Movements of Ocean Water

2.1 Core concepts

Nature of ocean-water movement

• Ocean water is dynamic; its physical characteristics — temperature, salinity and density — and the external forces of the sun, moon and winds together influence its movement. Horizontal motion includes ocean currents and waves; vertical motion includes tides plus upwelling of cold water from the subsurface and the sinking of surface water (NCERT §Introduction, p. 108).
• Currents move water from one place to another; in waves, the water itself does not travel forward — only the wave train (the energy form) moves ahead. Tides are the rise and fall of sea level twice a day due to the attraction of the sun and moon (NCERT p. 108). Waves
• Waves are energy, not water as such, moving across the ocean surface. Water particles only travel in a small circle as a wave passes — the wave form moves but the water particles do not progress. Wind provides energy to the waves and the energy is released on shorelines. The motion of surface water seldom affects the stagnant deep bottom water (NCERT p. 108).
• As a wave approaches the beach it slows down because of friction with the sea floor. When the depth of water becomes less than half the wavelength, the wave breaks. The largest waves are found in the open oceans, where they continue to grow as they absorb energy from the wind (NCERT p. 108).
• Most waves are caused by wind driving against water. A breeze of two knots or less generates small ripples that grow as wind speed increases, until white caps appear in breaking waves. Waves may travel thousands of km before rolling ashore, breaking and dissolving as surf (NCERT p. 108).
• A wave's size and shape reveal its origin — steep waves are young, locally generated; slow, steady waves originate from far-away places, possibly another hemisphere. Maximum wave height depends on (i) strength of the wind, (ii) duration of the wind, and (iii) the fetch — the area over which the wind blows in a single direction (NCERT p. 108).
• Waves travel because wind pushes the water body forward while gravity pulls the crests downward; the falling water pushes the former troughs upward, and the wave moves to a new position. Below the surface the motion of water is circular — particles are carried up and forward as the crest approaches, down and back as it passes (NCERT p. 109, Fig. 13.1).
• Wave characteristics (NCERT box, p. 109): crest (highest point) and trough (lowest); wave height = vertical distance from trough bottom to crest top; wave amplitude = half the wave height; wavelength = horizontal distance between two successive crests; wave period = time interval between two successive crests passing a fixed point; wave speed = rate at which the wave moves through water, measured in knots; wave frequency = number of waves passing a fixed point in one second. Tides
• A tide is the periodical rise and fall of sea level, once or twice a day, mainly due to the gravitational attraction of the moon and (to a lesser extent) the sun (NCERT p. 109).
• Surges are water movements caused by meteorological effects — winds and atmospheric pressure changes; they are not regular like tides (NCERT p. 109).
• The study of tides is complex spatially and temporally because tides vary greatly in frequency, magnitude and height (NCERT p. 109).
• A third factor besides the moon's and sun's gravitational pull is centrifugal force, which acts to counter-balance gravity. Together, gravitational pull and centrifugal force create two major tidal bulges on the earth — one on the side facing the moon (where gravitational pull exceeds the centrifugal force, creating a net force toward the moon), and one on the opposite side (where centrifugal force dominates and creates a net force away from the moon) (NCERT p. 109, Fig. 13.2).
• The 'tide-generating' force is the difference between the moon's gravitational pull and the centrifugal force. On the surface of the earth, the horizontal tide-generating forces are more important than the vertical forces in generating the tidal bulges (NCERT p. 109).
• Tidal bulges on wide continental shelves have greater height; when bulges hit mid-oceanic islands, they become low. Funnel-shaped bays and estuaries greatly magnify tidal magnitudes. Tidal currents are tides channelled between islands or into bays and estuaries (NCERT pp. 109–110).
• Bay of Fundy, Canada has the highest tides in the world — tidal bulge 15–16 m. Two high and two low tides per ~24-hour day mean a tide comes in within about a six-hour period, rising about 240 cm per hour (1,440 cm ÷ 6 h) (NCERT box, p. 110). Types of tides
• By frequency — Semi-diurnal tide: most common; two high and two low tides per day, successive tides approximately equal in height. Diurnal tide: one high and one low tide per day, approximately equal in height. Mixed tide: variations in height; common along the west coast of North America and many islands of the Pacific Ocean (NCERT p. 110).
• By sun–moon–earth positions — Spring tides: sun, moon and earth in a straight line (syzygy); height of tide is higher; occur twice a month — at full moon and new moon. Neap tides: sun and moon at right angles to earth; forces partially counteract; occur about 7 days after spring tides. The moon's attraction is more than twice as strong as the sun's, but is diminished by the counter-acting pull of the sun (NCERT p. 110).
• Perigee — once in a month, when the moon's orbit is closest to the earth, unusually high and low tides occur and the tidal range is greater than normal. Apogee — two weeks later, when the moon is farthest from the earth, its gravitational force is limited and tidal ranges are less than average (NCERT p. 110).
• Perihelion — when the earth is closest to the sun, around 3rd January each year, tidal ranges are much greater, with unusually high and low tides. Aphelion — when the earth is farthest from the sun, around 4th July, tidal ranges are much less than average (NCERT p. 110).
• Ebb — the time between high tide and low tide, when the water level is falling. Flow / Flood — the time between low tide and high tide, when the tide is rising (NCERT p. 110). Importance of tides
• Tides are caused by accurately known earth–moon–sun positions and so can be predicted well in advance, helping navigators and fishermen plan their activities (NCERT p. 110).
