🌡️Climatology Unit 4 – Ocean Circulation and Climate

Key Concepts and Terminology

• Ocean circulation plays a crucial role in regulating Earth's climate by redistributing heat, nutrients, and gases across the globe
• Gyres are large systems of rotating ocean currents, primarily driven by wind patterns and the Coriolis effect
• Upwelling occurs when deep, cold, nutrient-rich water rises to the surface, often along coastlines or due to diverging surface currents
• Downwelling involves the sinking of dense, cold water from the surface to deeper layers of the ocean
• Thermocline refers to the distinct layer in the ocean where temperature changes rapidly with depth, separating the mixed layer from deeper waters
• Halocline is a layer in the ocean where salinity changes rapidly with depth, often influencing water density and circulation patterns
• Ekman transport describes the net movement of surface water perpendicular to the wind direction due to the Coriolis effect
• Langmuir circulation consists of counter-rotating vortices aligned with the wind direction, causing surface convergence and downwelling

Ocean Circulation Patterns

• Surface currents are primarily driven by global wind patterns, forming large-scale gyres in each ocean basin
  • Gyres rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere due to the Coriolis effect
• Boundary currents are intense, narrow currents that flow along the edges of continents or basins, such as the Gulf Stream or Kuroshio Current
• Equatorial currents flow westward near the equator, driven by trade winds, and include the North and South Equatorial Currents
• Antarctic Circumpolar Current is the world's largest ocean current, flowing eastward around Antarctica and connecting the Atlantic, Pacific, and Indian Oceans
• Coastal upwelling occurs along the western coasts of continents, where prevailing winds cause surface water to move offshore, allowing deep, nutrient-rich water to rise
• Subtropical gyres are characterized by central regions of relatively calm, stable waters known as the doldrums or horse latitudes
• Deep ocean circulation is primarily driven by density differences due to temperature and salinity, forming the global thermohaline circulation

Drivers of Ocean Currents

• Wind stress is the primary driver of surface ocean currents, as friction between the wind and the ocean surface transfers energy and momentum
• The Coriolis effect, caused by Earth's rotation, deflects ocean currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
• Density differences in seawater, influenced by temperature and salinity, drive thermohaline circulation and deep ocean currents
  • Colder, saltier water is denser and sinks, while warmer, fresher water is less dense and rises
• Tides, caused by the gravitational pull of the moon and sun, can influence coastal currents and mixing
• Topography and bathymetry, such as continental shelves, underwater ridges, and seamounts, can steer or disrupt ocean currents
• Atmospheric pressure gradients can influence surface currents, with high-pressure systems causing divergence and low-pressure systems causing convergence
• Freshwater input from rivers, precipitation, and ice melt can create localized density differences and affect coastal circulation patterns

Thermohaline Circulation

• Thermohaline circulation, also known as the global conveyor belt, is a large-scale ocean circulation pattern driven by temperature and salinity differences
• The process begins with the formation of deep water in the North Atlantic and the Southern Ocean, where cold, dense water sinks and flows along the ocean floor
  • North Atlantic Deep Water (NADW) forms in the Labrador and Greenland Seas, while Antarctic Bottom Water (AABW) forms in the Weddell and Ross Seas
• Deep water masses slowly flow southward in the Atlantic, eventually joining the Antarctic Circumpolar Current and spreading into the Indian and Pacific Oceans
• Upwelling of deep water occurs in the Indian and Pacific Oceans, where the water gradually warms and becomes less dense
• The warmer, less dense water returns to the surface and flows northward, eventually returning to the North Atlantic to complete the circulation pattern
• Thermohaline circulation plays a crucial role in redistributing heat, nutrients, and dissolved gases throughout the global ocean
• Changes in temperature or salinity, such as from increased freshwater input or altered precipitation patterns, can disrupt the thermohaline circulation and impact global climate

