11.2 Ocean Circulation and Coastal Processes
5 min read•july 30, 2024
Ocean circulation shapes Earth's climate and coastal landscapes. Wind-driven currents and density differences create complex patterns, transporting heat and nutrients globally. These processes influence weather, marine ecosystems, and coastline formation.
Coastal processes, including waves, tides, and currents, sculpt shorelines through and deposition. Human activities can alter these natural processes, impacting coastal environments. Understanding these dynamics is crucial for managing coastal resources and adapting to climate change.
Ocean Circulation Patterns
Wind-Driven Circulation and Ekman Transport
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- Wind-driven circulation is caused by the frictional force of wind on the ocean surface, creating surface currents that move in the direction of the prevailing winds
- Ekman transport is the net movement of water 90 degrees to the right of the wind direction in the Northern Hemisphere (90 degrees to the left in the Southern Hemisphere) due to the Coriolis effect
- Example: In the North Atlantic, winds blowing from east to west cause surface waters to move northward due to Ekman transport
- This process contributes to the formation of ocean gyres and / regions
Density Differences and Thermohaline Circulation
- Density differences in ocean water, caused by variations in temperature and salinity, lead to
- Denser water sinks, while less dense water rises, creating deep ocean currents
- Example: The formation of North Atlantic Deep Water (NADW) in the Labrador and Nordic Seas due to cooling and increased salinity
- The Coriolis effect, resulting from Earth's rotation, deflects ocean currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
- This creates circular patterns in ocean basins called gyres (North Atlantic Gyre, North Pacific Gyre)
- Upwelling occurs when wind-driven surface currents move water away from the coast, causing deeper, colder, nutrient-rich water to rise to the surface
- Example: Coastal upwelling along the west coast of South America (Peru-Chile Current)
- Downwelling occurs when surface currents converge and push water downward
Surface and Deep Ocean Currents
Major Surface Currents and Gyres
- Surface currents are primarily driven by wind and are influenced by the Coriolis effect, forming large circular patterns called gyres
- Examples: North Pacific Gyre (California Current, North Pacific Current, Kuroshio Current, North Equatorial Current), North Atlantic Gyre (, North Atlantic Current, Canary Current, North Equatorial Current)
- The Antarctic Circumpolar Current (ACC) is a strong, eastward-flowing surface current that encircles Antarctica, connecting the Atlantic, Pacific, and Indian Oceans
- The ACC is driven by strong westerly winds and plays a crucial role in global ocean circulation and heat transport
Deep Ocean Currents and Thermohaline Circulation
- Deep ocean currents are driven by density differences in water masses, forming thermohaline circulation
- The global conveyor belt is a simplified model of this circulation, depicting the interconnected flow of surface and deep currents
- The formation of deep water occurs in high-latitude regions, such as the North Atlantic (NADW) and the Southern Ocean (Antarctic Bottom Water, AABW), where cold, dense water sinks and flows along the ocean bottom
- Example: NADW formation in the Labrador and Nordic Seas, which then flows southward at depths of 2,000-4,000 meters
- Upwelling of deep water occurs in regions where wind-driven surface currents diverge, such as along the equator and the west coasts of continents
- Example: Equatorial upwelling in the Pacific Ocean, which brings nutrient-rich deep water to the surface, supporting high biological productivity
Ocean Circulation and Climate
Heat Transport and Climate Moderation
- Ocean currents redistribute heat from the equator to the poles, moderating Earth's climate
- Warm currents, such as the Gulf Stream, transport heat poleward, while cold currents, such as the Labrador Current, transport cold water toward the equator
- Example: The Gulf Stream carries warm water from the Caribbean to the North Atlantic, contributing to the mild climate of Western Europe
- The global conveyor belt plays a crucial role in regulating Earth's climate by transporting heat and nutrients around the globe
- Changes in the conveyor belt circulation can have significant impacts on regional and global climate
- Example: A slowdown of the Atlantic Meridional Overturning Circulation (AMOC) could lead to cooling in the North Atlantic and shifts in precipitation patterns
Climate Variability and Ocean-Atmosphere Interactions
- The -Southern Oscillation (ENSO) is a periodic fluctuation in ocean temperatures and atmospheric pressure in the equatorial Pacific, affecting global weather patterns
- El Niño events are characterized by warmer ocean temperatures, while events are characterized by cooler temperatures
