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Chapter 13 Movements Of Ocean Water
Ocean water is constantly in motion, a dynamic characteristic influenced by its physical properties like temperature, salinity, and density, as well as external forces from celestial bodies (sun and moon) and the atmosphere (wind). The movements of ocean water can be categorized into horizontal and vertical motions.
- **Horizontal Motion:** Refers to the large-scale movement of water across the ocean surface. This includes **ocean currents** (continuous flow of vast amounts of water in a definite direction) and **waves** (horizontal transfer of energy across the surface). In ocean currents, water itself moves from one place to another, while in waves, it is primarily the energy that travels, not the water particles over long distances.
- **Vertical Motion:** Refers to the upward and downward movement of ocean water. The most significant vertical motion is the periodic rise and fall of sea level called **tides**, driven primarily by the gravitational pull of the sun and moon. Other forms of vertical motion include the **upwelling** of cold water from deeper levels to the surface and the **sinking** (downwelling) of surface water to deeper levels, driven by density differences.
Waves
**Waves** are generated when energy is transferred to the water surface. The most common source of this energy is **wind**. When wind blows over the water, it transfers some of its energy through friction, creating ripples that can grow into larger waves. As a wave travels across the ocean surface, it's important to understand that it is primarily the **energy** that is moving forward, not the mass of water itself over long distances in open water. Water particles within a wave move in circular or elliptical paths, returning to roughly their original position as the wave passes. (This refers to Figure 13.1).
Wind speed, the duration for which the wind blows, and the distance over which the wind blows in a single direction (the fetch) determine the size and characteristics of the waves generated. Small ripples form even with a light breeze (2 knots or less), growing into larger waves with increasing wind speed. The largest waves are typically found in the open ocean with strong, consistent winds over long distances.
As a wave approaches the shoreline and enters shallower water, it undergoes changes. The base of the wave begins to drag against the seafloor, causing friction that slows the wave down. As the wave's speed decreases and its crest outruns its base, the wave becomes unstable and "breaks" when the water depth is less than about half of the wave's wavelength. The breaking wave releases its energy onto the shoreline as **surf**.
The shape and size of waves can provide clues about their origin. Steep, choppy waves are usually young and generated by local winds. Smooth, long-period swells (waves that travel great distances) are typically older and originate from storm systems far away, potentially even in another hemisphere.
Waves propagate forward due to the interplay of wind (transferring energy) and gravity (pulling the raised wave crests downwards). The downward pull on the crests forces the water into the adjacent troughs, pushing the troughs upwards and causing the wave form to move to a new position. The circular motion of water particles beneath a wave means that anything floating on the surface is lifted and moved slightly forward as a crest approaches, and moved down and slightly backward as a trough passes.
Characteristics Of Waves
Key terms are used to describe the anatomy and properties of waves:
- **Wave Crest:** The highest point of a wave.
- **Wave Trough:** The lowest point of a wave.
- **Wave Height:** The vertical distance measured from the bottom of a wave trough to the top of the following wave crest.
- **Wave Amplitude:** Half of the wave height; the vertical distance from the undisturbed sea level to the crest or trough.
- **Wavelength:** The horizontal distance between two successive wave crests or two successive wave troughs.
- **Wave Period:** The time it takes for two successive wave crests (or troughs) to pass a fixed point.
- **Wave Speed:** The rate at which the wave form travels across the water surface, typically measured in knots or meters per second. It is related to wavelength and period (Speed = Wavelength / Period).
- **Wave Frequency:** The number of waves that pass a fixed point in a one-second time interval. (Frequency = 1 / Period).
Tides
**Tides** are the regular, periodic rise and fall of sea level that occur most places on Earth once or twice each day. Tides are primarily caused by the gravitational attraction of the **moon** and, to a lesser extent, the **sun**, acting on the Earth's oceans. Movements of water caused by strong winds and atmospheric pressure changes (like during storms) are called **surges** or storm surges, which are irregular and not tidal in nature.
The primary forces responsible for tides are:
- **Gravitational Pull:** The moon's gravity exerts a pull on the Earth, including its oceans. This pull is strongest on the side of the Earth facing the moon and decreases with distance. The sun also exerts a gravitational pull, but it is weaker than the moon's tidal influence because the sun is much farther away.
- **Centrifugal Force:** As the Earth and moon orbit around a common center of mass, a centrifugal force is generated. This force acts equally on all parts of the Earth, directed outwards, away from the center of the Earth-moon system. It acts to counterbalance gravity and keep the Earth in orbit.
