12.2 Ocean currents and waves

Ocean currents and waves shape our planet's marine environment. These powerful forces distribute heat, nutrients, and marine life across vast distances, influencing climate and ecosystems worldwide.

Understanding ocean currents and waves is crucial for predicting weather patterns, managing coastal areas, and navigating the seas. From surface currents to deep ocean circulation, these dynamic processes play a vital role in Earth's complex oceanic system.

Types of ocean currents

• Ocean currents are continuous, directed movements of seawater that play a crucial role in the global distribution of heat, nutrients, and marine life
• Currents can be classified based on their location in the water column, the forces that drive them, and their role in global ocean circulation

Surface vs deep currents

Top images from around the web for Surface vs deep currents

• 

  Surface Currents | Physical Geography View original

  Is this image relevant?

• 

  Ocean Currents | Earth Science View original

  Is this image relevant?

• 

  Deep Currents | Physical Geography View original

  Is this image relevant?

• 

  Surface Currents | Physical Geography View original

  Is this image relevant?

• 

  Ocean Currents | Earth Science View original

  Is this image relevant?

1 of 3

Top images from around the web for Surface vs deep currents

• 

  Surface Currents | Physical Geography View original

  Is this image relevant?

• 

  Ocean Currents | Earth Science View original

  Is this image relevant?

• 

  Deep Currents | Physical Geography View original

  Is this image relevant?

• 

  Surface Currents | Physical Geography View original

  Is this image relevant?

• 

  Ocean Currents | Earth Science View original

  Is this image relevant?

1 of 3

• Surface currents occur in the upper 400 meters of the ocean and are primarily driven by wind stress on the ocean surface
• Deep currents flow below the thermocline and are driven by density differences resulting from variations in temperature and salinity
• Surface currents generally move faster than deep currents and have a more direct impact on climate and marine ecosystems

Geostrophic currents

• Geostrophic currents are large-scale, steady flows that result from the balance between the Coriolis force and the pressure gradient force
• These currents flow along lines of constant pressure (isobars) and are perpendicular to the pressure gradient
• Examples of geostrophic currents include the Gulf Stream and the Kuroshio Current

Ekman currents

• Ekman currents are wind-driven currents that arise due to the balance between wind stress, Coriolis force, and friction
• In the Northern Hemisphere, Ekman transport is 90° to the right of the wind direction; in the Southern Hemisphere, it is 90° to the left
• Ekman currents contribute to upwelling and downwelling processes, which have significant implications for marine productivity

Thermohaline circulation

• Thermohaline circulation, also known as the global conveyor belt, is a large-scale ocean circulation pattern driven by density differences caused by temperature and salinity variations
• This circulation involves the sinking of cold, dense water at high latitudes and the upwelling of warmer, less dense water in other regions
• Thermohaline circulation plays a vital role in the global distribution of heat, nutrients, and carbon dioxide

Causes of ocean currents

• Ocean currents are driven by a combination of forces, including wind stress, density differences, tidal forces, and the Earth's rotation
• Understanding the causes of ocean currents is essential for predicting their behavior and their impact on climate, marine ecosystems, and human activities

Wind-driven currents

• Wind-driven currents are generated by the friction between the wind and the ocean surface, which creates a stress that sets the water in motion
• The direction and strength of wind-driven currents depend on the prevailing wind patterns, such as the trade winds and westerlies
• Examples of wind-driven currents include the Antarctic Circumpolar Current and the North Atlantic Drift

Density-driven currents

• Density-driven currents, also known as thermohaline currents, arise from differences in water density caused by variations in temperature and salinity
• Cold, saline water is denser than warm, less saline water and tends to sink, creating deep ocean currents
• Density-driven currents are a key component of the global conveyor belt and contribute to the distribution of heat and nutrients in the ocean

Tidal currents

• Tidal currents are caused by the gravitational pull of the moon and sun on the Earth's oceans
• These currents are most pronounced in coastal areas and can be either flood currents (flowing towards the shore) or ebb currents (flowing away from the shore)
• Tidal currents can influence coastal erosion, sediment transport, and the distribution of marine organisms

