Water (Oceans) (Advanced)

Hydrological Cycle

The hydrological cycle, commonly known as the water cycle, is the continuous and dynamic process that describes the circulation of water on, above, and below the surface of the Earth. It is driven by solar energy and gravity and involves the transformation of water between its solid, liquid, and gaseous states. This cycle is fundamental for all life on Earth and plays a crucial role in shaping our planet's climate, weather patterns, and surface features.

Key Components and Processes:

1. Evaporation: The transformation of liquid water into water vapour, primarily driven by solar radiation. It occurs from oceans, seas, lakes, rivers, and soil moisture.
2. Transpiration: The release of water vapour from plants into the atmosphere through their leaves. This is a significant component of evapotranspiration, especially in vegetated areas.
3. Evapotranspiration: The combined process of evaporation from surfaces and transpiration from plants, representing the total water flux from land to atmosphere.
4. Sublimation: The direct transition of water from solid (ice/snow) to gas (water vapour), bypassing the liquid phase. It is most significant in cold, dry, and windy conditions.
5. Condensation: The change of water vapour into liquid water droplets or ice crystals. This occurs when moist air cools to its dew point, typically forming clouds or fog. Condensation nuclei are essential for this process.
6. Precipitation: Water released from clouds in the form of rain, snow, sleet, hail, or drizzle, falling to the Earth's surface due to gravity. It occurs when cloud particles grow large enough.
7. Transportation: The movement of water in its various forms (vapour, liquid, solid) through the atmosphere by winds.
8. Infiltration: The process by which precipitation or surface water soaks into the ground, entering the soil.
9. Percolation: The downward movement of infiltrated water through soil and rock layers, leading to groundwater recharge.
10. Runoff: The flow of water over the land surface, typically when precipitation exceeds the infiltration capacity of the soil or when the ground is saturated. This water collects in streams, rivers, and lakes, eventually reaching the oceans.
11. Groundwater Flow: The movement of water beneath the Earth's surface through porous rock and soil layers (aquifers).
12. Storage: Water is stored in various reservoirs, including oceans (the largest reservoir), ice caps and glaciers, groundwater aquifers, lakes, rivers, soil moisture, and the atmosphere.

The Cycle's Driving Forces:

• Solar Energy: The primary energy source driving evaporation, transpiration, and atmospheric circulation, which transports water vapour.
• Gravity: Causes precipitation to fall to Earth and drives runoff and groundwater flow.

Significance: The hydrological cycle is vital for:

• Maintaining Earth's climate system by distributing heat and moisture.
• Supplying freshwater for all terrestrial and aquatic ecosystems, as well as for human consumption, agriculture, and industry.
• Shaping landscapes through erosion and deposition by water.
• Purifying water through evaporation.

Relief Of The Ocean Floor

The ocean floor is not a flat, featureless plain but possesses a varied topography, much like the continents. Its relief features are shaped by geological processes like plate tectonics, volcanic activity, and sedimentation.

Divisions Of The Ocean Floors

The ocean floor can be broadly divided into two major parts:

1. The Continental Margin: The submerged edge of the continent.
2. The Deep Ocean Basin: The vast, deep plains beyond the continental margin.

Continental Shelf

Definition: The gently sloping, submerged edge of a continent, extending from the coastline to the continental slope. It is essentially a shallow, submerged extension of the continent.

Characteristics:

• Width: Varies considerably, from very narrow off some rocky coasts to hundreds of kilometers wide off continental landmasses.
• Depth: Typically extends to a depth of about 100-200 meters (average about 130 meters).
• Gradient: Very gentle, usually less than 1 degree.
• Geology: Composed of continental crust, covered by sediments derived from land and marine sources.
• Significance: Rich in mineral resources (oil, natural gas) and important fishing grounds due to sunlight penetration (allowing photosynthesis) and nutrient availability.

Continental Slope

Definition: The steeper, underwater slope that marks the seaward edge of the continental shelf. It is the transition zone between the continental shelf and the deep ocean floor.

Characteristics:

• Gradient: Much steeper than the continental shelf, averaging about 4 degrees, but can range from 1 to 25 degrees.
• Depth: Extends from the continental shelf break down to the deep ocean floor, typically at depths of 1,500 to 3,000 meters.
• Formation: Represents the true edge of the continent, often dissected by submarine canyons.

Deep Sea Plain (Abyssal Plains)

Definition: Vast, flat, and gently sloping areas of the deep ocean floor, found beyond the continental margin. They are among the flattest and smoothest regions on Earth.