Together, the gravitational attraction and the centrifugal force create two main tidal bulges on the Earth's surface. A bulge forms on the side of the Earth facing the moon due to the moon's stronger gravitational pull dominating over the centrifugal force. A second bulge forms on the opposite side of the Earth. On this far side, the moon's gravitational pull is weakest, allowing the centrifugal force to dominate and pull the water away from the Earth's center, creating a bulge. (This refers to Figure 13.2 illustrating the tidal bulges).
The "tide-generating force" is the net force resulting from the difference between the moon's gravitational pull and the centrifugal force at different points on Earth's surface. While there are vertical components to these forces, the **horizontal components** are more significant in generating the tidal bulges and causing the movement of water that results in the rise and fall of sea level observed as tides.
The shape of the coastline and seafloor can greatly influence tidal magnitudes. Wide continental shelves can amplify tidal bulges, leading to higher tides. Funnel-shaped bays, like the Bay of Fundy, can significantly magnify tidal range as the incoming tide is squeezed into a progressively narrower and shallower area. When tidal flow is concentrated in channels between islands or within bays/estuaries, it creates relatively strong currents known as **tidal currents**.
Tides Of Bay Of Fundy, Canada
The **Bay of Fundy**, located between New Brunswick and Nova Scotia in Canada, is famous for having the **highest tidal range in the world**. The difference between high tide and low tide can reach an astonishing 15-16 meters (around 50 feet). Due to the geometry of the bay and the arrival of the tidal bulge, water rushes in and out very rapidly. With a typical semi-diurnal tide (two high and two low tides per day), the water level can rise or fall by several meters per hour during the peak flow, creating strong tidal currents. This rapid and extreme change in water level poses navigational challenges and highlights the power of tides in specific geographic settings.
Types Of Tides
Tides can be classified based on their frequency (how many high and low tides occur per day) and their height variations.
Tides Based On Frequency
- **Semi-diurnal Tide:** The most common type globally, characterized by **two high tides and two low tides** of roughly equal height each day (approximately every 12 hours and 25 minutes).
- **Diurnal Tide:** Occurs in some areas, featuring only **one high tide and one low tide** each day (approximately every 24 hours and 50 minutes). The successive high and low tides usually have similar heights.
- **Mixed Tide:** A pattern where tides vary significantly in height. There might be two high tides and two low tides per day, but the heights of the two high tides (or two low tides) are noticeably different. This pattern 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 influence the magnitude of the tidal bulges and thus the height of the tides. The combined gravitational pulls enhance or counteract each other.
- **Spring Tides:** Occur when the sun, moon, and Earth are aligned in a straight line (during **new moon** and **full moon** phases, approximately twice a month). In this alignment, the gravitational pulls of the sun and moon are combined, resulting in the **highest high tides** and the **lowest low tides**. The tidal range (difference between high and low tide) is maximum during spring tides.
- **Neap Tides:** Occur when the sun, moon, and Earth are positioned at right angles to each other (during the **first and third quarter moon** phases, approximately a week after spring tides). In this configuration, the gravitational pulls of the sun and moon partially counteract each other, resulting in the **lowest high tides** and the **highest low tides**. The tidal range is minimum during neap tides.
Additional factors influencing tidal height based on distance:
- **Perigee/Apogee:** The moon's orbit around Earth is elliptical. When the moon is closest to Earth (perigee), its gravitational pull is strongest, resulting in higher tidal ranges. When it is farthest (apogee), its pull is weaker, resulting in lower tidal ranges. This cycle occurs roughly once a month.
- **Perihelion/Aphelion:** Earth's orbit around the sun is also elliptical. When Earth is closest to the sun (perihelion, around January 3rd), the sun's gravitational pull is strongest, leading to slightly higher tidal ranges overall. When Earth is farthest (aphelion, around July 4th), the sun's pull is weaker, resulting in slightly lower tidal ranges.
The phase when the tide is falling from high to low is called the **ebb tide**. The phase when the tide is rising from low to high is called the **flow tide** or **flood tide**.
Importance Of Tides
Predicting tides accurately is very important due to their various practical applications:
- **Navigation:** Knowledge of high and low tides is essential for ships entering or leaving harbors, especially those with shallow entrances or bars that might limit access at low tide. Tidal currents can also affect navigation.
- **Fishing:** Fishermen often use tidal information to plan their trips, as fish behavior can be influenced by tidal cycles and associated currents.
- **Coastal Processes:** Tidal currents help in removing sediments from estuaries and harbors (desilting) and flushing out polluted water, aiding in coastal sanitation.
- **Tidal Energy Generation:** The rise and fall of tides can be harnessed to generate electricity using tidal power plants, such as projects in Canada, France, Russia, and China. A tidal power project is also planned or underway in the Sunderbans region of West Bengal, India.
