Oceanography

Introduction

The oceans, encompassing around 70% of Earth's surface, stand as the largest and most prominent feature on our planet, defining its unique character. With a single interconnected body of water known as the world ocean, it is divided into five principal oceans viz. the Pacific, Atlantic, Indian, Southern, and Arctic Oceans. 

Marine Resources:

Marine resources consist of the oceans and seas, covering about 70% of the Earth's surface. These vast bodies of saltwater harbor an immense wealth of resources.

• Marine Organisms: Oceans are home to a diverse array of organisms, many of which are harvested for human consumption. This includes fish, shellfish, and seaweed.
• Mineral Resources: Oceans contain vast amounts of mineral resources. These include oil and natural gas reserves, sand and gravel, and various polymetallic nodules containing manganese, copper, cobalt, and nickel.
• Energy Resources: Marine resources also include renewable energy sources like wind, wave, tidal, and thermal energy. 

Divisions of the Ocean Floor

The ocean floor can be classified into several major divisions that play important roles in the Earth's geography - Continental-Oceanic margin, Continental Shelf, Continental Slope, Continental Rise, Deep ocean plains, and Oceanic Ridges. 

Continental-Oceanic Margin

The continental-oceanic margin is a significant division of the ocean floor, characterized by its unique features and geological processes.

Continental Shelf

The continental shelf, an extension of the continent, exhibits various characteristics that vary across different regions. Its width, angle, and depth play crucial roles in shaping coastal areas.

• Angle: The continental shelf has a slight inclination, typically around 1⁰.
• Depth: The depth of the continental shelf ranges from shallow areas of about 30 meters to deeper regions of up to 600 meters.
• Width: The width of the continental shelf varies greatly, with examples such as the wide shelves in the Bay of Bengal and the East Coast of North America, contrasting with the virtually absent shelf on the West Coast of South America.
• Sedimentary Deposits: The continental shelf is covered with sediments, including those brought down by rivers and glaciers.
• Shelf Break: The continental shelf ends with a steep slope known as the shelf break. 

Continental Slope

The continental slope connects the continental shelf to the ocean basins and exhibits distinct features and characteristics.

• Steep Slope: The continental slope steepens abruptly at the edge of the continental shelf.
• Gradient: The slope region's gradient ranges from 2° to 5°.
• Depth: The depth of the slope region varies between 200 meters and 3 kilometers.
• Continental Rise: The seaward edge of the continental slope gradually loses gradient, giving rise to the continental rise.
• Canyons and Trenches: Canyons and trenches are prominent features observed in the continental slope region. 

Continental Rise

• The continental rise is a sediment underwater feature located between the continental slope and the abyssal plain.
• It forms through the gradual deposition of sediments transported by rivers and other sources. 

Deep Ocean Plains (Abyssal Plain)

The deep ocean plains, also known as abyssal plains, cover a significant portion of the ocean floor and possess distinctive characteristics.

• Gentle Slope: At the end of the continental slope, the slope becomes gentler, ranging from 5⁰ to 1⁰.
• Extent: Abyssal plains lie 2-3 miles below sea level and cover approximately 40% of the ocean floor.
• Sediment Cover: These plains are covered with fine-grained sediments like clay and silt.
• Distribution: Abyssal plains are found between the foot of a continental rise and a mid-ocean ridge, constituting over 50% of the Earth's surface. 

Oceanic Ridges

Oceanic ridges are continuous underwater mountain ranges formed by tectonic activity and volcanic processes.

• Formation: Oceanic ridges are created when magma rises between diverging plates of the lithosphere, resulting in the formation of a new layer of crust.
• Structure: They consist of two chains of mountains separated by a large depression, which marks a divergent boundary.

Minor Relief Features

In addition to the major divisions, the ocean floors host various minor relief features that contribute to the overall complexity and diversity of underwater landscapes. 

Marginal Seas

• Marginal seas are divisions of oceans, partially enclosed by land formations such as islands, archipelagos, or peninsulas. They are either open to the open ocean or bounded by submarine ridges on the sea floor.
• Examples of Marginal Seas: Arabian Sea, Persian Gulf, Red Sea, Gulf of Oman, Gulf of Aden, Gulf of Kutch, Gulf of Khambat, Bay of Bengal, Andaman Sea, Malacca Strait, Mozambique Channel, Great Australian Bight, Gulf of Mannar, Laccadive Sea. 

