Movements of Ocean Water

Introduction

Ocean water is constantly in motion. These movements can be broadly classified into waves, tides, and currents. Each type of movement has different causes, characteristics, and geographical significance. Oceanic circulation plays a vital role in redistributing heat, nutrients, and marine life around the globe.

Waves

Waves are oscillatory movements of water at the ocean surface, primarily caused by wind friction on the water surface. Waves transfer energy but not water — water molecules move in circular orbits but return to nearly the same position after each wave passes.

• Parts of a wave:
• Crest: Highest point of a wave
• Trough: Lowest point
• Wave height: Vertical distance between crest and trough
• Wavelength: Horizontal distance between two successive crests
• Wave period: Time for one complete wave cycle to pass a fixed point
• Wave velocity: Speed of wave propagation (v = wavelength / period)

Tides

Tides are the periodic rise and fall of sea level due to the gravitational pull of the Moon and, to a lesser extent, the Sun.

The Moon exerts a stronger tidal force than the Sun despite being much smaller, because it is much closer to Earth. Tidal force follows an inverse cube law.

• Types of tides:
• Spring tides: When the Sun, Earth, and Moon are aligned (new moon and full moon). Solar and lunar tidal forces add together → very high high tides and very low low tides. Greatest tidal range.
• Neap tides: When the Moon is at first or last quarter (Sun–Earth–Moon form a right angle). Solar and lunar forces partly cancel → smaller tidal range. Least tidal range.

• Importance of tides:
• Enable ships to enter and leave shallow ports.
• Tidal energy is a renewable power source.
• Tidal flats and estuaries support unique ecosystems.
• Flushing action prevents stagnation of harbours.

Ocean Currents

• Ocean currents are continuous, directed flows of ocean water following definite paths. They can be:
• Surface currents: Driven by wind; affect upper ~200 m (about 10% of ocean volume).
• Deep (thermohaline) currents: Driven by density differences due to temperature and salinity; affect the remaining 90%.

1. 1.Causes of ocean currents:
2. 2.Prevailing winds: Most important factor for surface currents. Trade winds drive equatorial currents westward; westerlies drive mid-latitude currents eastward.
3. 3.Coriolis effect: Deflects currents to the right in NH and left in SH, creating circular gyres.
4. 4.Density differences: Cold, salty water sinks; warm, less salty water rises — drives thermohaline circulation.
5. 5.Shape of ocean basins: Coastlines deflect currents.
6. 6.Earth's rotation: Contributes to gyres.

Major Ocean Currents

• Pacific Ocean:
• North Equatorial Current (NEC) → Kuroshio (Japan) Current (warm, northward along Asia east coast) → North Pacific Current → California Current (cold, southward) → back to NEC.
• South Equatorial Current → East Australian Current (warm).

• Atlantic Ocean:
• North Equatorial Current → Gulf Stream (warm, powerful, 50–80 km wide along east coast of USA) → North Atlantic Drift (warm, moderates NW Europe) → Canary Current (cold, southward along NW Africa) → NEC.
• South Atlantic: Brazil Current (warm) and Benguela Current (cold).

• Indian Ocean:
• The North Indian Ocean reverses seasonally with monsoons (NE monsoon drives currents SW in winter; SW monsoon reverses them NE in summer).
• South Indian Ocean: South Equatorial Current, Agulhas Current (warm), West Australian Current (cold).

Effects of Ocean Currents on Climate

• Warm currents along east coasts of continents (in tropics) and west coasts of continents (in mid-latitudes) raise temperatures, increase humidity, and bring rainfall.
• Cold currents along west coasts of continents (in tropics) lower temperatures, stabilise the atmosphere, produce fog, and create desert conditions (Namib, Atacama).
• North Atlantic Drift makes NW Europe much warmer than equivalent latitudes elsewhere (Bergen, Norway at 60 degrees N has ice-free ports; equivalent latitude in Canada is frozen).

Upwelling

Upwelling occurs where wind drives surface water away from the coast (Ekman transport), and cold, nutrient-rich deep water rises to replace it. Upwelling zones are among the world's richest fishing grounds (Peru/Humboldt Current, Benguela Current, Canary Current, Somali Coast during SW monsoon).