Movements of Ocean Water
1. Learning Objectives
After reading these notes, you will be able to:
2. Overview โ Movements of Ocean Water
Horizontal motion
Energy moves,
not water
Vertical motion
Rise and fall
twice a day
Horizontal motion
Continuous flow
in definite direction
Vertical motion
Cold water rises,
surface water sinks
Waves: Water does NOT move forward โ only the wave train moves ahead. Water particles move in a small circle.
Tides: Rise and fall of ocean water due to gravitational attraction of Sun and Moon โ twice a day.
3. Waves
- As a wave approaches the beach, it slows down due to friction between the dynamic water and the sea floor.
- When the depth of water is less than half the wavelength of the wave, the wave breaks.
- The largest waves are found in the open oceans. Waves grow larger as they move and absorb more energy from wind.
- Most waves are caused by wind driving against water. When a breeze of two knots or less blows over calm water, small ripples form โ grow into waves โ white caps appear.
- Waves may travel thousands of km before rolling ashore, breaking and dissolving as surf.
- Steep waves = fairly young, formed by local wind. Slow and steady waves originate from far away places, possibly from another hemisphere.
- The maximum wave height is determined by the strength of wind, how long it blows and the area over which it blows in a single direction.
- The actual motion of water beneath the waves is circular โ things are carried up and forward as wave approaches, and down and back as it passes.
๐ Characteristics of Waves
| Term | Definition |
|---|---|
| Wave Crest | The highest point of a wave |
| Wave Trough | The lowest point of a wave |
| Wave Height | Vertical distance from the bottom of a trough to the top of a crest |
| Wave Amplitude | One-half of the wave height |
| Wave Period | Time interval between two successive wave crests or troughs as they pass a fixed point |
| Wavelength | Horizontal distance between two successive crests |
| Wave Speed | Rate at which the wave moves through water โ measured in knots |
| Wave Frequency | Number of waves passing a given point during a one-second time interval |
4. Tides
๐ Causes of Tides
๐ Moon’s Gravitational Pull
The Moon’s gravitational pull (to a great extent) and the Sun’s gravitational pull (to a lesser extent) are the major causes of tides. On the side of Earth facing the Moon โ gravitational attraction > centrifugal force โ tidal bulge towards the Moon.
๐ Centrifugal Force
The force that acts to counter balance gravity. On the opposite side of Earth from the Moon โ Moon’s gravitational attraction is less (farther away) โ centrifugal force is dominant โ creates a second tidal bulge away from the Moon.
๐ข Types of Tides โ Based on Frequency
๐ 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 the same height.
1๏ธโฃ Diurnal Tide
Only one high tide and one low tide during each day. Successive high and low tides are approximately of the same height.
ใฐ๏ธ Mixed Tide
Tides having variations in height โ known as mixed tides. Generally occur along the west coast of North America and on many islands of the Pacific Ocean.
โ๏ธ๐ Types of Tides โ Based on Sun-Moon-Earth Positions
๐ Spring Tides
When the Sun, Moon and Earth are in a straight line โ the height of the tide will be higher. These are called spring tides. Occur twice a month โ once on full moon and once on new moon period. Sun and Moon pull in the same direction โ combined gravitational force โ extra high tides.
๐ Neap Tides
Normally, there is a seven day interval between spring and neap tides. At this time the Sun and Moon are at right angles to each other โ forces of Sun and Moon counteract one another โ lower tides. Moon’s attraction is more than twice as strong as Sun’s but is diminished by Sun’s counteracting pull.
๐ Special Tidal Positions
| Position | Term | Effect on Tides | Date |
|---|---|---|---|
| Moon closest to Earth | Perigee | Unusually high and low tides โ tidal range greater than normal | Once a month |
| Moon farthest from Earth | Apogee | Moon’s gravitational force limited โ tidal ranges less than average | Two weeks after perigee |
| Earth closest to Sun | Perihelion | Tidal ranges much greater โ unusually high and low tides | Around 3rd January |
| Earth farthest from Sun | Aphelion | Tidal ranges much less than average | Around 4th July |
Flow / Flood: The time between the low tide and high tide, when the tide is rising.
โ Importance of Tides
- Since tides are caused by Earth-Moon-Sun positions which are known accurately, tides can be predicted well in advance โ helps navigators and fishermen plan activities.
- Tidal heights are very important for harbours near rivers and estuaries with shallow bars at the entrance โ prevent ships from entering at low tide.
- Tides are helpful in desilting sediments and in removing polluted water from river estuaries.
- Tides are used to generate electrical power โ in Canada, France, Russia and China.
- A 3 MW tidal power project at Durgaduani in Sunderbans of West Bengal is under way.
- When tide is channelled between islands or into bays and estuaries โ called tidal currents.