- Example: During El Niño years, the weakening of trade winds and the eastward shift of warm water in the Pacific can cause increased rainfall in the eastern Pacific and droughts in the western Pacific
- Climate change can affect ocean circulation patterns, potentially leading to changes in heat distribution, sea level rise, and shifts in marine ecosystems
- Example: Melting of the Greenland Ice Sheet due to global warming could lead to an influx of freshwater into the North Atlantic, potentially disrupting the AMOC and causing regional climate changes
Coastal Processes and Coastline Formation
Waves, Tides, and Longshore Currents
- Waves are generated by wind blowing over the ocean surface; wave characteristics (height, wavelength, period) depend on wind speed, duration, and fetch (the distance over which the wind blows)
- occurs when waves approach the shore at an angle, causing the part of the wave in shallower water to slow down and bend toward the shore
- This process concentrates wave energy on headlands and disperses it in bays
- Example: Wave refraction around a headland can lead to the formation of a tombolo (a sand or gravel bar connecting an island to the mainland)
- Tides are the regular rise and fall of sea level caused by the gravitational pull of the Moon and Sun
- Tidal range varies depending on the location and the alignment of the Moon and Sun (spring tides vs. neap tides)
- Example: The Bay of Fundy in Canada has one of the highest tidal ranges in the world, with tides reaching up to 16 meters
- Longshore currents are generated when waves approach the shore at an angle, creating a zigzag pattern of water movement that transports sediment along the coast ()
- This process can lead to the formation of features such as spits and
- Example: The Outer Banks of North Carolina are a series of barrier islands formed by longshore drift
Coastal Erosion, Deposition, and Human Impacts
- occurs when waves, currents, and tides remove sediment from the shoreline
- Example: The White Cliffs of Dover in England are being eroded by wave action, causing the cliffs to retreat landward
- Coastal deposition occurs when sediment is added to the shoreline, leading to the formation of beaches, dunes, and other coastal features
- Example: The Nile Delta in Egypt is a depositional feature formed by sediment carried by the Nile River
- Human activities, such as the construction of jetties, groins, and seawalls, can interfere with natural coastal processes and alter the balance between erosion and deposition
- Example: The construction of groins along a beach can trap sediment on the updrift side, leading to erosion on the downdrift side
Key Terms to Review (18)
Barrier islands: Barrier islands are elongated landforms situated parallel to the mainland coast, formed primarily from sand and sediment. These islands act as a protective barrier against storm surges, high waves, and coastal erosion, creating a unique ecosystem that supports diverse wildlife and plant species. Their formation and ongoing evolution are influenced by ocean currents, wave action, and tidal forces.
Benthic zone: The benthic zone refers to the ecological region at the lowest level of a body of water, including the sediment surface and sub-surface layers. This zone is crucial for various aquatic ecosystems, as it supports a diverse community of organisms, from microscopic bacteria to larger benthic animals, all interacting with their environment. The benthic zone plays a significant role in nutrient cycling, energy flow, and the overall health of aquatic habitats.
Coastal Erosion: Coastal erosion is the process by which coastlines are gradually worn away by the action of waves, currents, tides, and human activities. This natural phenomenon can reshape coastal landscapes, leading to the loss of land and changes in marine ecosystems. Understanding coastal erosion is crucial for managing coastal zones and protecting infrastructure from the impacts of changing sea levels and increased storm intensity.
Downwelling: Downwelling is the process where surface waters sink to deeper ocean layers, typically occurring in areas where water converges or where winds push surface water away. This movement plays a crucial role in ocean circulation and is essential for transporting nutrients from the surface down into the depths, thus affecting marine ecosystems and climate patterns.
El Niño: El Niño is a climate pattern characterized by the periodic warming of sea surface temperatures in the central and eastern Pacific Ocean, significantly impacting global weather and climate systems. This phenomenon disrupts normal weather patterns, affecting rainfall, temperature, and storm activity across various regions of the world, leading to both positive and negative environmental consequences.
Erosion: Erosion is the process by which soil and rock are removed from one location and transported to another by natural forces such as wind, water, and ice. This process plays a vital role in shaping landscapes and influencing various Earth systems through the movement of sediments and materials.
