Introduction
Ocean water is dynamic. Its temperature, salinity and density, together with external forces from the sun, moon and winds, make it move.
Horizontal motion: ocean currents (continuous flow of huge amounts of water in a definite direction) and waves. In waves the water itself does not move ahead; only the wave trains move.Vertical motion: tides — the rise and fall of the sea twice a day due to the attraction of the sun and moon. Upwelling of cold subsurface water and sinking of surface water are also vertical motions.Key Concepts
1. Waves
Waves are really energy, not water, moving across the ocean surface. Water particles only travel in a small circle as a wave passes — up and forward as the wave approaches, down and back as it passes.Wind provides the energy, which is released on shorelines. Surface motion seldom affects the stagnant deep water.As a wave nears the beach it slows down because of friction with the sea floor. When the water depth is less than half the wavelength, the wave breaks.The largest waves are in the open oceans; waves grow as they absorb wind energy.A breeze of two knots or less over calm water makes ripples, which grow as wind speed increases until white caps appear. Waves may travel thousands of km before breaking as surf.A wave's size and shape reveal its origin: steep waves are young, formed by local wind; slow, steady waves come from far away, possibly another hemisphere.Maximum wave height depends on the strength of the wind, how long it blows and the area over which it blows in one direction.Waves travel because wind pushes the water, gravity pulls the crests down, and the falling water pushes the former troughs up.Characteristics of waves
Crest and trough — the highest and lowest points.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 between two successive crests (or troughs) passing a fixed point.Wavelength — horizontal distance between two successive crests.Wave speed — rate at which the wave moves through water, measured in knots.Wave frequency — the number of waves passing a point in one second.2. Tides
A tide is the periodic rise and fall of sea level, once or twice a day, mainly due to the attraction of the sun and moon.Surges — water movements caused by meteorological effects (winds, pressure changes); they are not regular like tides.Causes: the moon's gravity (major), the sun's gravity (lesser) and centrifugal force, which counterbalances gravity.On the side facing the moon, the moon's pull exceeds the centrifugal force, creating a bulge towards the moon. On the opposite side, the pull is weaker and the centrifugal force dominates, creating a second bulge. The tide-generating force is the difference between the two. Horizontal tide-generating forces matter more than vertical ones.Tidal bulges are higher on wide continental shelves and lower at mid-oceanic islands. Funnel-shaped bays and estuaries magnify tides; tides channelled between islands or into bays are tidal currents.Bay of Fundy, Canada (Nova Scotia) — the highest tides in the world, with a tidal bulge of 15–16 m. With two high and two low tides a day, the tide comes in within about six hours — roughly 240 cm an hour (1,440 cm ÷ 6 hours).
Types of tides by frequency
Semi-diurnal — the most common: two high and two low tides each day, of about the same height.Diurnal — one high and one low tide each day.Mixed — tides with variations in height; common on the west coast of North America and many Pacific islands.Types by sun, moon and earth positions
Spring tides — when the sun, moon and earth are in a straight line; higher tides; twice a month, at full moon and new moon.Neap tides — about seven days after spring tides, when the sun and moon are at right angles and counteract each other. The moon's attraction is more than twice the sun's but is reduced by the sun's pull.Perigee (moon closest to earth, once a month) → unusually high and low tides; apogee (farthest, two weeks later) → smaller tidal ranges.Perihelion (earth closest to sun, around 3 January) → greater tidal ranges; aphelion (around 4 July) → smaller ranges.Ebb — the falling water level between high and low tide. Flow or flood — the rising tide between low and high tide.Importance of tides
They can be predicted well in advance, helping navigators and fishermen.Tidal heights matter for harbours near rivers and estuaries with shallow 'bars' at the entrance.Tides help desilt sediments and remove polluted water from estuaries.Tides generate electricity — in Canada, France, Russia and China; a 3 MW tidal power project at Durgaduani in the Sunderbans, West Bengal, is under way.3. Ocean Currents
