5.3 Thermohaline circulation and deep ocean currents

Ocean circulation is a complex dance of currents driven by temperature and salinity differences. These movements transport heat, salt, and nutrients across vast distances, shaping global climate patterns and marine ecosystems.

Deep water formation and the global conveyor belt are key players in this oceanic ballet. As climate change alters these processes, it could lead to significant shifts in weather patterns, marine life, and even the Earth's carbon cycle.

Ocean Circulation Fundamentals

Thermohaline circulation in oceans

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• Large-scale ocean circulation driven by temperature and salinity differences moves heat, salt, and nutrients across ocean basins
• "Thermo" refers to temperature, "haline" refers to salinity influences global climate patterns
• Key components include surface currents, deep water formation, upwelling and downwelling processes connect surface and deep ocean waters

Deep water formation process

• Occurs primarily in North Atlantic and Southern Ocean when surface water cools and becomes more saline due to sea ice formation
• Increased density causes water to sink creating vertical mixing and initiating deep ocean currents
• North Atlantic Deep Water (NADW) forms in Labrador and Greenland Seas drives Atlantic Meridional Overturning Circulation (AMOC)
• Antarctic Bottom Water (AABW) forms in Weddell and Ross Seas constitutes densest water mass in global ocean

Global conveyor belt concept

• Interconnected system of surface and deep ocean currents circulates water through all major ocean basins
• Takes ~1,000 years for complete cycle redistributing heat, nutrients, and dissolved gases
• Moves warm water from equatorial regions to higher latitudes influencing regional and global climate patterns
• Brings nutrient-rich deep waters to surface through upwelling supporting marine ecosystems and primary productivity
• Facilitates absorption and release of gases (oxygen, carbon dioxide) playing crucial role in global carbon cycle

Climate change impacts on circulation

• Increased freshwater input in polar regions from melting ice sheets and changing precipitation patterns alters water density
• Ocean warming reduces density differences between surface and deep waters potentially weakening circulation
• Weakening of Atlantic Meridional Overturning Circulation (AMOC) may lead to:
  1. Cooling of Northern Europe
  2. Shifts in tropical rainfall patterns
  3. Changes in upwelling intensity and location
• Alterations in heat distribution could intensify extreme weather events (hurricanes, droughts)
• Feedback loops may accelerate global warming as changes in ocean circulation affect atmospheric CO2 concentrations