Oceans and continents: five oceans and the unequal hemispheres

Detail

1. How much of the Earth is water

• About 71% of the Earth's surface is water. About 29% is land.
• All the oceans are joined together. They form one World Ocean, which is a single linked body of salt water.
• Because water covers so much of the surface, the Earth looks blue from space. This is why it is called the "Blue Planet".

2. Distribution of land and water: the unequal hemispheres

• Hemisphere means one half of the globe. The Equator divides it into a Northern and a Southern Hemisphere.
• Land and water are not shared equally between the two halves. The NCERT figure (Fig. 2.2) shows this through two views, one from above each pole.

Feature	Northern Hemisphere	Southern Hemisphere
Nickname	"Land hemisphere"	"Water hemisphere"
Share of land	About two-thirds of all land	Much less land
Share of ocean	Smaller	About 81% ocean
Centre of the hemisphere with most land/water	Near western France (land hemisphere)	Near New Zealand (water hemisphere)
Ocean temperature	Warmer oceans	Cooler, open seas
Annual temperature range	Larger	Smaller
Winds	Westerlies are broken up by land	Strong, steady westerlies (Roaring Forties)

• Land hemisphere and water hemisphere are not just the north and south halves. They are the two halves of the globe that hold the most land and the most water.
• The half with the most land is centred near western France.
• The opposite half, centred near New Zealand, is mostly ocean.

Why the Northern Hemisphere has bigger temperature ranges

• Land heats up fast in summer and cools fast in winter. Water heats and cools slowly.
• The north has more land, so its summers are hotter and its winters colder.
• Its oceans are partly enclosed by land, and land warms the water near it. So the northern oceans are warmer. (Temperature is covered in Section 5.)

Why the Southern Hemisphere has the Roaring Forties

• The westerlies are winds that blow from west to east in the middle latitudes (about 35°–60°).
• Between about 40°S and 50°S, there is almost no land to slow the wind down.
• So the westerlies blow strong and steady over the open sea. Sailors named these winds the Roaring Forties. Further south come the "Furious Fifties" and the "Shrieking Sixties".

3. The five oceans, largest to smallest

Rank	Ocean	Key facts
1	Pacific	Largest and deepest. Covers about one-third of the Earth's surface, more than all the land put together
2	Atlantic	Second largest
3	Indian	Third largest. The only ocean named after a country
4	Southern	Surrounds Antarctica, south of about 60°S
5	Arctic	Smallest. Lies around the North Pole

Pacific Ocean in numbers

• It covers more than 155 million km² and covers more than 30% of the Earth's surface [3].
• Its average depth is about 4,000 m [3].
• It holds almost twice as much water as the Atlantic, the second-largest ocean [3].
• The deepest known point on Earth is the Challenger Deep.
• Depth: about 10,935 m [4].
• Location: the southern end of the Mariana Trench in the western Pacific, south-west of Guam [4].
• The Mariana Trench is about 2,500 km long and curves in an arc beside islands such as Guam and Saipan [4].

Ocean boundaries are only conventions

• The oceans flow into one another, so no natural line separates them. Their boundaries are drawn by agreement.
• Seas, bays and gulfs are smaller parts of oceans that land partly encloses. Examples are the Arabian Sea, the Bay of Bengal and the Gulf of Mannar.
• Watch out: Class 11 NCERT names five oceans and then says "four large oceans". This is a slip in the textbook. Use five oceans in answers.

4. The Southern Ocean: the newest ocean on the map

• Definition: the Southern Ocean is the ocean around Antarctica, stretching from the Antarctic coast north to about 60°S.

• The US Board on Geographic Names also uses the line from the Antarctic coast to 60°S [2].

• What defines it: the Antarctic Circumpolar Current (ACC).

• The ACC is a current that flows from west to east all the way around Antarctica [2].
• It is the largest ocean current in the world [2].
• It links the waters of the Pacific, Atlantic and Indian Oceans into one network of currents [2].

• Water inside this current has its own temperature, plants and animals, including whales and penguins. So scientists treat it as a separate ecological region, meaning an area with its own living world and climate [5].

• How it was recognised:

Year	Event
Early 20th century	Some oceanographers begin using the name "Southern Ocean" for the seas around Antarctica [5]
2000	The IHO (International Hydrographic Organization, the body that sets ocean and sea limits) drafts limits that name the Southern Ocean. The draft was never formally ratified
2021	On World Ocean Day (8 June), National Geographic changes its map policy and recognises the Southern Ocean as Earth's fifth ocean [5]

• Most countries, including the United States, now accept the Southern Ocean as the fifth ocean [2].
• Before this, the ocean was usually split into four: the Pacific, Atlantic, Indian and Arctic [5].

