Movements Of Ocean Water (Advanced)

Waves

Ocean waves are surface phenomena representing the transfer of energy through the water, primarily driven by wind. They are a crucial component of ocean dynamics, influencing coastal processes, marine ecosystems, and even global weather patterns.

Characteristics Of Waves

Waves are described by several key characteristics:

• Wave Crest: The highest point of a wave.
• Wave Trough: The lowest point of a wave.
• Wave Height (H): The vertical distance from the trough to the crest. It is a measure of the wave's energy.
• Wavelength (L): The horizontal distance between two successive crests or troughs.
• Wave Period (T): The time it takes for two successive crests or troughs to pass a fixed point.
• Wave Frequency (f): The number of waves passing a fixed point per unit of time (f = 1/T).
• Wave Speed (C): The speed at which a wave crest travels. In deep water, speed is primarily dependent on wavelength: $C = \sqrt{\frac{gL}{2\pi}}$, where $g$ is acceleration due to gravity and $L$ is wavelength. In shallow water, speed is dependent on depth: $C = \sqrt{gd}$, where $d$ is depth.
• Wave Steepness: The ratio of wave height to wavelength (H/L). When steepness exceeds a critical limit (approximately 1/7), the wave becomes unstable and breaks.

Orbital Motion of Water Particles:

• In deep water, water particles beneath a wave move in circular orbits. The diameter of these orbits is roughly equal to the wave height and decreases rapidly with depth.
• In shallow water, the orbits become flattened into ellipses as the wave interacts with the seabed.

Types of Waves Based on Generating Force:

• Wind Waves: The most common type, generated by the friction of wind blowing over the water surface. Their characteristics (height, period, speed) depend on wind speed, duration, and fetch.
• Tidal Waves (Tides): Caused by the gravitational pull of the Moon and Sun. They are essentially very long-wavelength waves.
• Tsunamis: Caused by sudden displacement of large volumes of water, often due to earthquakes, volcanic eruptions, or landslides. They have very long wavelengths and can travel across entire ocean basins.

Wave Breaking:

• Deep Water Breaking: Occurs when the wave's steepness becomes too great (H/L > 1/7).
• Shallow Water Breaking (Spilling Breaker): Occurs when a wave approaches a gently sloping shore. The wave crest slides down the front of the wave, creating a spilling breaker.
• Plunging Breaker: Occurs on steeper beaches. The crest outruns the base, and the wave plunges forward, creating a curling breaker.

Longshore Drift: Waves approaching the shore at an angle create a current parallel to the beach, known as the longshore current. This current transports sediment along the coastline.

Tides

Tides are the periodic rise and fall of sea levels caused by the gravitational forces exerted by the Moon and the Sun, combined with the Earth's rotation.

Types Of Tides

Tides are classified based on several criteria, including their frequency and the alignment of the Sun, Moon, and Earth.

Tides Based On Frequency

This classification describes the number of high and low tides that occur in a typical day (approximately 24 hours and 50 minutes, the time it takes for the Moon to appear in the same position in the sky).

• Diurnal Tides:
  • Description: One high tide and one low tide each lunar day.
  • Frequency: Approximately 24 hours and 50 minutes between successive high tides.
  • Location: Uncommon, found in specific locations like the Gulf of Mexico and parts of Southeast Asia.
• Semidiurnal Tides:
  • Description: Two high tides and two low tides each lunar day, with the heights of successive high tides and low tides being approximately equal.
  • Frequency: Approximately 12 hours and 25 minutes between successive high tides.
  • Location: Common along the Atlantic coasts of North America and Europe.
• Mixed Tides:
  • Description: Two high tides and two low tides each lunar day, but the heights of successive high tides and low tides are significantly different. This is a combination of diurnal and semidiurnal characteristics.
  • Location: Common along the Pacific coasts of North America and Asia.

Tides Based On The Sun, Moon And The Earth Positions

This classification relates to the combined gravitational influence of the Sun and Moon on Earth's tides.

• Spring Tides:
  • Alignment: Occur when the Sun, Moon, and Earth are aligned in a straight line. This happens during the new moon and full moon phases.
  • Gravitational Effect: The gravitational pulls of both the Sun and the Moon combine, creating stronger tidal forces.
  • Characteristics: Result in higher than average high tides and lower than average low tides. The tidal range is at its maximum.
• Neap Tides:
  • Alignment: Occur when the Sun and Moon are positioned at right angles to each other relative to the Earth. This happens during the first and third quarter moon phases.
  • Gravitational Effect: The gravitational pulls of the Sun and Moon partially counteract each other.
  • Characteristics: Result in lower than average high tides and higher than average low tides. The tidal range is at its minimum.

