Chapter 14. Movements of Ocean Water

Ocean Movements
• movements that occur in oceans are categorized as waves, tides & currents. Waves are formed due to friction between wind and surface water layer. stronger wind, bigger wave. They die out quickly on reaching shore or shallow waters.
• Horizontal currents arise mainly due to friction between wind and water. Rotation of earth, Coriolis force and differences in water level gradient play a major role. Vertical currents arise mainly due to density differences caused by temperature and salinity changes. Tsunamis, storm surges and tides are tidal waves with large wavelengths that have greater intensity and destructive power.

• Waves are nothing but oscillatory movements that result in rise and fall of water’s surface. They are a kind of horizontal movement of ocean water. They are energy, not water as such, which moves across ocean surface. This energy for waves is provided by wind.
• In a wave, movement of each water particle is circular. A wave has two major parts: raised part is known as crest while lower point is known as trough.

Characteristics of Waves
Wave crest and trough: highest and lowest points of a wave are known as crest and trough respectively.
Wave height: This is vertical distance from bottom of a trough to top of a crest of a wave.
Wave period: This is merely time interval between two successive wave crests or troughs as they pass a fixed point.
Wave amplitude: This is one-half of wave height.
Wavelength: This is horizontal distance between two successive crests.
Wave speed: This is rate at which wave moves through water, and is measured in knots.
Wave frequency: This is number of waves passing through a given point during a one-second time interval.

• periodical rise and fall of sea level, once or twice a day, mainly due to attraction of sun and moon, is known as a tide. movement of water caused by meteorological effects is known as surges.
• moon’s gravitational pull to a great extent and a lesser extent sun’s gravitational pull, are major causes for occurrence of tides. Another factor is a centrifugal force which acts opposite to gravitational pull of earth. Tides occur due to a balance between all these forces.
• The ‘tide-generating’ force is difference between these two forces; i.e., gravitational attraction of moon and centrifugal force. On surface of earth, nearest moon, pull or attractive force of moon is greater than centrifugal force, and so there is a net force causing a bulge towards moon.
• On opposite side of earth, attractive force is less, as it is farther away from moon, centrifugal force is dominant. Hence, there is a net force away from moon. It creates second bulge away from moon.

Factors Controlling Nature and Magnitude of Tides
• movement of moon concerning earth, changes in position of sun and moon about earth, uneven distribution of water over globe and irregularities in configuration of oceans.
• On surface of earth, horizontal forces that cause tides are more important than vertical forces that cause them. Tides can be made stronger by shape of bays and estuaries along a coast. size of tides changes a lot in bays that look like funnels. Tidal currents happen when tide flows between islands or into bays and estuaries.

Tides Based on Frequency Semi-Diurnal Tide
• most common tidal pattern features two high tides and two low tides each day. successive high or low tides are approximately of same height. Although tides occur twice a day, their interval is not exactly 12 hours. But 12 hours and 25 minutes because moon each day moves a bit to east.

Diurnal Tide
• There is only one high tide and one low tide during each day. successive high and low tides are approximately of same height.

Mixed Tides
• Tides having variations in height based on Sun, Moon & Earth Positions are called mixed tides. Spring tides and neap tides come under this category.

Neap Tides
• Normally, there is a seven-day interval between spring tides and neap tides. At this time sun and moon are at right angles to each other and forces of sun and moon tend to counteract one another. Moon’s attraction, though more than twice as strong as sun’s, is diminished by counteracting force of sun’s gravitational pull. Like spring tides, these tides occur twice a month.

Spring Tides
• When sun, moon & earth are in a straight line, height of tide will be higher. These are known as spring tides and they occur twice a month, one during full moon period and another during new moon period.

Magnitude of Tides Based on Perigee and Apogee of Moon
• Once in a month, when moon’s orbit is closest to earth [perigee], unusually high and low tides occur. During this time tidal range is greater than normal. Two weeks later, when moon is farthest from Earth [apogee], moon’s gravitational force is limited and tidal ranges are less than their average heights.

EBB and Flood
• time between high tide and low tide, when water level is falling, is known as ebb. time between low tide and high tide, when tide is rising, is known as flow or flood.

