2.2 Ocean circulation and currents

Ocean circulation patterns shape our planet's climate and marine ecosystems. Surface currents like gyres and deep ocean currents form a global conveyor belt, redistributing heat and nutrients. Wind, density differences, and Earth's rotation drive these complex movements.

Thermohaline circulation plays a crucial role in regulating global climate by transporting heat from the equator to the poles. It also impacts marine life by distributing nutrients and organisms. Understanding these patterns is key to predicting climate change impacts and protecting ocean biodiversity.

Ocean Circulation Patterns and Driving Forces

Major ocean currents and patterns

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• Surface currents
  • Gyres: large systems of rotating ocean currents (North Pacific, South Pacific, North Atlantic, South Atlantic, Indian Ocean)
  • Antarctic Circumpolar Current: flows eastward around Antarctica connecting the Atlantic, Pacific, and Indian Oceans
  • Kuroshio Current: warm current flowing northeastward off the coast of Japan
  • Gulf Stream: warm current originating in the Gulf of Mexico and flowing along the eastern coast of the United States
  • Agulhas Current: warm current flowing southward along the eastern coast of Africa
• Deep ocean currents
  • Thermohaline circulation: global system of deep ocean currents driven by temperature and salinity differences
    • Antarctic Bottom Water: cold, dense water that forms near Antarctica and flows northward along the ocean floor
    • North Atlantic Deep Water: cold, dense water that forms in the North Atlantic and flows southward at depths
    • Circumpolar Deep Water: deep water that circulates around Antarctica

Driving forces of ocean circulation

• Wind-driven circulation
  • Trade winds: steady winds blowing from east to west near the equator
  • Westerlies: prevailing winds blowing from west to east in the middle latitudes
  • Ekman transport: net movement of water 90° to the right (left) of the wind direction in the Northern (Southern) Hemisphere
    • Ekman spiral: decreasing current velocity with depth resulting from the balance between wind stress and Coriolis force
• Density-driven circulation
  • Temperature and salinity gradients: differences in water density due to variations in temperature and salinity
  • Thermohaline circulation: global circulation driven by density differences
    • Deep water formation: sinking of cold, dense water at high latitudes (North Atlantic, Antarctic)
    • Upwelling of deep water: rising of deep water to the surface in certain regions (equatorial, coastal)
• Coriolis effect
  • Deflection of currents due to Earth's rotation
    • Northern Hemisphere: currents deflected to the right, resulting in clockwise circulation
    • Southern Hemisphere: currents deflected to the left, resulting in counterclockwise circulation
  • Geostrophic flow: balance between Coriolis force and pressure gradient force, causing currents to flow along lines of constant pressure

Thermohaline Circulation and Its Impacts

Thermohaline circulation in climate

• Global conveyor belt: interconnected system of surface and deep currents that redistributes heat and nutrients globally
• Heat transport
  • Poleward transport of warm surface waters: currents carry heat from the equator towards the poles (Gulf Stream, Kuroshio Current)
  • Equatorward transport of cold deep waters: deep currents return cold water from the poles towards the equator (Antarctic Bottom Water, North Atlantic Deep Water)
  • Moderation of climate extremes: ocean circulation helps to reduce temperature differences between the equator and poles
• Climate regulation
  • Influence on global temperature patterns: thermohaline circulation affects the distribution of heat across the planet
  • Impact on atmospheric circulation: changes in ocean circulation can alter atmospheric pressure systems and wind patterns
  • Potential changes due to climate change
    • Slowing of thermohaline circulation: increased freshwater input from melting ice and changes in precipitation patterns may weaken the circulation
    • Consequences for global climate: a slowdown in thermohaline circulation could lead to regional cooling in Europe and North America, and changes in precipitation patterns worldwide

Ocean circulation for marine ecosystems

• Nutrient distribution
  • Upwelling zones: regions where nutrient-rich deep waters are brought to the surface (coastal upwelling, equatorial upwelling)
    • High primary productivity: upwelled nutrients support the growth of phytoplankton, forming the base of marine food webs
  • Downwelling zones: regions where nutrient-poor surface waters sink (subtropical gyres)
    • Low primary productivity: limited nutrient availability results in lower phytoplankton growth
• Marine organism distribution
  • Dispersal of larvae and juveniles: ocean currents transport early life stages of marine organisms, connecting populations across distant regions
    • Connectivity between populations: exchange of individuals among geographically separated populations, maintaining genetic diversity
  • Transport of plankton: currents carry plankton, which serve as a food source for higher trophic levels (fish, marine mammals)
  • Migration routes: ocean circulation patterns influence the distribution and migration of various marine species (sea turtles, whales, tuna)