5. Ocean Currents
โก Primary Forces Initiating Ocean Currents
โ๏ธ 1. Heating by Solar Energy
Causes water to expand. Near the equator, ocean water is about 8 cm higher in level than in the middle latitudes โ very slight gradient โ water flows down the slope.
๐จ 2. Wind
Wind blowing on the surface of the ocean pushes the water to move. Friction between the wind and the water surface affects the movement of the water body in its course.
โฌ๏ธ 3. Gravity
Gravity tends to pull the water down the pile and create gradient variation. It drives the movement of water from higher to lower levels.
๐ 4. Coriolis Force
Causes water to move to the right in NH and to the left in SH. Large accumulations of water and the flow around them are called Gyres โ these produce large circular currents in all ocean basins.
๐ Types of Ocean Currents
๐ต Surface Currents
Constitute about 10% of all water in the ocean. These are the upper 400 m of the ocean. Driven mainly by wind and Coriolis force. Usually strongest near the surface โ may attain speeds over 5 knots.
๐ฃ Deep Water Currents
Make up the other 90% of the ocean water. Move around ocean basins due to variations in density and gravity. Deep waters sink into deep ocean basins at high latitudes where temperatures are cold enough โ density increases. Speed generally less than 0.5 knots.
โ๏ธ Cold Currents
Bring cold water into warm water areas. Usually found on the west coast of continents in low and middle latitudes (both hemispheres) and on the east coast in higher latitudes in the Northern Hemisphere.
๐ด Warm Currents
Bring warm water into cold water areas. Usually observed on the east coast of continents in low and middle latitudes (both hemispheres). In the Northern Hemisphere โ found on the west coasts of continents in high latitudes.
๐ Characteristics of Ocean Currents
- Currents are referred to by their “drift” โ drift is measured in terms of knots.
- Usually strongest near the surface โ may attain speeds over 5 knots.
- At depths, currents are generally slow โ speeds less than 0.5 knots.
- A current is usually strongest at the surface and decreases in strength with depth.
- Major ocean currents are greatly influenced by prevailing winds and Coriolis force.
- The oceanic circulation pattern roughly corresponds to Earth’s atmospheric circulation pattern. Air circulation in middle latitudes is mainly anticyclonic โ oceanic circulation follows the same.
- The oceanic circulation transports heat from one latitude belt to another โ similar to the general circulation of the atmosphere.
- Due to Coriolis force, warm currents from low latitudes tend to move right in NH and left in SH.
๐ Effects of Ocean Currents
๐ก๏ธ West Coasts โ Tropical/Subtropical Latitudes
Bordered by cool waters (cold currents). Average temperatures relatively low, narrow diurnal and annual ranges. There is fog, but generally the areas are arid.
๐ค๏ธ West Coasts โ Middle/Higher Latitudes
Bordered by warm waters โ distinct marine climate. Characterised by cool summers and mild winters with a narrow annual range of temperatures.
๐ง๏ธ East Coasts โ Tropical/Subtropical Latitudes
Warm currents flow parallel to east coasts โ warm and rainy climates. These areas lie in the western margins of the subtropical anticyclones.
๐ Best Fishing Grounds
The mixing of warm and cold currents helps to replenish oxygen and favour the growth of planktons โ the primary food for fish population. The best fishing grounds of the world exist mainly in these mixing zones.
Summary โ Quick Revision
Waves: Energy moves, not water. Water particles move in a small circle. Wind provides energy. Waves slow down near beach due to friction with sea floor. Wave breaks when water depth < half the wavelength. Largest waves in open oceans.
Wave Terms: Crest (highest point), Trough (lowest), Wave Height (trough to crest), Amplitude (ยฝ of height), Wavelength (distance between two crests), Wave Period (time between two crests), Wave Speed (in knots), Wave Frequency (waves per second).
Tides: Periodical rise and fall of sea level once or twice a day โ due to Moon’s gravitational pull (main) + Sun’s gravitational pull + Centrifugal force. Two tidal bulges form on opposite sides of Earth.
Types of Tides by Frequency: Semi-diurnal (2 high + 2 low per day โ most common), Diurnal (1 high + 1 low per day), Mixed (variable heights โ W. coast of N. America, Pacific islands).
Spring vs Neap Tides: Spring tides โ Sun + Moon + Earth in straight line โ higher tides (twice a month: full moon + new moon). Neap tides โ Sun and Moon at right angles โ lower tides (7 days after spring tide).
Special Positions: Perigee (Moon closest โ highest tides), Apogee (Moon farthest โ lowest tides), Perihelion (Earth closest to Sun, ~3 Jan โ greater tides), Aphelion (Earth farthest from Sun, ~4 July โ less tides). Ebb = falling tide. Flow/Flood = rising tide.