Importance of Tides
• Since tides are caused by earth-moon-sun positions which are known accurately, tides can be predicted well in advance. This helps navigators and fishermen plan their activities.

• Tidal heights are very important, especially harbours near rivers and within estuaries having shallow ‘bars’ at entrance, which prevent ships and boats from entering harbour. High tides help in navigation. They raise water level close to shores. This helps ships to arrive at harbour more easily. Tides usually help in making some of rivers navigable for ocean-going vessels.

Fishing and Desilting
• high tides help in fishing. Much more fish come closer to shore during high tide. This enables fishermen to get a plentiful catch. Tides are helpful in desilting sediments and in removing polluted water from river estuaries.

Characteristics of Tides
• tidal bulges on wide continental shelves have greater height. In open ocean, tidal currents are relatively weak. When tidal bulges hit midoceanic islands, they become low. shape of bays and estuaries along a coastline can magnify intensity of tides. Funnel-shaped bays greatly change tidal magnitudes. Tidal patterns differ greatly from ocean to ocean and from location to location.

Ocean Currents
• Ocean currents are most important ocean movements because of their influence on climatology of various regions. They are like rivers flowing in oceans. They represent a regular volume of water in a definite path and direction.

General Characteristics of Ocean Currents
• general movement of currents in northern hemisphere is clockwise and in southern hemisphere, anti-clockwise. warm currents move towards cold seas and cool currents towards warm seas.
• shape and position of coasts play an important role in guiding direction of currents. currents flow not only at surface but below sea surface.

Ocean Currents are Influenced by Two Types of Forces
• Primary forces that initiate movement of water are influenced by heating by solar energy; wind; gravity; and Coriolis force.
• Secondary forces that influence currents to flow are Temperature difference; and Salinity difference.

Influence of Insolation
• Heating by solar energy causes water to expand. That is why near equator ocean water is about 8 cm higher in level than in middle latitudes. This causes a very slight gradient and water tends to flow down slope. flow is normally from east to west.

Influence of Wind [Atmospheric Circulation]
• Friction between wind and water surface affects movement of water body in its course. Winds are responsible for both magnitude and direction of ocean currents. air circulation over oceans in middle latitudes is mainly anticyclonic.
• oceanic circulation pattern corresponds with same. At higher latitudes, where wind flow is mostly cyclonic, oceanic circulation follows this pattern. In regions of pronounced monsoonal flow, monsoon winds influence current movements which change directions according to seasons.

Influence of Gravity and Coriolis Force
• Gravity tends to pull water down to pile and create gradient variation. Coriolis force intervenes and causes water to move to right in northern hemisphere and to left in southern hemisphere.
• These large accumulations of water and flow around them are known as Gyres. These produce large circular currents in all ocean basins. One such circular current is Sargasso Sea.

Warm and Cold Ocean Currents
• Differences in water density affect vertical mobility of ocean currents. Water with high salinity is denser than water with low salinity and in same way cold water is denser than warm water. Denser water tends to sink, while relatively lighter water tends to rise.
• Cold-water ocean currents occur when cold water at poles sinks and slowly moves towards equator. Warm-water currents travel out from equator along surface, flowing towards poles to replace sinking cold water.

Climate Results
• Warm and rainy climates in tropical and subtropical latitudes, Cold & dry climates on western margins in sub-tropics due to desiccating effect, Foggy weather and drizzle in mixing zones, and moderate climate along with western coasts in sub-tropics.

Phytoplankton, Fishing & Navigation
• Phytoplanktons are primary producer in marine food chain and hence they are known as grass of sea.
• Phytoplanktons are predominantly microscopic, single-celled organisms. It is one of reasons why cold and warm currents convergence zones and upwelling zones are very productive.
• mixing of cold and warm currents bears richest fishing grounds in world and favours growth of plankton, primary food for fish population. best fishing grounds in world exist mainly in these mixing zones.
• Currents are referred to by their ‘drift’. Usually, currents are strongest near surface and may attain speeds over five knots. Ships generally follow routes that are aided by ocean currents and winds.

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