Estuary: An estuary is a coastal area where freshwater from rivers and streams meets and mixes with saltwater from the ocean. This unique environment is characterized by brackish water, which supports diverse ecosystems and serves as crucial habitats for various species of fish, birds, and other wildlife. Estuaries are vital for nutrient cycling and act as buffers against storm surges, making them important both ecologically and for human communities.
Gulf stream: The Gulf Stream is a powerful warm ocean current originating in the Gulf of Mexico, flowing along the eastern coast of the United States and across the Atlantic Ocean towards Europe. This current plays a significant role in regulating climate by transferring warm water to higher latitudes, impacting weather patterns and oceanic conditions. Its movement influences atmospheric circulation patterns, contributes to global climate variability, and is essential for understanding coastal processes.
La Niña: La Niña is a climatic phenomenon characterized by cooler-than-average sea surface temperatures in the central and eastern tropical Pacific Ocean. It plays a significant role in influencing global weather patterns, often leading to increased precipitation in some regions while causing droughts in others. This phenomenon is essentially the counterpart to El Niño and is crucial for understanding variations in climate and weather across different parts of the world.
Longshore drift: Longshore drift is the process by which sediment is transported along the coastline, parallel to the shore, by the action of waves approaching at an angle. This movement occurs due to the combination of wave direction and the angle at which waves hit the beach, creating a zigzag pattern of sediment transport. Understanding longshore drift is crucial for grasping coastal dynamics and the formation of various coastal landforms.
Maritime pollution: Maritime pollution refers to the contamination of ocean waters and coastal environments due to human activities, which can adversely affect marine ecosystems and coastal communities. It encompasses various pollutants, including oil spills, plastic debris, sewage discharge, and hazardous materials from ships. The patterns of ocean circulation play a significant role in spreading these pollutants across vast distances, affecting coastal processes and marine life.
Matthew Fontaine Maury: Matthew Fontaine Maury was a prominent American astronomer, oceanographer, and cartographer known for his pioneering work in the study of ocean currents and winds in the mid-19th century. He is often referred to as the 'Father of Modern Oceanography' due to his extensive research and the establishment of the first systematic charts of oceanic currents, which greatly advanced maritime navigation and understanding of ocean processes.
Neritic zone: The neritic zone is the shallow part of the ocean that extends from the low tide mark to the edge of the continental shelf, typically reaching depths of up to 200 meters. This area is characterized by sunlight penetration, which supports a diverse array of marine life and productivity. The neritic zone plays a vital role in coastal ecosystems, influencing both ocean basins and seafloor topography, as well as patterns of ocean circulation and coastal processes.
Sediment transport: Sediment transport refers to the movement of solid particles, such as sand, silt, and clay, from one location to another by natural forces, primarily water, wind, and ice. This process plays a critical role in shaping landforms along coastlines and riverbanks, influencing ecosystem dynamics and sedimentary environments. The mechanisms of sediment transport include erosion, transportation, and deposition, which collectively contribute to the evolution of landscapes and geological features.
Thermohaline circulation: Thermohaline circulation refers to the global system of ocean currents driven by differences in temperature and salinity, which affect water density. This process plays a crucial role in regulating climate patterns, distributing heat across the oceans, and influencing weather systems around the world. It connects various ocean basins and is integral to understanding oceanic processes and their impacts on coastal environments.
Upwelling: Upwelling is the process where deep, nutrient-rich water rises to the surface of the ocean, often due to wind patterns and ocean currents. This phenomenon plays a critical role in enhancing marine productivity, supporting diverse ecosystems by providing essential nutrients for phytoplankton growth. The presence of upwelling zones is vital for fisheries and can significantly affect local weather patterns.
Walter Munk: Walter Munk was a renowned oceanographer known for his groundbreaking work on ocean circulation and wave dynamics. His research has greatly influenced our understanding of the interactions between the ocean and the atmosphere, including how ocean currents affect climate and coastal processes. Munk's findings have provided essential insights into oceanic patterns that are crucial for predicting weather and understanding sea-level rise.
Wave refraction: Wave refraction is the bending of ocean waves as they approach the shore at an angle, which causes the waves to change direction and energy distribution. This phenomenon plays a critical role in shaping coastal landscapes, influencing erosion, sediment transport, and the formation of various coastal features like beaches and tidal flats.