Characteristics:

• Depth: Typically found between 3,000 and 6,000 meters.
• Formation: Formed by the accumulation of fine sediments (like clay and silt) that have been transported from land or from the decay of marine organisms, effectively burying the underlying topography.
• Extent: Cover vast areas of the ocean basins.

Oceanic Deeps Or Trenches

Definition: Extremely deep, narrow depressions on the ocean floor. They represent the deepest parts of the ocean.

Formation: Formed at convergent plate boundaries where one oceanic plate subducts (slides) beneath another continental plate or another oceanic plate.

Characteristics:

• Depth: Can exceed 10,000 meters in some places. The Mariana Trench is the deepest known trench, reaching nearly 11,000 meters.
• Shape: Typically V-shaped or U-shaped.
• Location: Found along the margins of the Pacific Ocean (the "Ring of Fire") and in other areas of active plate tectonics.

Minor Relief Features

Besides the major divisions, the ocean floor features numerous other significant landforms:

Mid-Oceanic Ridges

Definition: Underwater mountain ranges that form a continuous system of mountain ranges extending across all the world's oceans. They are sites of active seafloor spreading.

Characteristics:

• Formation: Created by divergent plate boundaries where magma rises from the mantle, cools, and solidifies to form new oceanic crust.
• Structure: Characterized by a central rift valley where seafloor spreading is actively occurring.
• Extent: The longest mountain range on Earth, extending for about 65,000 km. The Mid-Atlantic Ridge is a prominent example.

Seamount

Definition: Isolated, volcanic mountains that rise from the ocean floor but do not reach the sea surface. They are often found in chains.

Formation: Typically formed by volcanic activity over hotspots or along mid-oceanic ridges.

Submarine Canyons

Definition: Steep-sided valleys that cut into the continental shelf and slope, often resembling canyons found on land.

Formation: Believed to be formed by turbidity currents – dense, sediment-laden flows of water that move downslope along the ocean floor.

Guyots

Definition: Seamounts that have a flattened, table-like top. Also known as tablemounts.

Formation: Thought to be extinct volcanoes that once rose above sea level and were eroded flat by wave action before sinking beneath the sea due to crustal cooling and subsidence, or due to sea-level changes.

Atoll

Definition: A ring-shaped coral reef island, enclosing a lagoon. They are typically found in tropical waters.

Formation: Believed to form around a volcanic island that gradually subsides. As the island sinks, coral reefs grow upwards around its edge, eventually forming a ring after the island has completely submerged.

Temperature Of Ocean Waters

The temperature of ocean waters varies significantly horizontally and vertically, influencing marine life, weather patterns, and global climate.

Factors Affecting Temperature Distribution

The temperature of the ocean surface is primarily determined by the amount of solar radiation received, but several other factors modify this:

• Latitude: Insolation is highest at the equator and decreases towards the poles. This is the primary factor causing a decrease in surface water temperature from the tropics to the poles.
• Altitude: While not directly applicable to the ocean surface itself, landmasses at higher altitudes are generally colder, influencing coastal air temperatures and potentially impacting nearby sea surface temperatures through air masses.
• Distribution of Land and Water: Continents heat up and cool down faster than oceans. This causes coastal areas to have more moderate temperatures than interiors at the same latitude.
• Ocean Currents: Currents transport heat. Warm currents (e.g., Gulf Stream) warm the surface waters they flow through, while cold currents (e.g., Labrador Current) cool them.
• Prevailing Winds: Winds can drive surface waters, leading to upwelling of cold deep water or downwelling of warmer surface water, thus influencing local temperatures. Winds also cause evaporation, which has a cooling effect.
• Depth: Temperature generally decreases with depth. Surface waters are warmed by solar radiation, while deeper waters are much colder and more stable.
• Seasonality: Surface water temperatures fluctuate seasonally due to changes in insolation and wind patterns.
• Cloud Cover and Transparency of Atmosphere: Cloud cover reduces the amount of solar radiation reaching the ocean surface, leading to cooler temperatures.

Horizontal And Vertical Distribution Of Temperature

Horizontal Distribution:

• Equator: Highest surface water temperatures, typically around 27-28°C (81-82°F).
• Subtropics: Slightly lower temperatures than the equator due to the influence of subtropical highs and sinking air.
• Mid-Latitudes: Experience seasonal variations, with warmer summers and cooler winters.
• Polar Regions: Lowest surface water temperatures, often near freezing point (around -1.8°C or 28.8°F), where ice forms.
• Isotherms: Lines connecting points of equal temperature. On surface temperature maps, isotherms generally run parallel to lines of latitude but are significantly deflected by ocean currents and landmasses.
• Seasonal Variations: Surface temperatures are highest in summer and lowest in winter, with the greatest variations occurring in mid-latitudes.