Ocean Currents
**Ocean currents** are continuous, directed movements of large volumes of ocean water. They can be thought of as "rivers" flowing within the oceans. Ocean currents are initiated and influenced by a combination of forces:
Primary forces that start the movement:
- **Solar Heating:** Uneven heating causes temperature and density differences. Warmer water near the equator expands, creating a slightly higher sea level than in colder regions. This very slight gradient drives a slow flow of water downslope.
- **Wind:** Prevailing winds blowing over the ocean surface exert friction on the water, pushing it and creating surface currents.
- **Gravity:** Pulls denser water downwards and influences the flow of water down density or height gradients.
Secondary forces that influence the direction and flow pattern:
- **Coriolis Force:** The Earth's rotation deflects moving water (like winds). It causes currents to deflect to the **right** in the **Northern Hemisphere** and to the **left** in the **Southern Hemisphere**. This force plays a major role in creating large, circular current systems called **gyres** in the major ocean basins.
- **Differences in Density:** Differences in water density (caused by variations in temperature and salinity) drive **thermohaline circulation** (deep ocean currents). Cold, saline water is denser than warmer, less saline water and tends to sink. This sinking and subsequent flow of deep water are crucial components of global ocean circulation.
Types Of Ocean Currents
Ocean currents can be classified based on their depth or temperature:
Classification by Depth:
- **Surface Currents:** Constitute about the upper 10% of ocean water (typically the top 400 meters). These are primarily driven by wind and the Coriolis force.
- **Deep Water Currents:** Make up the remaining 90% of ocean water and flow below the surface layer. These currents are driven by differences in density (thermohaline circulation), caused by variations in temperature and salinity. Cold, dense water formed at high latitudes sinks and moves slowly across the ocean basins.
Classification by Temperature:
- **Cold Currents:** Bring colder water from higher latitudes towards lower latitudes, or from deep areas to the surface (upwelling). They are usually found on the **west coasts** of continents in low and middle latitudes (e.g., California Current, Peru Current) and on the east coasts in high latitudes (e.g., Labrador Current).
- **Warm Currents:** Bring warmer water from lower latitudes towards higher latitudes. They are usually found on the **east coasts** of continents in low and middle latitudes (e.g., Gulf Stream, Brazil Current, Kuroshio Current) and on the west coasts in high latitudes (e.g., North Atlantic Drift).
Characteristics Of Ocean Currents
Ocean currents are described by their speed, often referred to as their "drift." Speed is typically measured in knots (1 knot = 1 nautical mile per hour $\approx 1.85 \text{ km/h}$). Surface currents are generally the fastest, sometimes exceeding 5 knots, while deep currents are much slower, often less than 0.5 knots. The strength of a current is related to its speed.
Major Ocean Currents
Major ocean currents are strongly influenced by global wind patterns (prevailing winds) and the Coriolis force. The large-scale circulation patterns in the oceans broadly mirror the general atmospheric circulation patterns (e.g., mid-latitude oceanic gyres correspond to subtropical anticyclonic atmospheric circulation). (Figure 13.3 shows the main global ocean currents).
Due to the Coriolis force, warm currents moving away from the equator are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, contributing to the circular flow in gyres. Conversely, cold currents moving towards the equator are deflected accordingly.
The global ocean circulation system, including both surface and deep currents, plays a vital role in transporting heat from the tropics towards the poles and cold water from the poles towards the tropics, contributing significantly to global heat distribution and moderating climates.
Effects Of Ocean Currents
Ocean currents have numerous impacts on climate, ecosystems, and human activities:
- **Climate Influence:**
- **West Coasts (Tropical/Subtropical):** Bordered by **cold currents** (except very near the equator). These currents cool and stabilize the air, leading to lower average temperatures, small annual/diurnal temperature ranges, frequent fog, and often arid or semi-arid conditions (coastal deserts like Atacama or Namib).
- **West Coasts (Mid-High Latitudes):** Bordered by **warm currents** (e.g., North Atlantic Drift warming northwestern Europe). These currents bring mild, humid conditions, resulting in a **marine climate** with cool summers and relatively mild winters and small annual temperature ranges.
- **East Coasts (Tropical/Subtropical):** Bordered by **warm currents** (e.g., Gulf Stream). These currents bring warm, moist air, resulting in warm and rainy climates, often lying on the western side of subtropical high-pressure systems.
- **Fishing Grounds:** Areas where warm and cold currents meet are often excellent fishing grounds. The mixing of waters brings nutrients from the deep ocean to the surface, supporting the growth of plankton (the base of the marine food web), which in turn attracts large fish populations.
- **Navigation:** Currents can affect the speed and direction of ships, requiring adjustments in navigation. Knowing current patterns can optimize routes to save time and fuel.
Exercises
Multiple Choice Questions
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Answer The Following Questions In About 30 Words
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Answer The Following Questions In About 150 Words
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Project Work
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