Chapter 14 Movements Of Ocean Water

Ocean water is constantly in motion. The dynamic nature of ocean water is influenced by its physical characteristics, such as temperature, salinity, and density, as well as external forces exerted by the sun, moon, and winds.

Ocean water exhibits both horizontal and vertical movements:

• Horizontal Motion: This includes waves and ocean currents.
  • Waves: Primarily involve the horizontal transfer of energy across the water surface, while the water particles themselves largely move in circular paths.
  • Ocean Currents: Represent the continuous, large-scale flow of massive volumes of water in a distinct path and direction across ocean basins. Unlike waves, in currents, the water itself moves from one place to another.
• Vertical Motion: This includes the rhythmic rise and fall of sea level known as tides, caused mainly by the gravitational pull of the moon and sun. Vertical motion also encompasses the slower processes of upwelling (cold water rising from the depths to the surface) and downwelling (surface water sinking to deeper levels), often driven by density differences and wind.

Waves

Ocean waves are essentially the movement of energy across the surface of the water, not the mass movement of water itself over large distances. As a wave passes, water particles move in circular orbits, returning close to their original position (Figure 14.1).

Diagram showing how waves propagate energy horizontally while water particles move in circular orbits beneath the surface, with the orbit size decreasing with depth.

The primary source of energy for most ocean waves is the wind. Wind blowing over the water surface transfers energy to the water, generating waves. This energy is carried across the ocean and ultimately released onto coastlines as waves break.

The motion of waves is mainly confined to the surface layers and typically does not significantly affect the calm water found in the deep ocean below. As a wave approaches shallow water near the shore, it slows down due to friction with the seabed. When the water depth becomes less than half the wave's wavelength, the wave becomes unstable and breaks, forming surf.

Waves start as small ripples on calm water when a light breeze blows. As wind speed increases and blows over a greater distance and for a longer duration, these ripples grow into larger waves. Larger waves accumulate more energy from the wind and can travel thousands of kilometers across the open ocean before breaking on shore.

The size and shape of a wave can give clues about its origin. Steep waves are generally younger and created by local winds, whereas smoother, more regular waves (swells) have traveled long distances from their generation area. The maximum height a wave can reach is determined by the wind's strength, how long it blows (duration), and the distance over which it blows in a consistent direction (fetch).

Waves travel because wind pushes the water, and gravity acts on the water to pull the crests down. The falling water pushes the water in the preceding trough upwards, allowing the wave form to propagate forward.

Characteristics Of Waves

Waves are described using several standard characteristics:

• Wave Crest and Trough: The highest point of a wave is the crest; the lowest point is the trough.
• Wave Height: The vertical distance from the bottom of a trough to the top of a crest.
• Wave Amplitude: Half of the wave height.
• Wave Period: The time it takes for two consecutive wave crests or troughs to pass a fixed point.
• Wavelength: The horizontal distance between two successive wave crests or troughs.
• Wave Speed: The rate at which a wave propagates across the water surface.
• Wave Frequency: The number of waves that pass a fixed point in a given unit of time (usually one second). It is the inverse of the wave period.

Tides

Tides are the periodic, rhythmic rise and fall of the sea level. This phenomenon occurs once or twice daily and is primarily caused by the gravitational attraction of the moon and, to a lesser extent, the sun on the Earth's ocean waters. Wind and atmospheric pressure changes can also cause short-term sea level changes called surges, but these are not regular tidal movements.

The study of tides is complex due to variations in their frequency, height, and timing depending on location and time. The key forces driving tides are the gravitational pull of the moon and sun and the centrifugal force generated by the Earth-moon system's rotation around a common center of mass.

These forces create two major tidal bulges on the Earth. One bulge forms on the side of the Earth directly facing the moon, where the moon's gravitational pull is strongest. A second bulge forms on the opposite side of the Earth. Although the moon's gravitational pull is weakest here, the centrifugal force (an outward-acting inertial force) is dominant, pulling water away from the Earth's center and creating a bulge (Figure 14.2).

Diagram showing how the gravitational attraction of the Moon and the centrifugal force of the Earth-Moon system combine to create two tidal bulges on opposite sides of the Earth.

The "tide-generating force" at any point on Earth is the difference between the gravitational attraction of a celestial body (moon or sun) and the average centrifugal force acting on the Earth as a whole. On the side closest to the moon, gravity dominates, causing a bulge. On the opposite side, the weaker gravity is overcome by the stronger centrifugal force, also causing a bulge. Water flows horizontally across the Earth's surface towards these bulges.

The magnitude of tides can be influenced by local factors such as the shape of the coastline and the seafloor. Wide continental shelves can amplify tidal bulges, while mid-oceanic islands have smaller tides. Funnel-shaped bays or estuaries can significantly increase tidal ranges. When tidal flow is channeled through narrow passages or into bays, these are called tidal currents.