Ocean-Atmosphere Interactions

• The ocean and atmosphere are tightly coupled systems that constantly exchange heat, moisture, and momentum
• Ocean surface temperatures influence atmospheric circulation patterns, as warm water promotes rising air, convection, and the formation of low-pressure systems
• Atmospheric winds drive surface ocean currents, transferring energy and momentum to the upper ocean layers
• Evaporation from the ocean surface provides moisture to the atmosphere, contributing to cloud formation and precipitation
  • This moisture transport is a key component of the global water cycle
• The ocean absorbs and stores large amounts of heat from the atmosphere, helping to regulate Earth's temperature and mitigate short-term climate fluctuations
• Climate phenomena such as El Niño and La Niña involve changes in ocean-atmosphere interactions, affecting global weather patterns and ocean circulation
• Sea ice formation and melting influence ocean-atmosphere heat exchange, albedo, and thermohaline circulation, particularly in polar regions
• Ocean uptake of atmospheric carbon dioxide helps to mitigate the greenhouse effect, but also leads to ocean acidification

Climate Impacts of Ocean Circulation

• Ocean currents redistribute heat from the equator to the poles, moderating Earth's climate and reducing temperature gradients
• The poleward transport of warm water by western boundary currents, such as the Gulf Stream and Kuroshio Current, helps to keep higher latitudes warmer than they would be otherwise
• Upwelling regions, such as along the coasts of Peru, California, and West Africa, support highly productive marine ecosystems and fisheries by bringing nutrient-rich water to the surface
• Changes in ocean circulation patterns can affect regional climates, such as the collapse of the North Atlantic cod fishery due to shifts in the Labrador Current
• Slowdown or disruption of the thermohaline circulation could lead to cooling in the North Atlantic and Europe, as well as changes in global precipitation patterns
• Ocean circulation plays a role in the global carbon cycle by transporting dissolved carbon dioxide and organic matter, influencing atmospheric CO2 levels and climate
• Sea level rise due to thermal expansion and melting land ice can be influenced by changes in ocean circulation and heat distribution
• Shifts in ocean currents can affect the distribution and survival of marine species, particularly those with specific temperature or habitat requirements

Measuring and Modeling Ocean Currents

• Drifters and floats, such as the Argo program, provide direct measurements of ocean currents, temperature, and salinity at various depths
• Satellite altimetry measures sea surface height, which can be used to infer surface currents and ocean circulation patterns
• Acoustic Doppler Current Profilers (ADCPs) use sound waves to measure current velocities at different depths
• Moored buoys and arrays, such as the Tropical Atmosphere Ocean (TAO) array, provide long-term, fixed-point measurements of ocean and atmospheric variables
• Ocean general circulation models (OGCMs) simulate large-scale ocean circulation patterns and their interactions with the atmosphere and climate
  • These models incorporate physical equations, bathymetry, and forcing factors to predict ocean currents, temperature, and salinity
• Coupled atmosphere-ocean models, such as those used in climate projections, account for the complex feedbacks and interactions between the ocean and atmosphere
• Data assimilation techniques combine observations with numerical models to improve the accuracy and reliability of ocean circulation simulations
• Tracers, such as chlorofluorocarbons (CFCs) and radiocarbon, can be used to track water masses and estimate the age and mixing of ocean waters

Current Research and Future Projections

• Scientists are investigating the potential impacts of climate change on ocean circulation patterns, particularly the stability of the thermohaline circulation
• Increased freshwater input from melting glaciers and ice sheets could weaken or disrupt the Atlantic Meridional Overturning Circulation (AMOC), with consequences for regional and global climate
• Research is focused on understanding the complex interactions between the ocean, atmosphere, and cryosphere in polar regions, as these areas are particularly sensitive to climate change
• Studies are examining the role of ocean circulation in the global carbon cycle and its potential to mitigate or amplify the effects of anthropogenic carbon emissions
• Advances in ocean observing systems, such as autonomous underwater vehicles (AUVs) and remote sensing technologies, are improving our ability to monitor and understand ocean circulation
• High-resolution ocean models are being developed to better simulate mesoscale and submesoscale processes, such as eddies and fronts, which play important roles in heat and nutrient transport
• Interdisciplinary research is investigating the impacts of changing ocean circulation on marine ecosystems, fisheries, and biogeochemical cycles
• Climate projections are exploring the potential effects of different greenhouse gas emission scenarios on future ocean circulation patterns and their consequences for global climate and sea level rise