Coriolis effect on currents

• The Coriolis effect is an apparent force that arises due to the Earth's rotation and influences the direction of ocean currents
• In the Northern Hemisphere, the Coriolis force deflects moving objects to the right; in the Southern Hemisphere, it deflects them to the left
• The Coriolis effect contributes to the formation of large-scale circulation patterns, such as gyres and boundary currents

Major ocean currents

• Major ocean currents are large-scale, persistent flows that transport vast amounts of water, heat, and nutrients across ocean basins
• These currents play a crucial role in regulating global climate, influencing marine ecosystems, and facilitating maritime transportation

Gyres in ocean basins

• Gyres are large, circular ocean currents that span entire ocean basins and are driven by a combination of wind stress and the Coriolis effect
• There are five major gyres: the North Pacific, South Pacific, North Atlantic, South Atlantic, and Indian Ocean gyres
• Gyres are composed of multiple currents and typically rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere

Gulf Stream

• The Gulf Stream is a powerful, warm ocean current that originates in the Gulf of Mexico and flows along the eastern coast of the United States before crossing the Atlantic Ocean
• This current transports warm, saline water from the tropics to the higher latitudes, influencing the climate of the eastern United States and western Europe
• The Gulf Stream is part of the North Atlantic Gyre and plays a crucial role in the global heat budget

Kuroshio Current

• The Kuroshio Current, also known as the Japan Current, is a warm ocean current that flows northeastward along the coast of Japan
• This current is the western boundary current of the North Pacific Gyre and is characterized by its high velocity and large volume transport
• The Kuroshio Current influences the climate of Japan and the North Pacific and is an important factor in the distribution of marine life

Antarctic Circumpolar Current

• The Antarctic Circumpolar Current (ACC) is the strongest ocean current in the world, flowing eastward around Antarctica and connecting the Atlantic, Pacific, and Indian Oceans
• The ACC is driven by strong westerly winds and is not constrained by continental boundaries, allowing it to flow uninterrupted around the globe
• This current plays a crucial role in the global circulation of heat, nutrients, and carbon dioxide and influences the climate of the Southern Hemisphere

Properties of ocean waves

• Ocean waves are oscillations of the sea surface that transfer energy through the water without significant net movement of water particles
• Understanding the properties of ocean waves is essential for predicting their behavior, assessing their impact on coastal environments, and designing marine structures

Wave height vs wavelength

• Wave height is the vertical distance between the crest (highest point) and the trough (lowest point) of a wave
• Wavelength is the horizontal distance between two consecutive crests or troughs
• The ratio of wave height to wavelength influences the steepness of a wave and its potential to break

Wave period vs frequency

• Wave period is the time it takes for two consecutive crests or troughs to pass a fixed point
• Wave frequency is the number of wave crests that pass a fixed point per unit time and is the reciprocal of the wave period
• Waves with shorter periods (higher frequencies) tend to have less energy than waves with longer periods (lower frequencies)

Wave speed vs water depth

• Wave speed, also known as phase speed, is the rate at which a wave crest moves through the water
• In deep water, wave speed depends only on the wave period or wavelength and is not influenced by water depth
• In shallow water, wave speed is influenced by both wavelength and water depth, with waves slowing down as they approach the shore

Energy transport by waves

• Ocean waves transport energy across the sea surface without significant net transport of water particles
• The energy of a wave is proportional to the square of its height and is divided into potential energy (due to the displacement of water from its equilibrium position) and kinetic energy (due to the motion of water particles)
• As waves propagate, they can transfer energy over long distances and influence coastal processes, such as erosion and sediment transport

Types of ocean waves

• Ocean waves can be classified based on their generation mechanisms, propagation characteristics, and physical properties
• Understanding the different types of ocean waves is crucial for predicting their behavior, assessing their potential impacts, and developing strategies for coastal management and protection

Wind-generated waves

• Wind-generated waves are the most common type of ocean waves and are created by the friction between wind and the sea surface
• As wind blows over the ocean, it transfers energy to the water, causing small ripples to form and eventually grow into larger waves
• The size and shape of wind-generated waves depend on the wind speed, duration, and fetch (the distance over which the wind blows)