The Bay of Fundy in Canada is famous for having the highest tides in the world, reaching 15-16 meters due to its specific shape and the resonance of tidal movements.

Types Of Tides

Tides are classified based on their frequency of occurrence per day or their height variations.

Tides Based On Frequency

• Semi-diurnal Tide: The most common type globally, featuring two high tides and two low tides of approximately equal height within a tidal day (roughly 24 hours and 52 minutes).
• Diurnal Tide: Occurs when there is only one high tide and one low tide during each tidal day. The successive high and low tides are usually of similar height.
• Mixed Tide: Exhibits variations in height, with successive high or low tides having different heights. This type is common along the west coast of North America and in parts of the Pacific Ocean.

Tides Based On The Sun, Moon And The Earth Positions

The relative positions of the Sun, Moon, and Earth significantly influence the height of the tides because the Sun's gravity also contributes to the tide-generating force. Tides based on these alignments are:

• Spring Tides: Occur when the Sun, Moon, and Earth are aligned in a straight line. In this configuration (syzygy), the gravitational pulls of the Sun and Moon combine (either both on the same side of Earth during New Moon, or on opposite sides during Full Moon), resulting in reinforced tidal bulges and thus higher high tides and lower low tides than average. Spring tides occur twice a month, during the New Moon and Full Moon phases.
• Neap Tides: Occur when the Sun and Moon are positioned at right angles ($90^\circ$) relative to the Earth. In this configuration, the gravitational forces of the Sun and Moon partially counteract each other. This results in lower high tides and higher low tides than average, meaning a reduced tidal range. Neap tides occur twice a month, during the first and third quarter moon phases, approximately seven days after spring tides.

Other astronomical factors also slightly affect tidal range:

• When the Moon is closest to the Earth in its orbit (perigee, approximately once a month), its gravitational pull is strongest, resulting in slightly higher spring tides and lower neap tides (greater tidal range).
• When the Moon is farthest from the Earth (apogee, approximately once a month), its gravitational pull is weaker, resulting in smaller than average tidal ranges.
• Similarly, when the Earth is closest to the Sun (perihelion, around January 3rd), solar tidal forces are stronger, contributing to larger tidal ranges.
• When the Earth is farthest from the Sun (aphelion, around July 4th), solar tidal forces are weaker, resulting in smaller tidal ranges.

The period when the tide is falling (from high to low tide) is called the ebb tide or ebb. The period when the tide is rising (from low to high tide) is called the flow tide or flood tide.

Importance Of Tides

Tides are predictable phenomena because the positions of the Earth, Moon, and Sun are known in advance. This predictability makes tides important for various human activities:

• Navigation: Tidal predictions (tide tables) are crucial for shipping and navigation, especially in harbors and shallow coastal areas. Knowledge of high tide is necessary for large ships to enter or leave ports, particularly those with shallow entrance channels or sandbars.
• Harbour Operations: Tidal currents can assist ships entering or leaving harbors.
• Coastal Processes: Tidal flows help to flush sediments out of estuaries and harbors and disperse polluted water.
• Fishing: Fishermen use tidal information to plan their fishing trips, as tidal movements can affect fish behavior and distribution.
• Tidal Power Generation: The energy of rising and falling tides can be harnessed to generate electricity. Tidal power projects, while not widespread, exist in some countries like Canada, France, Russia, and China.

Ocean Currents

Ocean currents are continuous, directed movements of seawater within ocean basins. They are like vast rivers flowing through the ocean, carrying massive amounts of water over long distances in defined paths and directions. Ocean currents are influenced by a combination of primary forces that initiate water movement and secondary forces that modify their flow.

Primary forces that initiate ocean currents:

• Heating by Solar Energy: Uneven solar heating of the ocean surface causes water to expand (become less dense) in warmer regions, creating slight differences in sea level (e.g., water level near the equator is slightly higher). This creates a minor pressure gradient that drives water to flow from higher to lower levels.
• Wind: Wind blowing over the ocean surface exerts stress and drags the surface water along. Persistent prevailing winds (like the Trade Winds and Westerlies) drive the major surface currents.
• Gravity: Gravity pulls water downwards, helping to create pressure gradients due to density differences (denser water sinks, less dense water rises) and also influences the flow of water down slopes created by differences in sea level.
• Coriolis Force: The Earth's rotation deflects moving ocean currents (like winds) to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This force is crucial in causing currents to flow in large circular patterns (Gyres) within ocean basins.

Secondary forces that influence current flow include friction (resistance from surrounding water and the seabed), and the shape of coastlines and ocean floor topography which can deflect or channel currents. Differences in water density, caused by variations in temperature and salinity, drive deep-water currents. Cold water is denser than warm water, and saltier water is denser than less salty water. Denser water tends to sink, while lighter water rises, creating vertical water movements and driving deep ocean circulation (thermohaline circulation).