Vertical Distribution:

• Surface Layer (Mixed Layer): The top layer of the ocean, typically extending from the surface down to about 100-200 meters (or more in some regions). It is warmed by solar radiation and mixed by winds and waves. Temperatures are relatively uniform within this layer.
• Thermocline: Below the surface layer, there is a zone where temperature decreases rapidly with increasing depth. This sharp temperature gradient is called the thermocline. It acts as a barrier to vertical mixing between the warm surface waters and the cold deep waters. The depth and intensity of the thermocline vary with latitude and season.
• Deep Ocean Layer: Below the thermocline, extending to the ocean floor, the temperature is very cold and relatively uniform, typically ranging from 0°C to 4°C (32°F to 39°F). This cold temperature is due to the lack of solar heating and the slow circulation of deep, cold water masses originating from polar regions.

Salinity Of Ocean Waters

Salinity is the measure of the total amount of dissolved salts in ocean water. It is a critical factor influencing water density, freezing point, and marine ecosystems.

Factors Affecting Ocean Salinity

Salinity is not uniform throughout the oceans and is influenced by several factors that add or remove freshwater:

• Evaporation: As water evaporates from the ocean surface, it leaves the salts behind, thus increasing salinity. Evaporation rates are highest in hot, dry, windy regions, particularly in subtropical latitudes.
• Precipitation: Rainfall adds freshwater to the ocean surface, diluting the salts and decreasing salinity. Precipitation is highest in equatorial regions and mid-latitudes.
• Freezing of Sea Ice: When seawater freezes, the salt is largely excluded from the ice crystals, making the remaining unfrozen water saltier and denser. This process increases salinity in polar regions where ice formation is significant.
• Melting of Sea Ice: Conversely, the melting of sea ice adds freshwater to the surface layer, diluting the water and decreasing salinity.
• Inflow of Freshwater: Rivers discharge large amounts of freshwater into the oceans, particularly near coastlines and in estuaries, reducing salinity in those areas.
• Inflow of Saline Water: In enclosed seas with high evaporation and limited freshwater inflow (e.g., the Mediterranean Sea, Red Sea), salinity can be higher than in the open ocean.
• Submarine Springs: In some locations, freshwater may emerge from the seabed, reducing local salinity.

Horizontal Distribution Of Salinity

Surface salinity varies significantly across the globe due to the interplay of the factors mentioned above:

• Equatorial Regions: Salinity is relatively low (around 34-35 ppt) due to high rainfall and low evaporation.
• Subtropical Regions (around 20°-30° N/S): Salinity is highest (often exceeding 37 ppt) due to high evaporation rates and low precipitation, coupled with sinking air that inhibits cloud formation.
• Mid-Latitudes: Salinity is moderate, influenced by a balance of evaporation, precipitation, and wind-driven circulation.
• Polar Regions: Salinity is generally lower due to low evaporation rates and the influx of freshwater from melting ice and rivers.
• Coastal Areas: Salinity can be lower near the mouths of large rivers.
• Enclosed Seas: Seas like the Mediterranean and Red Seas have higher salinities due to high evaporation and limited exchange with the open ocean.

Average Salinity: The average salinity of the world's oceans is about 35 parts per thousand (ppt), or 3.5%. This means there are approximately 35 grams of dissolved salts in every kilogram of seawater.

Vertical Distribution Of Salinity

Salinity also changes with depth, though generally less dramatically than temperature:

• Surface Layer: Salinity here is influenced by evaporation, precipitation, river inflow, and ice formation/melting.
• Halocline: Below the surface layer, there can be a zone where salinity changes rapidly with depth. This zone is called the halocline. It acts as a barrier to vertical mixing, similar to the thermocline for temperature.
• Deep Ocean Layer: In the deep ocean, salinity is generally uniform and slightly higher than at the surface in the tropics, but lower than the peak subtropical values. This uniformity is due to the slow mixing of deep water masses that originate from processes in polar regions.

Relationship with Temperature and Density: Salinity, along with temperature, determines the density of seawater. Denser water tends to sink, driving deep ocean circulation (thermo-haline circulation).