Importance Of Tides

Tides have significant impacts on coastal environments and human activities:

• Coastal Ecosystems: Tides create intertidal zones, which are unique habitats exposed to air at low tide and submerged at high tide. These zones are rich in biodiversity and support specialized organisms.
• Navigation: Tides influence the depth of water in harbors and estuaries, affecting the draft of ships and dictating safe times for navigation.
• Tidal Power Generation: The rise and fall of tides can be harnessed to generate electricity using tidal barrages or turbines, providing a renewable energy source.
• Coastal Morphology: Tidal currents can shape coastlines, erode beaches, and transport sediment, influencing coastal landforms.
• Marine Life Cycles: Many marine organisms synchronize their reproductive and feeding activities with tidal cycles.

Ocean Currents

Ocean currents are continuous, directed movements of seawater, forming vast circulatory systems that play a crucial role in distributing heat, nutrients, and dissolved gases across the globe.

Types Of Ocean Currents

Ocean currents are broadly classified based on their driving forces and depth:

• Surface Currents:
  • Definition: Currents that flow in the upper layer of the ocean (down to about 400 meters).
  • Driving Force: Primarily driven by friction between the wind and the water surface.
  • Characteristics: They follow prevailing wind patterns and are influenced by the Coriolis effect, continental boundaries, and the shape of the ocean floor. They form large, circular patterns called ocean gyres.
  • Speed: Generally faster than deep ocean currents.
• Deep Ocean Currents (Thermo-haline Circulation):
  • Definition: Currents that flow deep beneath the surface, driven by differences in water density.
  • Driving Force: Variations in temperature (thermo) and salinity (haline) of seawater. Colder, saltier water is denser and sinks, while warmer, less saline water is less dense and rises.
  • Characteristics: This circulation is very slow but moves enormous volumes of water across the globe, connecting all oceans. It originates in polar regions where water cools and becomes saltier (due to ice formation) and sinks.
• Tidal Currents:
  • Definition: Horizontal movements of water associated with the rise and fall of tides.
  • Driving Force: Gravitational pull of the Moon and Sun.
  • Characteristics: Occur in coastal areas, estuaries, and narrow straits. They are often strongest during spring tides and weakest during neap tides.

Major Ocean Currents

Ocean currents are often named based on their location or the direction they flow. They are integral parts of the large ocean gyres:

• North Atlantic Gyre:
  • Gulf Stream: A warm current flowing northeastward from the Gulf of Mexico, significantly warming Western Europe.
  • North Atlantic Drift: An extension of the Gulf Stream, continuing to warm Northern Europe.
  • Labrador Current: A cold current flowing southward from the Arctic along the coast of North America.
  • Canary Current: A cold current flowing southward along the coast of North Africa.
• North Pacific Gyre:
  • Kuroshio Current: A warm current flowing northward along the coast of Japan, similar to the Gulf Stream.
  • North Pacific Current: An extension of the Kuroshio, flowing eastward across the Pacific.
  • California Current: A cold current flowing southward along the coast of California.
  • Oyashio Current: A cold current flowing southward from the Arctic along the coast of Eastern Siberia and the Kamchatka Peninsula.
• South Atlantic Gyre:
  • Brazil Current: A warm current flowing southward along the coast of Brazil.
  • Benguela Current: A cold current flowing northward along the coast of Southwest Africa.
• South Pacific Gyre:
  • East Australian Current: A warm current flowing southward along the east coast of Australia.
  • West Australian Current: A cold current flowing northward along the west coast of Australia.
• Indian Ocean Gyre:
  • Agulhas Current: A warm current flowing southward along the east coast of Africa.
  • West Australian Current: (Mentioned above as part of South Pacific Gyre context, but flows north).
  • Somali Current: A unique current that reverses direction seasonally due to monsoons.
• Southern Ocean Currents:
  • Antarctic Circumpolar Current (ACC): The largest ocean current in the world, flowing eastward around Antarctica, driven by the Westerlies. It is the only current that flows unimpeded around the globe.
• Equatorial Currents: Flow westward near the equator, driven by the Trade Winds.

Deep Ocean Currents: For example, the North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) are major components of the thermohaline circulation.

Effects Of Ocean Currents

Ocean currents have profound impacts on Earth's climate, weather, and ecosystems:

• Heat Transport: They are the primary mechanism for transporting heat from the tropics towards the poles and cold from the poles towards the tropics. This significantly moderates global temperatures and makes higher latitudes more habitable. For example, the Gulf Stream warms Western Europe.
• Climate Regulation: Currents influence regional climates by transferring heat and moisture. Coastal areas influenced by warm currents tend to have milder, wetter climates, while those influenced by cold currents are often cooler and drier.
• Weather Patterns: Temperature differences between currents can influence atmospheric stability and contribute to the formation of fog or influence the intensity of storms.
• Nutrient Distribution: Deep ocean currents bring nutrient-rich waters to the surface (upwelling), supporting highly productive marine ecosystems and fisheries.
• Marine Life Migration: Currents act as highways for the migration of marine animals, influencing their distribution and abundance.
• Pollution Dispersal: Currents can transport pollutants (like oil spills or plastic debris) over vast distances, impacting marine environments worldwide.