Swell waves

• Swell waves are long-period, regular waves that have propagated away from their area of generation and are no longer influenced by the local wind
• These waves are characterized by their smooth, sinusoidal shape and can travel vast distances across ocean basins with little energy loss
• Swell waves are an important factor in coastal processes, such as beach erosion and longshore sediment transport

Tsunamis

• Tsunamis are long-wavelength, shallow-water waves generated by sudden, large-scale disturbances of the sea surface, such as earthquakes, landslides, or volcanic eruptions
• These waves can travel at high speeds (up to 800 km/h) in the open ocean and have extremely long wavelengths (up to hundreds of kilometers)
• As tsunamis approach shallow water, they slow down and increase in height, potentially causing severe damage and loss of life in coastal areas

Internal waves

• Internal waves are gravity waves that oscillate within the interior of the ocean, rather than on the sea surface
• These waves are generated by the interaction of tidal currents or other flow disturbances with underwater topography, such as seamounts or continental shelves
• Internal waves can have large amplitudes (up to hundreds of meters) and play a crucial role in mixing and energy transfer within the ocean

Wave dynamics

• Wave dynamics encompasses the various processes and interactions that govern the behavior of ocean waves as they propagate, transform, and dissipate
• Understanding wave dynamics is essential for predicting wave conditions, assessing the impact of waves on coastal environments, and designing effective coastal protection measures

Wave-current interactions

• Wave-current interactions occur when ocean waves propagate through areas with significant currents, such as tidal flows or ocean circulation patterns
• Currents can modify the speed, direction, and height of waves, leading to phenomena such as wave refraction, shoaling, and breaking
• These interactions have important implications for navigation safety, coastal erosion, and the distribution of sediments and pollutants

Wave refraction vs diffraction

• Wave refraction occurs when waves propagate over varying water depths or encounter changes in current velocity, causing the wave crests to bend and align themselves with the depth contours
• Wave diffraction occurs when waves encounter obstacles, such as islands or breakwaters, and bend around them, spreading energy into the sheltered regions behind the obstacles
• Both refraction and diffraction can significantly alter the distribution of wave energy along the coast and influence coastal morphology

Wave reflection vs transmission

• Wave reflection occurs when waves encounter a rigid, impermeable boundary, such as a seawall or cliff, and are redirected back towards the sea
• Wave transmission occurs when waves pass through or over a permeable or submerged structure, such as a breakwater or reef, with a portion of the wave energy being transferred to the other side
• The balance between reflection and transmission depends on the properties of the boundary and the characteristics of the incident waves

Wave breaking mechanisms

• Wave breaking is the process by which waves become unstable and dissipate their energy through turbulence and air entrainment
• There are several mechanisms that can lead to wave breaking, including:
  1. Steepness-induced breaking, which occurs when the wave height becomes too large relative to the wavelength
  2. Depth-induced breaking, which occurs when waves propagate into shallow water and the water depth becomes too small relative to the wave height
  3. Wind-induced breaking, which occurs when strong winds blow against the direction of wave propagation, causing the waves to steepen and break
• Wave breaking plays a crucial role in energy dissipation, sediment transport, and the generation of nearshore currents

Coastal processes

• Coastal processes refer to the various physical, chemical, and biological phenomena that shape and modify coastlines over time
• Understanding coastal processes is essential for managing coastal resources, mitigating the impacts of natural hazards, and developing sustainable coastal development strategies

Longshore currents

• Longshore currents are shore-parallel currents that are generated by the breaking of obliquely incident waves and the resulting longshore component of the radiation stress
• These currents flow along the coast in the direction of the prevailing wave approach and can transport large amounts of sediment, leading to the formation of features such as spits and barrier islands
• Longshore currents play a crucial role in the distribution of sediments, pollutants, and marine organisms along the coast

Rip currents

• Rip currents are strong, narrow, seaward-flowing currents that originate near the shore and extend perpendicular to the coastline
• These currents are generated by the convergence of longshore currents or the channeling of water through gaps in nearshore sandbars
• Rip currents can pose a significant hazard to swimmers and are responsible for many drowning incidents on beaches worldwide