Ocean Temperature

• The study of ocean temperature is important for understanding ocean currents, marine organism distribution, and coastal climate.
• Insolation (incoming solar radiation) is the primary energy source for ocean temperature.
• Oceans play a crucial role in energy and temperature regulation due to their high heat capacity.
• The average temperature of the oceans is around 3-5 degrees Celsius, while the average surface temperature of ocean water is about 25 degrees Celsius. 

Factors Affecting Temperature Distribution

Variation in Ocean Temperature

• The equator receives about four times more average incoming solar energy than the poles. Solar radiation can penetrate below the ocean's surface due to water's transparency.
• Shorter wavelengths (high energy) penetrate deeper than longer wavelengths, transferring heat to deeper levels through mixing.
• Diurnal and seasonal temperature variations in water are relatively small compared to land due to water's high specific heat.
• Most solar energy is absorbed near the ocean surface, providing energy for photosynthesis by marine plants and algae. 

Vertical Variation in Oceanic Temperature

• The vertical distribution of temperature in the deep ocean is influenced by density-driven water movements.
• The maximum temperature of oceans is found at the surface due to direct solar energy.
• Heat conduction alone transfers only a small proportion of heat downward; convection plays a crucial role in transmitting heat to lower sections of the oceans. 

Thermal Layer Distribution in the Ocean

• 1st layer: The top layer consists of warm oceanic water with a thickness of about 500 meters and a temperature range of 20-25°C.
• This layer exists throughout the year in tropical regions but develops only during summer in mid-latitudes.
• 2nd layer: Temperature rapidly declines between depths of about 200 meters to 1000 meters, forming the permanent thermocline.
• About 90% of the total volume of water lies below the thermocline, with temperatures approaching 0°C.
• The thermocline is less pronounced in Polar Regions due to surface temperatures close to 0°C.
• 3rd layer: Beyond 1000 meters, there is virtually no seasonal variation, and temperatures remain around 2°C.
• This layer extends to the deep ocean floor and is influenced by the temperature of cold, dense water sinking at the polar regions and flowing toward the equator.

Horizontal Variation in Oceanic Temperature

• The average temperature of surface water in the ocean is around 27°C. Average temperature gradually decreases from the equator towards the poles.
• The southern hemisphere generally records lower temperatures than the northern hemisphere due to unequal land and water distribution.
• The highest temperature is usually slightly away from the equator in the northern direction. 

Salinity

• Salinity refers to the amount of salt (in grams) dissolved in 1,000 grams (1 kg) of seawater. It is commonly expressed as parts per thousand (ppt) or o/oo.
• A salinity of 7 ppt is considered the upper limit for "brackish water."
• Even slight variations in ocean surface salinity can have significant impacts on the water cycle and ocean circulation. 

Factors Affecting Salinity 

Sources of Salts in Ocean Water

• Sediments carried by rivers contribute to the salt content.
• Submarine volcanism at Oceanic Ridges releases minerals into the water.
• Chemical reactions between rocks from geothermal vents or volcanoes and cold water.
• Erosion of oceanic rocks. 

Distribution of Salinity

• Vertical Distribution: Salinity changes with depth, leading to stratification. The halocline is a distinct zone where salinity increases sharply.
• Horizontal Distribution: Salinity is highest near the tropics and decreases towards the equator and poles. Heavier rainfall near the equator incorporates freshwater, while less evaporation near the poles prevents water molecule removal. 

Relationship Between Salinity, Temperature, and Density

• Temperature and density have an inverse relationship. As temperature increases, the space between water molecules increases, reducing salinity. Water reaches its maximum density at 4°C.
• Density and salinity have a positive relationship. As density increases, so does salinity.
• Differences in density between warm and cold seawater drive ocean currents and upwelling. Warm seawater floats, while cold and dense seawater sinks. 

Variation of Density, Salinity, and Temperature with Oceanic Depth

• Rapid changes in temperature, density, or salinity create distinct regions known as clines.
• Thermoclines represent areas of rapid temperature change, pycnoclines indicate rapid density change, and haloclines reflect rapid salinity change. 

Ocean currents 

Introduction

• Ocean movements are classified into waves, tides, and currents.
• Waves form due to friction between wind and the ocean's surface. They diminish near the shore or shallow waters.
• Horizontal currents result from wind-water friction, Earth's rotation, Coriolis force, and differences in water level gradient.
• Vertical currents are driven by density variations caused by temperature and salinity changes.
• Ocean currents are crucial movements that significantly impact regional climatology. Similar to river flows, they represent a regular volume of water flowing in a specific path and direction.
• Ocean currents are influenced by two types of forces: primary forces that initiate the movement and secondary forces that influence the flow. 

