocean
• Stability of stratification affects circulation patterns

Thermohaline circulation

Density-driven ocean circulation

• Thermohaline circulation driven by density differences
• Results from temperature (thermo-) and salinity (haline) variations
• Global conveyor belt model illustrates interconnected deep ocean currents
• Key components include formation of:
  • North Atlantic Deep Water (NADW)
  • Antarctic Bottom Water (AABW)
• Transports heat, dissolved gases, and nutrients across ocean basins
• Crucial role in global climate regulation

Surface and deep circulation connections

• Surface currents integral to conveyor belt system (Gulf Stream)
• Connect shallow and deep circulation patterns
• Strength and pattern influenced by freshwater input changes
  • Increased ice melt in polar regions affects circulation

Timescales and global impact

• Operates on longer timescales than surface currents
• Influences global heat distribution
• Affects regional climates (western boundary currents)
• Plays role in global carbon cycle
  • Sequesters carbon dioxide in deep waters for long periods
• Changes in strength or pattern can lead to abrupt climate shifts

Density differences in ocean circulation

Factors affecting seawater density

• Temperature variations
  • Colder water generally denser
• Salinity variations
  • Saltier water generally denser
• Combined effect determines overall density
• Density differences drive convection and circulation

Regional density variations

• High-latitude regions experience significant cooling
  • North Atlantic
  • Southern Ocean near Antarctica
• Increased salinity in some areas due to:
  • Evaporation (subtropical gyres)
  • Sea ice formation (polar regions)
• These processes create dense water masses
  • North Atlantic Deep Water (NADW)
  • Antarctic Bottom Water (AABW)

Density-driven circulation processes

• Dense water masses sink at formation sites
• Displacement of less dense water initiates currents
• Horizontal density gradients create pressure differences
• Pressure gradients drive large-scale circulation patterns
• Caballing process adds complexity
  • Mixing of water masses can create even denser water

Importance of deep ocean circulation

Global heat distribution

• Transports warm water from equator towards poles
• Carries cold water from poles towards equator
• Influences regional climates
  • Western boundary currents (Gulf Stream, Kuroshio Current)
• Contributes to climate variability on various timescales
  • Seasonal
  • Decadal
  • Long-term trends

Climate regulation and carbon cycle

• Crucial role in global carbon cycle
  • Sequesters CO2 in deep waters for extended periods
• Ocean's heat storage capacity affects climate trends
• Changes in circulation can lead to abrupt climate shifts
  • Past examples: Younger Dryas cold period

Marine ecosystem impacts

• Influences distribution of nutrients in world's oceans
• Affects marine ecosystems globally
• Impacts global biogeochemical cycles
  • Nutrient transport
  • Oxygen distribution

Atmospheric-oceanic interactions

• Interacts with atmospheric processes
  • El Niño-Southern Oscillation (ENSO)
• Demonstrates complex relationship between oceanic and atmospheric systems
• Influences global weather patterns
  • Monsoons
  • Storm tracks