Oceanography: Currents, waves and tides

Ocean currents

Think of blowing across a bowl of soup: the surface starts to swirl in one direction. Winds do the same to the oceans, on a giant scale.

Prelims 2012 asked which factors influence ocean currents: rotation of the Earth, air pressure and wind, and density of ocean water, but not the Earth’s revolution (option b: 1, 2 and 3). Mains 2015 and Mains 2022 asked the same thing in Mains form.

A Mains answer can sort the forces into two groups.

• Primary forces start the water moving:
• Sun’s heat: warm water expands, so the sea surface near the equator stands about 8 cm higher than in middle latitudes, and water slides down the slope.
• Planetary winds, the big steady wind belts, drag the surface water.
• Gravity.
• Coriolis force, the turning effect of the Earth’s spin. It bends currents right in the northern hemisphere and left in the southern, so they form large loops called gyres.
• Secondary forces shape the flow: differences in density from temperature and salinity (saltiness), since cold, salty water sinks; and the shape of coastlines and the sea floor.

The equatorial counter-current. Between the westward north and south equatorial currents, a current flows east. Prelims 2015 asked why. The trade winds pile water up on the western side of each ocean where the two equatorial currents converge; in the belt of calm near the equator (the doldrums), with no strong wind to hold it back, that water flows back east.

Why currents matter: climate, fishing and navigation.

• Climate: warm currents give mild, wet coasts; north-west Europe is kept ice-free by the North Atlantic Drift. Cold currents along west coasts help make deserts (Atacama, Namib).
• Fishing: where warm and cold currents meet, plankton (tiny floating plants and animals that fish eat) thrive and fish gather: the Grand Banks off Newfoundland (Gulf Stream and Labrador) and the waters off Japan (Kuroshio and Oyashio).
• Upwelling: where winds push surface water away from the coast, cold water rises from below. Prelims 2011 said upwelling zones raise marine productivity by bringing nutrients to the surface (option b: 2 only). That is why Peru, California, Namibia and north-west Africa have huge fisheries.
• Navigation: ships save fuel by sailing with currents.

Currents vs water masses (Mains 2019). A current is a moving stream. A water mass is a large body of water with uniform temperature and salinity that may move slowly or stay put (North Atlantic Deep Water, Antarctic Bottom Water). Currents carry heat, larvae and nutrients far and fast. Water masses decide the conditions in which life settles (oxygen, temperature) and, when they sink or rise, drive the deep, slow conveyor belt that links all oceans.

The North Indian Ocean is special: its currents reverse with the monsoon, flowing clockwise in summer and anticlockwise in winter.

Waves, tsunamis and tides

Waves are energy, not water, moving across the ocean surface; the water itself only moves in small circles, like a football bobbing in place as waves pass. Most are made by wind: the stronger the wind, the longer it blows and the greater the open water (the fetch), the bigger the waves. In shallow water a wave’s base drags on the floor, so it slows, steepens and breaks.

A tsunami is different. It is a series of very long waves set off by an undersea earthquake, landslide or volcanic eruption that suddenly moves a huge column of water. In the deep sea it is low and fast (up to about 800 km per hour); near the shore it slows and rises into a wall of water.

Mains 2017 (GS III) recalled that in December 2004, a tsunami brought havoc on fourteen countries including India. On 26 December 2004 an earthquake of about magnitude 9.1 off Sumatra killed about 2.3 lakh people across the Indian Ocean, including over 10,000 in India (Tamil Nadu and the Andaman and Nicobar Islands). India then set up the Indian Tsunami Early Warning Centre at INCOIS, Hyderabad (2007).

Preparedness under the NDMA guidelines: warning systems, coastal zoning, mangrove and shelterbelt buffers, evacuation drills and community awareness.

Tides are the regular rise and fall of the sea, usually twice a day (about every 12 hours 25 minutes). They are caused mainly by the Moon’s gravity, and less by the Sun’s, together with the centrifugal force of the Earth–Moon system. The Moon is so much closer that its tide-raising force is about twice the Sun’s.

The tidal range, the gap between high and low tide, is larger in funnel-shaped bays: the Bay of Fundy in Canada has the world’s highest, about 16 m. In India, the Gulf of Khambhat and the Gulf of Kutch have the highest range and so the best potential for tidal energy; the Sundarbans also have it. Tides scour harbours and help river ports such as Kolkata–Haldia, and a tidal bore, a wall of water rushing upstream, runs up the Hooghly.

Prelims traps: spring tides have nothing to do with the season. Neap tides come at the half-moon. Tsunamis are not tidal waves and are not caused by wind. The Moon, not the Sun, is the main cause of tides.

What UPSC asks: factors behind currents, upwelling and fishing grounds, spring and neap tides, tidal energy sites in India, and tsunami preparedness. Next likely angle: warm and cold currents on the map and INCOIS warnings.