Primary Forces Responsible for Ocean Currents

1. Influence of Insolation: Solar heating causes water to expand, creating a slight gradient that leads to the flow of water from east to west.
2. Influence of Wind (Atmospheric Circulation): Wind pushes the ocean's surface water and affects its movement through friction. Magnitude and direction of ocean currents are influenced by wind, with monsoon winds playing a role in seasonal reversal of currents in the Indian Ocean.
3. Influence of Gravity: Gravity causes water to pile up and creates variations in gradient.
4. Influence of Coriolis Force: Coriolis force deflects water movement to the right in the northern hemisphere and to the left in the southern hemisphere. Gyres, large accumulations of water, form circular currents in all ocean basins. An example is the Sargasso Sea. 

Secondary Forces Responsible for Ocean Currents

• Secondary forces include temperature and salinity differences. Differences in water density impact vertical ocean currents.
• Water with higher salinity and colder temperature is denser and tends to sink, while lighter and warmer water rises. Cold-water currents form as cold water from the poles sinks and slowly moves towards the equator.
• Warm-water currents flow from the equator along the surface, replacing the sinking cold water and moving towards the poles. 

Types of Ocean Currents

Based on Depth

• Ocean currents can be classified into two types based on their depth: surface currents and deep water currents.

1. Surface currents: These currents make up about 10% of the ocean's water and occupy the upper 400 meters of the ocean.
2. Deep water currents: Accounting for the remaining 90% of ocean water, deep water currents circulate within the ocean basins due to density and gravity variations.

• At high latitudes, deep waters sink into the ocean basins where cold temperatures increase their density.

Based on Temperature

1. Cold currents: These currents bring cold water from high latitudes to low latitudes. They are typically found on the west coast of continents in low and middle latitudes in both the Northern and Southern Hemispheres. In the Northern Hemisphere, they are present on the east coast in higher latitudes.
2. Warm currents: Warm currents transport warm water from low to high latitudes. They are commonly observed on the east coast of continents in low and middle latitudes in both hemispheres. In the Northern Hemisphere, they flow along the west coasts of continents in high latitudes.

General Characteristics of Ocean Currents

• The movement of ocean currents follows a general pattern of clockwise circulation in the northern hemisphere and counterclockwise circulation in the southern hemisphere. This is due to the deflective force of the Coriolis force, following Ferrel's law.
• An exception to this pattern is seen in the northern Indian Ocean, where the current direction changes with the seasonal shift in monsoon winds. Warm currents tend to move towards cold seas and vice-versa.
• In lower latitudes, warm currents flow along the eastern shores and cold currents along the western shores. This situation is reversed in higher latitudes.
• Convergence occurs when warm and cold currents meet, while divergence happens when a single current splits into multiple currents flowing in different directions.
• The shape and position of coastlines play a significant role in guiding the direction of currents.
• Currents exist not only on the ocean's surface but also below it, influenced by differences in salinity and temperature. For example, the heavy surface water of the Mediterranean Sea sinks and forms a sub-surface current that flows westward past Gibraltar. 

Effects of Ocean Currents

Bays, Gulfs, Straits, and Isthmus

Continental Shelf Deposits

Poly Metallic Nodules (PMNs)

• Characteristics of PMNs: PMNs are potato-sized lumps of minerals found in the deep sea, ranging in size from millimeters to tens of centimeters in diameter.
• Composition: PMNs contain valuable metals such as nickel, copper, cobalt, lead, cadmium, vanadium, molybdenum, and titanium, with nickel, cobalt, and copper being economically significant.
• Abundance: PMNs are abundant and widely distributed across the sea floor of all oceans.
• India's Pioneering Efforts: India became the first country to receive the status of a pioneer investor for exploring and utilizing PMNs. It was allocated an exclusive area in the Central Indian Ocean Basin by the United Nations in 1987.
• Samudrayaan Project: India's National Institute of Ocean Technology (NIOT) is set to launch the "Samudrayaan project" by 2021-22 as part of the "Deep Ocean Mission."
  • The project aims to explore the deep sea region using an indigenously developed submersible vehicle with a capacity to carry three persons to a depth of about 6000 meters for underwater studies. 