5. The Indian Ocean

Position and boundaries

• To the north is Asia, with India in the middle.
• To the west is Africa.
• To the east is Australia.
• To the south is the Southern Ocean.
• Two arms of the sea lie on either side of the Indian Peninsula:
• the Arabian Sea to the west
• the Bay of Bengal to the east

Special features

• It is the third-largest ocean.
• It is the only ocean named after a country, India.

Why it has the monsoon and reversing currents

• Asia closes the Indian Ocean off to the north, so the ocean does not reach the cold Arctic.
• In summer, the large landmass heats up and air pressure over it falls. Moist winds blow in from the south-west and bring the south-west monsoon.
• In winter, the land cools and the winds reverse to blow from the north-east.
• Ocean currents in the northern Indian Ocean follow the winds, so they change direction with the seasons. No other ocean has currents that reverse like this. (See ocean-movements. The Indian Ocean Dipole is covered in indian-climate-monsoon.)

A highway of world trade

• About two-thirds of the world's oil shipments and half of its container traffic cross the Indian Ocean.
• Chokepoints are narrow sea passages that a lot of shipping must squeeze through:

Chokepoint	Connects	Lies between
Strait of Hormuz	Persian Gulf ↔ Gulf of Oman/Arabian Sea	Iran and Oman
Bab-el-Mandeb	Red Sea ↔ Gulf of Aden	Yemen and Djibouti/Eritrea
Strait of Malacca	Indian Ocean (Andaman Sea) ↔ South China Sea/Pacific	Malay Peninsula and Sumatra (Indonesia)

• For India, about 95% of trade by volume and about 70% by value goes by sea [7].

6. India's coastline and islands

• India has more than 1,300 islands. The official count is 1,382 islands [6][7].
• There are two main island groups:

Group	Sea	Type	Capital
Andaman and Nicobar Islands	Bay of Bengal	Large chain; parts of a drowned mountain range, with some volcanic islands (Barren Island)	Sri Vijaya Puram (formerly Port Blair)
Lakshadweep	Arabian Sea	Small coral islands and atolls (ring-shaped coral reefs)	Kavaratti

• Coastline:
• India's coastline was earlier given as 7,516.6 km.
• It has now been re-measured at 11,098.81 km. The National Hydrographic Office did this together with the Survey of India [6][7].

• The coast touches nine coastal states [7].

• Continents and islands as a map are in world-physical-features.

Prelims Hooks

• Water covers about 71% of Earth's surface and land about 29%. The Southern Hemisphere is about 81% ocean.
• Land hemisphere is centred near western France. Water hemisphere is centred near New Zealand.
• Oceans by size: Pacific > Atlantic > Indian > Southern > Arctic. The Arctic is the smallest.
• The Pacific covers more than 30% of the Earth's surface and is larger than all land combined. The Challenger Deep (Mariana Trench) is about 10,935 m deep [3][4].
• Roaring Forties: strong, steady westerlies at about 40°–50°S, caused by the lack of land in the Southern Hemisphere.
• The Southern Ocean lies south of 60°S and is defined by the Antarctic Circumpolar Current, which flows west to east. It is the world's largest current [2].
• Trap: the IHO's 2000 limits for the Southern Ocean were never formally ratified. National Geographic recognised the Southern Ocean in 2021 [5].
• The Indian Ocean is the only ocean named after a country.
• Lakshadweep is in the Arabian Sea and made of coral. Andaman and Nicobar is in the Bay of Bengal. India has 1,382 islands [6].
• Chokepoint pairs to remember: Hormuz → Iran–Oman; Bab-el-Mandeb → Red Sea–Gulf of Aden; Malacca → Malay Peninsula–Sumatra.

Mains Points

• Unequal distribution of land and water shapes climate (GS-I):
• The land-heavy north has larger temperature ranges and warmer oceans.
• The ocean-heavy south has steady westerlies (the Roaring Forties) and the unbroken Antarctic Circumpolar Current.

• The ACC links the three main oceans and moves heat between them [2].

• The Indian Ocean is landlocked to the north (GS-I):

• This one fact explains the monsoon, the seasonal reversal of currents and the Indian Ocean's unique climate.

• It is also why the ocean matters so much to farming and water supply across South Asia.

• Strategic and economic weight (GS-III):

• Most of the world's oil and much of its container trade passes through Hormuz, Bab-el-Mandeb and Malacca.
• About 95% of India's trade by volume goes by sea [7].

• India's 1,382 islands and its 11,098.81 km re-measured coastline extend its maritime reach [6]. They also call for sea-route security and port development.

• Ocean boundaries are set by agreement, not by nature (GS-I):

• The Southern Ocean shows that ocean limits change with science and with decisions by bodies such as the IHO and National Geographic.
• Its separate ecosystem supports the case for treating it as an ocean of its own [5].