Tides Importance: Can be predicted in advance โ helps navigators and fishermen. Harbour navigation. Desilting and removing polluted water. Tidal power generation (Canada, France, Russia, China). 3 MW project at Durgaduani, Sunderbans, West Bengal.
4 Primary Forces for Ocean Currents: Solar heating (equatorial water 8 cm higher โ gradient), Wind (pushes surface water), Gravity (pulls water downslope), Coriolis force (right in NH, left in SH โ Gyres).
Types of Currents: Surface (10%, upper 400 m), Deep water (90%, density-driven). Cold currents (west coast of continents, low-mid latitudes), Warm currents (east coast, low-mid latitudes). Speed: surface >5 knots, deep <0.5 knots.
Effects of Currents: West coast (tropical) + cold current โ fog, arid. West coast (mid-high latitude) + warm current โ marine climate, mild winters. East coast (tropical) + warm current โ warm and rainy. Mixing zones of warm + cold currents โ best fishing grounds (plankton growth).
Important Terms to Remember
- Waves: Energy that moves across the ocean surface โ not the water itself. Water particles move in small circles as a wave passes. Wind provides energy to waves.
- Surf: The broken, foamy waves that form when waves roll ashore and dissolve on the beach after travelling thousands of km.
- Wave Crest: The highest point of a wave.
- Wave Trough: The lowest point of a wave.
- Wave Height: Vertical distance from the bottom of a trough to the top of a crest.
- Wave Amplitude: One-half of the wave height.
- Wave Period: Time interval between two successive wave crests or troughs as they pass a fixed point.
- Wavelength: Horizontal distance between two successive crests.
- Wave Speed: Rate at which the wave moves through water โ measured in knots.
- Wave Frequency: Number of waves passing a given point during a one-second time interval.
- Tide: The periodical rise and fall of sea level, once or twice a day, mainly due to the attraction of the Sun and the Moon.
- Surge: Movement of water caused by meteorological effects like winds and atmospheric pressure changes. Not regular like tides.
- Tidal Bulge: The rise in ocean level on the side of Earth facing the Moon (due to gravitational pull) and on the opposite side (due to centrifugal force).
- Centrifugal Force: The force that acts to counterbalance gravity. Causes the second tidal bulge on the side of Earth opposite to the Moon.
- Tide-Generating Force: The difference between the Moon’s gravitational attraction and the centrifugal force โ responsible for creating tides.
- Tidal Currents: When the tide is channelled between islands or into bays and estuaries โ the resulting water movement is called tidal currents.
- Semi-Diurnal Tide: The most common tidal pattern โ two high tides and two low tides each day, with successive tides of approximately the same height.
- Diurnal Tide: Only one high tide and one low tide per day, with successive tides of approximately the same height.
- Mixed Tide: Tides with variations in height. Common along the west coast of North America and Pacific Ocean islands.
- Spring Tides: Higher-than-average tides occurring when the Sun, Moon and Earth are in a straight line. Occur twice a month โ at full moon and new moon.
- Neap Tides: Lower-than-average tides occurring when the Sun and Moon are at right angles to each other (quadrature). Occur 7 days after spring tides.
- Perigee: The position when the Moon is closest to Earth โ causes unusually high and low tides, greater than normal tidal range. Occurs once a month.
- Apogee: The position when the Moon is farthest from Earth โ Moon’s gravitational force is limited, tidal ranges less than average.
- Perihelion: The position when the Earth is closest to the Sun โ around 3rd January each year โ tidal ranges are much greater than average.
- Aphelion: The position when the Earth is farthest from the Sun โ around 4th July each year โ tidal ranges are much less than average.
- Ebb: The time between the high tide and low tide when the water level is falling.
- Flow / Flood: The time between the low tide and high tide when the tide is rising.
- Ocean Currents: Like river flow in oceans โ regular volume of water moving in a definite path and direction. Influenced by primary forces (solar heating, wind, gravity, Coriolis) and secondary forces.
- Gyres: Large accumulations of water and the circular flow around them โ produced by Coriolis force in all ocean basins.
- Drift: The speed of an ocean current โ measured in knots. A fast current is considered strong.
- Surface Currents: The upper 400 m of ocean โ constitute about 10% of all ocean water. Driven mainly by wind.
- Deep Water Currents: The other 90% of ocean water โ move due to variations in density and gravity. Generally slow (<0.5 knots).
- Cold Currents: Bring cold water into warm water areas. Found on west coasts of continents in tropical/mid-latitudes and on east coasts in higher latitudes (NH).
- Warm Currents: Bring warm water into cold water areas. Found on east coasts of continents in tropical/mid-latitudes and on west coasts in high latitudes (NH).
- Planktons: Microscopic organisms that grow in mixing zones of warm and cold currents. Primary food for fish โ the best fishing grounds of the world exist in these mixing zones.