Significance of Polymetallic Nodules

• Rare Earth Elements: PMNs contain rare earth elements and metals that are crucial for high-tech industries.
• Abundance of Copper: The CCZ nodules are estimated to hold approximately 20% of the copper reserves found in global land-based sources.
• Valuable Minerals: Rare earth minerals present in PMNs, such as gold, silver, and zinc, hold significant value.
• Reducing Dependence on China: With China currently controlling over 95% of rare earth metals, India's exploration efforts aim to reduce dependence on China's dominance in this sector. 

Challenges of Polymetallic Nodule Mining

• Economic Viability: Extracting metals from PMNs is currently not economically viable.
• Environmental Concerns: Deep sea mining must be approached with caution to prevent disturbances in the delicate aquatic ecosystem. 

UN Convention on the Law of the Sea

• The UN Convention on the Law of the Sea governs issues related to deep sea mining, environmental protection, maritime boundaries, and dispute settlement. 

International Seabed Authority

• An intergovernmental body established in 1994 under the Law of the Sea Convention. It regulates and controls all mineral-related activities in the international seabed area beyond national jurisdiction.
• Its primary goal is to protect the marine ecosystem while organizing and overseeing mineral exploration and exploitation.
• The International Seabed Authority operates under the United Nations Convention on the Law of the Sea (UNCLOS).
• It has its headquarters in Jamaica and holds an observer status to the UN. The total area under its regulation accounts for more than 54% of the world's oceanic surface.
• India is a member of the International Seabed Authority and has committed to sustainable development through Agenda 2030. 

Deserts and Trade Winds

• The aridity of hot deserts is primarily influenced by the effects of offshore Trade Winds, earning them the name Trade Wind Deserts.
• Major hot deserts, such as the Sahara Desert (3.5 million square miles), the Great Australian Desert, Arabian Desert, Iranian Desert, Thar Desert, Kalahari Desert, and Namib Desert, are located on the western coasts of continents between latitudes 15° and 30°N and S.
• These deserts lie along the Horse Latitudes or Sub-Tropical High Pressure Belts, where descending air suppresses precipitation.
• Rain-bearing Trade Winds blow offshore, while the onshore Westerlies blow outside the desert limits.
• Winds reaching the deserts blow from cooler to warmer regions, resulting in lowered relative humidity and minimal condensation.
• The absence of clouds and extremely low relative humidity, ranging from 60% in coastal districts to less than 30% in desert interiors, lead to permanent drought conditions. Precipitation is scarce and highly unpredictable.

Indian Ocean Currents and Monsoons

• The currents in the northern portion of the Indian Ocean exhibit seasonal changes in response to the rhythm of the monsoons. The influence of winds on the Indian Ocean currents is particularly significant. 

Winter Circulation

• Under the influence of prevailing easterly trade winds, the north equatorial current and the south equatorial current originate south of the Indonesian islands, moving from east to west.
• This elevation of the western Indian Ocean (southeast of the horn of Africa) raises the water level by a few centimeters, resulting in the formation of a counter equatorial current that flows in a west-east direction between the north equatorial current and the south equatorial current.
• During the northeast monsoons, the water along the coast of the Bay of Bengal circulates in an anticlockwise direction. Similarly, there is an anticlockwise circulation of water along the coast of the Arabian Sea. 

Summer Circulation - North Equatorial Current Counter-Equatorial Current are absent

• In summer, due to the strong southwest monsoon and the absence of the northeast trade winds, a strong current flow from west to east, completely overriding the north equatorial current.
• Consequently, there is no counter-equatorial current As a result, the circulation of water in the northern part of the ocean during this season is clockwise. 

Southern Indian Ocean Currents - Agulhas Current, Mozambique Current, West Australian Current

• The circulation pattern in the southern part of the Indian Ocean resembles that of the southern Atlantic and Pacific Oceans and is less affected by seasonal changes.
• The south equatorial current, partly influenced by its Pacific Ocean counterpart, flows from east to west.
• It splits into two branches: one flowing to the east of Madagascar known as the Agulhas current, and the other between Mozambique and the western coast of Madagascar known as the Mozambique current.
• These two branches merge at the southern tip of Madagascar, forming the Agulhas current, which remains a warm current until it joins the West Wind Drift.
• The West Wind Drift, flowing from west to east across the higher latitudes of the ocean, reaches the southern tip of the west coast of Australia.
• One branch of this cold current turns northwards along the west coast of Australia, known as the West Australian current, and flows northward to contribute to the south equatorial current.