Movements of Ocean Water: Waves, Tides and Currents

Waves

A wave is energy moving across the water surface. Most waves are made by wind. The water itself does not travel with the wave: each water particle moves in a small circle and comes back near where it started. That is why a floating bottle bobs up and down but stays nearly in place.

Parts of a wave

• Crest: the highest point. Trough: the lowest point.
• Wave height: vertical distance from trough to crest.
• Wave amplitude: half of the wave height.
• Wavelength: distance from one crest to the next.
• Wave period: time between two crests passing a point. Frequency: number of waves passing a point in one second.
• Wave speed: how fast the wave moves.

Bigger waves come from stronger winds that blow longer over a bigger stretch of sea. In shallow water a wave slows, grows steeper and breaks, throwing water up the beach. Waves shape coasts by erosion and deposition. Earthquakes under the sea make very long, fast waves called tsunamis.

Tides

Tides are the regular rise and fall of sea level, once or twice a day. They are caused mainly by the Moon's gravity, helped by the Sun's, and by the centrifugal force of the Earth–Moon system spinning around their common centre.

• On the side facing the Moon, gravity pulls water into a bulge.
• On the opposite side, the centrifugal effect is larger than the Moon's weak pull there, so a second bulge forms.
• The Earth turns under both bulges, so a place gets two high tides and two low tides in about 24 hours 50 minutes (the Moon moves a bit each day).

Types by how often

• Semi-diurnal: two high and two low tides a day, of nearly equal height (most common).
• Diurnal: one high and one low tide a day.
• Mixed: tides of different heights.

Types by Sun–Moon–Earth position

• Spring tides: Sun, Moon and Earth in a straight line (new moon and full moon) → highest high tides, lowest low tides.
• Neap tides: Sun and Moon at right angles (first and last quarter) → smaller tide range. Spring and neap tides each come twice a month, about seven days apart.
• When the Moon is nearest the Earth (perigee) tides are larger; at apogee they are smaller. When the Earth is nearest the Sun (perihelion, ~3 January) tides are a little larger than at aphelion (~4 July).

Tide ranges are large in funnel-shaped bays; the Bay of Fundy in Canada has one of the highest, about 15–16 m.

Importance of tides

• Navigation: high tide deepens water so large ships can enter ports and river mouths.
• Fishing: fishers time their trips with tides.
• Tides clear silt from harbours and river mouths and remove polluted water.
• Tidal energy can generate electricity.

Ocean currents

Ocean currents are large streams of sea water flowing in a steady direction, like rivers in the ocean. The strongest part of a current, its drift, is near the surface.

What makes currents flow

• Primary forces: heating by the Sun (warm water expands and stands slightly higher near the equator, so it flows down the slope), winds blowing over the sea, gravity pulling water down, and the Coriolis force turning currents right in the north and left in the south, which makes large circular loops called gyres.
• Secondary forces: differences in salinity and temperature change water density; dense (cold, salty) water sinks and less dense water flows over it.

Types

• By depth: surface currents (top ~10% of water, mostly wind-driven) and deep-water currents (the other 90%, driven by density; cold dense water sinks near the poles and creeps along the floor).
• By temperature: warm currents flow from low latitudes towards the poles, usually along the east coasts of continents in low and middle latitudes; cold currents flow from high latitudes towards the equator, usually along the west coasts in low and middle latitudes.

Major currents (examples)

• Warm: Gulf Stream and North Atlantic Drift, Kuroshio, Brazil, East Australian, Agulhas.
• Cold: Labrador, California, Humboldt (Peru), Benguela, Canaries, Oyashio, West Wind Drift (Antarctic Circumpolar).
• In the north Indian Ocean, currents reverse with the monsoon winds.

Effects of currents

• Climate: warm currents make coasts warmer and wetter (the North Atlantic Drift keeps north-west European ports ice-free in winter); cold currents make coasts cool and dry — many hot deserts (Atacama, Namib) lie next to cold currents.
• Fishing: where warm and cold currents meet, plankton grows and fish gather: the Grand Banks near Newfoundland and the seas off Japan are rich fishing grounds.
• Fog: where warm and cold currents meet, thick fog forms, a danger to ships.
• Shipping: ships save fuel by moving with currents.

Try it at home

Fill a long tray with water and float a small bottle cap. Blow across the surface: waves travel along, but the cap only bobs. Now look up the tide table for a port near you (or Mumbai) for one month and mark the dates of the highest tides. Check the Moon phase for those dates: they will be near new moon and full moon.

Key formulas and definitions

• Wave height = crest to trough; amplitude = height ÷ 2; wavelength = crest to crest
• Frequency = waves per second = 1 ÷ period
• Tides: ~2 high + 2 low in 24 h 50 min
• Spring tide: Sun, Moon, Earth in a line (new/full moon); Neap tide: at 90° (quarter moons)
• Warm currents: equator → poles (east coasts); Cold currents: poles → equator (west coasts)
• Coriolis: currents turn right in N, left in S → gyres

Worked examples

Common mistakes

• Thinking waves carry water across the ocean. The energy travels; the water mostly goes round in circles.
• Saying spring tides happen only in spring. They happen twice every month, at new and full moon.
• Thinking there is only one tidal bulge (facing the Moon). There are two: one on each side.
• Mixing up the coasts: warm currents usually run along east coasts and cold currents along west coasts in low and middle latitudes.