• Tidal flows are of great importance in navigation; tidal heights matter especially at harbours near rivers and within estuaries having shallow 'bars' at the entrance, which would otherwise prevent ships and boats from entering the harbour (NCERT p. 110).
• Tides are helpful in desilting sediments and removing polluted water from river estuaries (NCERT p. 111).
• Tides are used to generate electrical power in Canada, France, Russia and China. In India, a 3 MW tidal power project at Durgaduani in the Sunderbans of West Bengal is under way (NCERT p. 111). Ocean currents
• Ocean currents are like river flow in oceans — a regular volume of water in a definite path and direction (NCERT p. 111).
• Currents are influenced by two types of forces: (i) primary forces that initiate the movement of water — heating by solar energy, wind, gravity and Coriolis force; (ii) secondary forces that influence how currents flow (NCERT p. 111).
• Solar heating causes equatorial ocean water to expand, making it about 8 cm higher than in middle latitudes — a slight gradient down which water tends to flow. Wind blowing on the surface pushes water; friction between wind and water surface affects the movement; gravity pulls water down the pile; the Coriolis force deflects water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. These large accumulations of water and the flow around them are called Gyres — large circular currents in all ocean basins (NCERT p. 111).
• Differences in water density affect vertical mobility — water with high salinity is denser than low-salinity water, and cold water is denser than warm water. Denser water sinks; lighter water rises. Cold-water currents form when cold polar water sinks and slowly moves towards the equator; warm-water currents travel from the equator polewards along the surface to replace the sinking cold water (NCERT p. 111). Characteristics of ocean currents (NCERT box, p. 111)
• Currents are referred to by their "drift". They are strongest near the surface — up to 5 knots — and weaker at depth (generally less than 0.5 knots). Drift is measured in knots. Types of ocean currents
• By depth — Surface currents: upper ~400 m of the ocean; about 10% of ocean water. Deep-water currents: remaining 90%; move around ocean basins due to density and gravity variations; sink in deep ocean basins at high latitudes where temperatures are cold enough for density to increase (NCERT p. 111).
• By temperature — Cold currents: bring cold water into warm-water areas; usually found on the west coasts of continents in the low and middle latitudes (both hemispheres) and on the east coasts in higher latitudes of the Northern Hemisphere. Warm currents: bring warm water into cold-water areas; usually observed on the east coasts of continents in the low and middle latitudes (both hemispheres), and in the Northern Hemisphere also on the west coasts of continents at high latitudes (NCERT p. 111).
• Major currents are greatly influenced by prevailing winds and Coriolis force. Oceanic circulation roughly corresponds to atmospheric circulation — anticyclonic in middle latitudes (more pronounced in the southern hemisphere), cyclonic at higher latitudes, and monsoon-influenced in regions of pronounced monsoonal flow. Warm currents from low latitudes move to the right in the Northern Hemisphere and to their left in the Southern Hemisphere because of the Coriolis force (NCERT pp. 111–112).
• Oceanic circulation transports heat from one latitude belt to another, much like atmospheric circulation — cold Arctic and Antarctic waters move toward warmer tropical and equatorial regions, while warm waters of the lower latitudes move polewards (NCERT p. 112). Effects of ocean currents
• West coasts in tropical and subtropical latitudes (except near the equator) are bordered by cool waters — low average temperatures, narrow diurnal and annual ranges, fog, generally arid conditions.
• West coasts in middle and higher latitudes are bordered by warm waters — a distinct marine climate with cool summers and mild winters, narrow annual temperature range.
• East coasts in tropical and subtropical latitudes with parallel warm currents have warm and rainy climates; they lie in the western margins of subtropical anti-cyclones.
• The mixing of warm and cold currents replenishes oxygen and favours the growth of plankton — the primary food of fish — so the world's best fishing grounds lie mainly in these mixing zones (NCERT p. 112).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 13.1 (p. 109) — Motion of waves and water molecules. Shows the wave travelling to the right, with crests falling and troughs rising; below the surface the water molecules trace circular paths, carried up and forward as the wave approaches and down and back as it passes. Anchors the central NCERT point that energy travels forward but the water does not.
• Figure 13.2 (p. 109) — Relation between gravitational forces and tides. Three-panel diagram: (1) gravitational force alone produces one bulge toward the moon; (2) centrifugal force alone produces a bulge on the opposite side; (3) their combined effect produces two resultant tidal bulges with the sun, moon and earth aligned. Demonstrates why both sides of the earth simultaneously experience high tide.
• Figure 13.3 (p. 112) — Major currents in the Pacific, Atlantic and Indian oceans. Map showing named warm and cold currents — Gulf Stream, North Atlantic Drift, Labrador, Canaries, Brazilian, Falkland, Benguela (Atlantic); Alaska, California, North & South Equatorial, Eq. Counter, Kuroshio, Oyashio, N. Pacific Drift, Humboldt (Peru), West Wind Drift (Pacific); Agulhas, West Australian (Indian). Students should be able to identify direction and warm/cold classification.
• Process flow — wave generation and breaking: wind ≥ 2 knots → ripples → growth as energy is absorbed → travel across open ocean → friction with sea floor near shore → wave slows → at depth < ½ wavelength, wave breaks → surf.
• Process flow — spring/neap cycle: full or new moon (sun–earth–moon aligned) → spring tide (highest) → ~7 days → quadrature (sun–moon at right angles) → neap tide (lowest range) → repeat twice a month.
• Process flow — ocean current formation: solar heating raises equatorial water by ~8 cm → wind shears the surface → gravity pulls water down the gradient → Coriolis deflects (right in NH, left in SH) → circular gyres in each basin → density gradients (cold + saline sinks at poles) drive the deep-water leg.

