Water (Oceans)

Introduction

The oceans cover approximately 71% of the Earth's surface and contain about 97.3% of all water on Earth. They are central to regulating climate, the water cycle, and supporting life. This chapter examines the physical characteristics of ocean water — its distribution, temperature, salinity, and density — and their global patterns.

Ocean Basins and Distribution

• The world's oceanic waters are divided into four major oceans:
• Pacific Ocean: Largest (~165 million km²), deepest (Mariana Trench, ~11,034 m). Occupies nearly a third of Earth's surface.
• Atlantic Ocean: Second largest (~106 million km²), S-shaped, bounded by Americas on west and Europe/Africa on east.
• Indian Ocean: Third largest (~73 million km²), mostly in Southern Hemisphere; enclosed on three sides (Asia, Africa, Australia).
• Arctic Ocean: Smallest and shallowest; largely covered by sea ice.

Ocean Floor Relief

• The ocean floor has distinct landforms:
• Continental Shelf: Gently sloping submerged extension of continent; average width 70 km, depth 0–200 m. Rich in fish, oil, and gas. Most important zone for humans.
• Continental Slope: Steep descent from shelf edge to deep ocean floor; angle ~3–6 degrees.
• Deep Sea Plain (Abyssal Plain): Vast, flat regions at 3000–6000 m depth; covered in fine sediments.
• Ocean Ridges (Mid-Oceanic Ridges): Underwater mountain chains formed at diverging plate boundaries; site of new ocean floor creation. Examples: Mid-Atlantic Ridge, Indian Ocean Ridge.
• Ocean Trenches: Narrow, deep depressions at convergent plate boundaries (subduction zones); deepest parts of the ocean. Example: Mariana Trench.
• Seamounts: Isolated underwater volcanoes; flat-topped ones are guyots.

Ocean Temperature

• Factors affecting ocean temperature:
• Latitude: Surface water is hottest in tropics, coolest near poles. Highest surface temperatures (~27–28 degrees C) are in western Pacific tropics and Indian Ocean.
• Ocean currents: Warm currents raise temperature of adjacent water/coasts; cold currents lower them.
• Prevailing winds: Trade winds drive warm water westward; cold upwelling occurs on eastern coasts of oceans.
• Enclosed vs. open seas: Enclosed/marginal seas have more extreme temperature ranges.
• Depth: Temperature decreases with depth. Three layers:
• Surface (mixed) layer: 0–200 m, warmed by sun, well mixed by winds.
• Thermocline: 200–1000 m, rapid temperature decrease with depth.
• Deep water: Below 1000 m, uniformly cold (~2–4 degrees C), slow thermohaline circulation.

Ocean Salinity

Salinity is the concentration of dissolved salts in seawater, expressed in parts per thousand (ppt or ‰). Average ocean salinity is 35 ppt (35 grams of salt per 1000 grams of seawater).

Common dissolved salts (in order of abundance):
Sodium chloride (NaCl) is the most abundant (~77.7% of dissolved salts), followed by magnesium chloride, magnesium sulphate, calcium sulphate, and potassium sulphate.

1. 1.Factors affecting salinity:
2. 2.Evaporation: More evaporation increases salinity (removes water, leaves salt). Sub-tropical regions have highest salinity due to high evaporation and low rainfall.
3. 3.Precipitation: More rain dilutes surface water, lowering salinity. Equatorial regions have relatively lower salinity despite being warm.
4. 4.Freshwater input: River discharge dilutes coastal waters (Baltic Sea: ~7 ppt, near river mouths).
5. 5.Sea ice formation: When seawater freezes, salt is excluded, increasing salinity of surrounding water. When ice melts, it dilutes surrounding water.
6. 6.Wind and currents: Mix water and affect evaporation rates.

• Global salinity patterns:
• Highest salinity: Sub-tropical gyres (~37 ppt), Red Sea (~41 ppt — evaporation exceeds rainfall dramatically).
• Lowest salinity: High-latitude regions, near river mouths, Baltic Sea.
• The Mediterranean has high salinity (~38–39 ppt) due to excess evaporation.

Ocean Density

• Density of seawater depends on temperature and salinity:
• Lower temperature → higher density
• Higher salinity → higher density

Cold, salty water is the densest and sinks to form deep ocean water. This drives the thermohaline circulation (deep ocean conveyor belt). Most dense ocean water forms in the North Atlantic (North Atlantic Deep Water, NADW) and near Antarctica (Antarctic Bottom Water, AABW).