Ocean currents are like rivers in the ocean — a regular volume of water in a definite path and direction.Primary forces that start the movement: (i) heating by solar energy — water expands, so near the equator the ocean is about 8 cm higher than in middle latitudes, creating a slight slope; (ii) wind — friction pushes the water; (iii) gravity — pulls water down the pile; (iv) Coriolis force — turns water to the right in the NH and left in the SH.These large accumulations of water and the flow around them are gyres, producing large circular currents in all ocean basins.Secondary forces influence how currents flow.Characteristics
A current's speed is its "drift", measured in knots. Currents are strongest near the surface, sometimes over five knots; at depth usually less than 0.5 knots. Most currents are 5 knots or less.Density differences drive vertical movement: saltier and colder water is denser and sinks; lighter water rises. Cold water sinks at the poles and moves slowly to the equator; warm surface currents flow poleward to replace it.Types of currents
By depth: surface currents — about 10% of ocean water, the upper 400 m; deep water currents — the other 90%, moving due to density and gravity; deep waters sink at high latitudes.By temperature: cold currents — usually on west coasts in low and middle latitudes (both hemispheres) and on east coasts in higher latitudes of the NH. Warm currents — usually on east coasts in low and middle latitudes, and on west coasts in high latitudes of the NH.Major ocean currents
Influenced by prevailing winds and the Coriolis force. Ocean circulation roughly matches atmospheric circulation: anticyclonic in middle latitudes (stronger in the SH), cyclonic at higher latitudes, and driven by monsoon winds where monsoons are strong.Oceanic circulation transports heat from low to high latitudes: cold polar water moves towards the tropics and warm water moves poleward.Effects of ocean currents
West coasts in tropical and subtropical latitudes are bordered by cool waters — low temperatures, narrow ranges, fog, but generally arid.West coasts in middle and higher latitudes are bordered by warm waters — a marine climate with cool summers and mild winters (e.g., north-west Europe).Warm currents along east coasts of tropical and subtropical areas give warm, rainy climates.Where warm and cold currents mix, oxygen is replenished and plankton grow — the best fishing grounds are in these mixing zones.Diagrams to Know
Wave motion — water particles moving in circles as the wave passes; crest, trough, wavelength and height labelled.Tidal bulges — one towards the moon, one opposite, with the gravitational and centrifugal forces.Spring vs neap tides — sun, moon and earth in a line vs at right angles.Major currents of the Pacific, Atlantic and Indian Oceans (e.g., Gulf Stream, Labrador current, North Atlantic Drift, Peruvian current).Key-Facts Sheet
Wave amplitude = ½ wave height; wave speed in knots.A wave breaks when depth < ½ wavelength.Bay of Fundy tides: 15–16 m; about 240 cm per hour.Spring tides: full and new moon; neap tides about 7 days later.Surface currents: upper 400 m, about 10% of water; deep currents about 90%.Exam Focus
MCQ from the exercise: upward and downward movement of water = tide; spring tides occur when the moon and sun pull in the same direction; minimum earth–moon distance = perigee; perihelion is in January.30-word: How are tides caused? Outline: the moon's (and sun's) gravity plus centrifugal force create two bulges — one facing the moon and one opposite.How are tides related to navigation? Outline: predictable tides help ships cross shallow bars into harbours and help plan fishing.150-word: How do currents affect north-west Europe? Outline: the warm North Atlantic Drift keeps west-coast winters mild with a marine climate.Causes of currents: solar heating, wind, gravity, Coriolis force, and density differences from temperature and salinity.Key Terms
Wave (तरंग)Crest (शिखर), Trough (गर्त)Wavelength (तरंगदैर्ध्य)Wave period (तरंग काल)Tide (ज्वार-भाटा)Spring tide (दीर्घ ज्वार), Neap tide (लघु ज्वार)Perigee (उपभू), Apogee (अपभू)Ebb (भाटा), Flood tide (ज्वार)Ocean current (महासागरीय धारा)Gyre (वलय / गायर)Upwelling (उत्स्रवण)Drift (अपवाह)Common Mistakes
Thinking the water itself travels with a wave — it is energy that moves; water moves in circles.Confusing spring tides (straight line, full/new moon) with neap tides (right angles).Mixing up perigee/apogee (moon–earth) with perihelion/aphelion (earth–sun).Placing cold currents on east coasts in the tropics — they are usually on west coasts.Calling surges tides — surges come from weather and are irregular.