2.5 Key data table (NCERT figures from this chapter)

2.4 Common confusions / NTA trap points

• Perigee vs Perihelion: Perigee is the moon's closest approach to earth; Perihelion is earth's closest approach to the sun (around 3rd January). NTA frequently swaps these terms.
• Apogee vs Aphelion: Mirror confusion — Apogee is the moon farthest from earth (reduces tidal range); Aphelion is earth farthest from the sun on ~4 July (also reduces tidal range). Both reduce range but involve different bodies.
• Spring tides do not mean the spring season — they occur twice every lunar month at full moon and new moon, in every month of the year.
• Neap tides occur ~7 days after spring tides, with sun and moon at right angles (quadrature), not in a straight line.
• Cold currents lie on the west coasts and warm currents on the east coasts of continents in the low and middle latitudes of both hemispheres — students often reverse this. Remember Humboldt (cold, west coast of South America) and Gulf Stream (warm, east coast of North America).
• In the Northern Hemisphere only, cold currents are also found on east coasts in higher latitudes (e.g., Labrador), and warm currents on west coasts at high latitudes (e.g., North Atlantic Drift extension).
• Waves carry energy, not water — a frequent option claims water particles travel along with the wave; this is wrong. Particles trace circles in place.
• Surface vs deep water current shares: surface ≈ 10%, deep ≈ 90% — students often invert these.
• Surges are not tides — surges are caused by meteorology (winds, atmospheric pressure) and are irregular; tides are astronomical and predictable.
• The moon's gravitational pull is more than twice as strong as the sun's — NCERT emphasises this; "the sun's pull is stronger" is always wrong.
• Horizontal tide-generating forces > vertical in producing tidal bulges — NCERT explicitly states this; reversing it is a trap.
• Tidal power project in India is 3 MW at Durgaduani, Sunderbans, West Bengal — not Gulf of Khambhat or Gulf of Kachchh (those are proposed, not the figure NCERT gives).