Types Of Ocean Currents

Ocean currents can be classified based on their depth or temperature:

Based on Depth:

• Surface Currents: Make up about 10% of the total volume of ocean water. They are generally found in the upper 400 meters ($400 \, m$) of the ocean and are primarily driven by wind stress and modified by the Coriolis force.
• Deep Water Currents: Constitute the remaining 90% of ocean water. They move slower than surface currents and are driven by density differences (thermohaline circulation) caused by variations in temperature and salinity. Deep water masses form in high-latitude areas where surface water becomes cold and dense enough to sink into the deep ocean basins.

Based on Temperature:

• Cold Currents: Bring colder water from higher latitudes or from deep ocean areas into warmer water regions. In the low and middle latitudes, cold currents are typically found along the west coasts of continents (e.g., Peru Current off South America). In the Northern Hemisphere high latitudes, they can occur on the east coasts (e.g., Labrador Current).
• Warm Currents: Bring warmer water from lower latitudes into colder water regions. In the low and middle latitudes, warm currents are usually found along the east coasts of continents (e.g., Gulf Stream off North America, Brazil Current off South America). In the Northern Hemisphere high latitudes, they can occur on the west coasts (e.g., North Atlantic Drift warming Europe).

The speed or strength of a current is referred to as its "drift", measured in knots. Currents are generally strongest near the surface and their speed decreases with depth. Fast currents are considered strong; most currents have speeds less than or equal to 5 knots ($< 9.3 \, km/h$).

Major Ocean Currents

The large-scale patterns of major ocean currents (Figure 14.3) are strongly influenced by prevailing winds and the Coriolis force. The overall ocean circulation pattern broadly mimics the pattern of atmospheric circulation (the major wind belts). In the middle latitudes, the large circular currents (gyres) often follow the anticyclonic (clockwise in NH, anticlockwise in SH) circulation patterns of the subtropical atmospheric highs, while in higher latitudes, they may follow the cyclonic patterns associated with subpolar lows. In monsoon regions, seasonal wind reversals can cause seasonal reversals in ocean currents.

Map illustrating the general pattern and names of major warm (red arrows) and cold (blue arrows) ocean currents in the Pacific, Atlantic, and Indian Oceans.

Due to the Coriolis force, warm currents originating from low latitudes tend to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Conversely, cold currents originating from high latitudes are also deflected, contributing to the gyre circulation.

Major ocean currents play a vital role in transporting heat globally, moving energy from the energy-surplus regions in the tropics towards the energy-deficit polar regions, complementing the heat transport by atmospheric circulation. Cold waters from the Arctic and Antarctic move towards the tropics, and warm waters from the tropics move polewards.

Examples of currents influenced by prevailing winds (Figure 14.3):

• The North Equatorial Current and South Equatorial Current in the tropics are driven westward by the Trade Winds.
• The North Atlantic Current (North Atlantic Drift) and North Pacific Current in the mid-latitudes are driven eastward by the Westerlies.
• The seasonal change in wind direction during the Indian Monsoon causes a significant seasonal reversal in the currents of the northern Indian Ocean.

Effects Of Ocean Currents

Ocean currents have significant impacts on coastal climates, marine ecosystems, and human activities:

• Coastal Climate Modification:
  • West coasts of continents in tropical and subtropical latitudes (except near the equator) are often bordered by cold currents (e.g., Peru Current, California Current). These currents bring cooler temperatures than expected for the latitude, reduce evaporation, and are often associated with arid conditions and frequent fog (e.g., coastal deserts like Atacama, Namib).
  • West coasts of continents in middle and higher latitudes are often bordered by warm currents (e.g., North Atlantic Drift, Alaska Current). These currents bring warmer temperatures and significant moisture, resulting in a distinct marine climate characterized by cool summers, relatively mild winters, and narrow annual temperature ranges (e.g., the mild climate of northwestern Europe compared to inland areas at similar latitudes).
  • East coasts of continents in tropical and subtropical latitudes are often bordered by warm currents (e.g., Gulf Stream, Brazil Current). These currents contribute to warmer and more humid conditions and bring rainfall (e.g., southeastern USA, eastern South America).
• Marine Life and Fishing Grounds: Areas where warm and cold currents meet are often excellent fishing grounds. The mixing of these waters leads to turbulence that brings nutrients from the deep ocean to the surface. This nutrient enrichment supports the growth of phytoplankton (plankton), which are the base of the marine food web, leading to abundant fish populations. Examples include the meeting points of the Gulf Stream and Labrador Current near Newfoundland, or the Kuroshio and Oyashio Currents off Japan.
• Navigation: Currents can affect the speed and route of ships. Navigators utilize currents to save time and fuel, or avoid them if they are adverse.

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