Sediment transport by waves

• Waves are a primary driver of sediment transport in coastal environments, with the ability to erode, transport, and deposit sediments on beaches, dunes, and nearshore areas
• Sediment transport by waves occurs through several mechanisms, including:
  1. Bed load transport, where sediment particles roll, slide, or bounce along the seabed
  2. Suspended load transport, where sediment particles are lifted into the water column and carried by the wave-induced currents
  3. Swash transport, where sediment is moved up and down the beach face by the uprush and backwash of waves
• The rate and direction of sediment transport depend on factors such as wave height, period, and direction, as well as the size and composition of the sediment particles

Coastal erosion vs accretion

• Coastal erosion is the process by which sediment is removed from the shoreline, leading to the landward retreat of the coastline
• Coastal accretion is the process by which sediment is added to the shoreline, leading to the seaward advance of the coastline
• The balance between erosion and accretion determines the long-term evolution of coastal morphology and is influenced by factors such as:
  1. Wave climate and storm events
  2. Sediment supply from rivers, cliffs, and offshore sources
  3. Coastal structures and human interventions
  4. Sea-level rise and subsidence
• Managing coastal erosion and promoting accretion are key challenges for coastal communities and require a deep understanding of the underlying processes and their interactions

Measuring ocean currents and waves

• Accurate measurement of ocean currents and waves is essential for understanding their behavior, predicting their impacts, and supporting a wide range of marine activities, such as navigation, offshore engineering, and coastal management
• Various techniques and instruments are used to measure currents and waves, each with its own advantages and limitations

Lagrangian vs Eulerian methods

• Lagrangian methods involve tracking the motion of individual water particles or drifters as they move with the currents
• Eulerian methods involve measuring the velocity or other properties of the water at fixed locations over time
• Lagrangian methods provide a direct measure of the path and speed of water movement, while Eulerian methods provide a snapshot of the flow field at a given time and location

Current meters

• Current meters are instruments that measure the speed and direction of water flow at a specific point in the water column
• There are several types of current meters, including:
  1. Mechanical current meters, which use propellers or rotors to measure flow velocity
  2. Electromagnetic current meters, which measure the voltage induced by the movement of conductive seawater through a magnetic field
  3. Acoustic current meters, which use the Doppler effect to measure flow velocity based on the frequency shift of sound waves reflected by moving water particles
• Current meters can be deployed from ships, moorings, or autonomous underwater vehicles to provide time series of current velocity and direction

Acoustic Doppler current profilers

• Acoustic Doppler current profilers (ADCPs) are instruments that use sound waves to measure the velocity of water at multiple depths simultaneously
• ADCPs emit high-frequency sound pulses and measure the Doppler shift of the echoes reflected by suspended particles in the water, which are assumed to move with the same velocity as the water
• By measuring the Doppler shift at different time intervals and depths, ADCPs can provide a detailed profile of the current velocity and direction throughout the water column
• ADCPs can be mounted on ships, moorings, or seafloor platforms and are widely used for mapping ocean currents, studying circulation patterns, and monitoring flow in coastal and estuarine environments

Remote sensing techniques

• Remote sensing techniques involve measuring ocean currents and waves from a distance using sensors mounted on satellites, aircraft, or land-based platforms
• Some common remote sensing techniques for measuring currents and waves include:
  1. Altimetry, which uses radar to measure the height of the sea surface and infer the underlying currents based on the geostrophic balance
  2. Synthetic aperture radar (SAR), which uses high-resolution radar imagery to detect surface roughness patterns associated with currents and waves
  3. High-frequency (HF) radar, which uses shore-based radar systems to measure the speed and direction of surface currents based on the Doppler shift of the reflected radio waves
  4. Optical remote sensing, which uses visible and infrared imagery to track the movement of surface features, such as fronts, eddies, and slicks, that are associated with currents and waves
• Remote sensing techniques provide a synoptic view of ocean currents and waves over large areas and can complement in-situ measurements to improve our understanding of ocean dynamics