CDS Geography · Oceanography
Tides and Ocean Currents
What makes the tides and the kinds of tide, the forces that drive ocean currents, which currents are warm and which cold, and the great gyres with the Sargasso Sea inside one.
Why this matters
Eleven CDS questions from 2016 to 2026: 1 EASY, 9 MODERATE, 1 HARD. Three ask you to sort currents into warm and cold or match them to a coast, and the Sargasso Sea has come up twice. The 2026 (I) paper alone asked three of them in a row.
Concept 1 of 4: Tides and their kinds
The Moon pulls on the ocean and heaps the water up into a bulge on the side facing it, with a second bulge on the far side. As the Earth turns, a place passes through both bulges, so it gets two high tides a day. The Sun pulls too, but less, because it is so far away. When the Sun and Moon pull in the same line the tides are biggest; when they pull at right angles they are smallest.
Definition
The facts to hold:
- A tide is the regular rise and fall of the sea, once or twice a day, caused mainly by the pull of the Moon and the Sun. The Moon's pull is the stronger because the Moon is much nearer.
- Most coasts get two high tides and two low tides a day; high tides come about 12 hours 25 minutes apart.
- Spring tides: the Sun, Moon and Earth are in one line (new moon and full moon). The pulls add up, so high tides are highest and low tides lowest.
- Neap tides: the Sun and Moon are at right angles to the Earth (quadrature, at the first and last quarter moon). The pulls partly cancel, so the range is smallest. They come about seven days after a spring tide.
- Perigean tides: the Moon is nearest the Earth, so tides are higher than usual. Apogean tides: the Moon is farthest, so tides are lower.
- Equinoctial spring tides: extra-large spring tides near the March and September equinoxes, so they come about six months apart.
| Kind of tide | Position | Effect |
|---|---|---|
| Spring tide | Sun, Moon and Earth in a line (new and full moon) | Largest range: very high high tides |
| Neap tide | Sun and Moon at right angles to the Earth (quadrature) | Smallest range |
| Perigean tide | Moon nearest the Earth | Higher than usual |
| Apogean tide | Moon farthest from the Earth | Lower than usual |
| Equinoctial spring tide | Spring tide near an equinox | Recurs about every six monthsQ CDS 2026 (I): spring 4, neap 1, apogean 2, equinoctial spring 3. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (I) 2026 — General Knowledge · Q84Moderate
Example 1 · Oceanography · Tides and Ocean Currents
Match List-I with List-II and select the answer using the codes given below the Lists :
Codes :
| List-I (Type of Tide) | List-II (Characteristic) |
|---|---|
| A. Spring Tides | 1. Low tides caused when the Sun, Earth and Moon are in the position of quadrature |
| B. Neap Tides | 2. Occur when the Moon is the farthest from the Earth |
| C. Apogean Tides | 3. Tides recurring at an interval of six months |
| D. Equinoctial Spring Tides | 4. Very high tides caused when the Sun, Moon and Earth are almost in the same line |
Spring tides are not about the season
A spring tide has nothing to do with spring. It comes twice every month, at new moon and full moon, when the Sun, Moon and Earth are in one line.
The direction of the Earth's spin does not explain tides
Tides come from the pull of the Moon and Sun, with the Earth's turning carrying each place through the bulges. That the Earth turns west to east is true, but the direction is not the reason tides happen.
Concept 2 of 4: What drives ocean currents
An ocean current is a river of water flowing through the sea. NCERT names four forces that start it moving: the Sun's heat, the wind, gravity and the Coriolis force. Differences in density then make water sink or rise. The pull of the Moon and Sun is not on the list: it makes tides, which go up and down, not currents.
Definition
From NCERT (Movements of Ocean Water):
- Heating by the Sun makes water expand, so near the equator the sea stands about 8 cm higher than in the middle latitudes, and water slowly flows down that slope.
- Wind blowing on the surface pushes the water along. The great surface currents follow the prevailing winds.
- Gravity (the Earth's) pulls the heaped water down and keeps it flowing.
- Coriolis force turns moving water to the right in the Northern Hemisphere and to the left in the Southern.
- Density: cold or salty water is denser and sinks; warm or fresh water is lighter and rises. This drives the slow deep circulation (the thermohaline circulation).
- The gravitational pull of the Sun and Moon raises tides, not ocean currents.
| Force | What it does to sea water |
|---|---|
| Heating by the Sun | Expands water near the equator, making a gentle slope |
| Wind | Drags the surface water along: the main push behind surface currents |
| Gravity of the Earth | Pulls heaped-up water down the slope |
| Coriolis force | Turns currents right in the north, left in the south |
| Differences in density | Make cold, salty water sink and drive the deep circulation |
| Pull of the Sun and Moon | Raises tides; not a cause of currentsQ CDS 2022 (II): the gravitational pull of the Sun and Moon is the one that does not influence currents. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2022 · CDS (II) 2022 — General Knowledge · Q44Moderate
Example 2 · Oceanography · Tides and Ocean Currents
Which one of the following do not influence the ocean currents ?
Gravity is on the list, the Moon's pull is not
NCERT lists gravity among the forces behind currents, meaning the Earth's gravity pulling heaped water down. The pull of the Sun and Moon is a different thing: it makes tides.
Concept 3 of 4: Warm and cold currents, and El Niño
One rule sorts almost every current. Water flowing from the warm tropics towards the poles is a warm current; water flowing from the cold high latitudes towards the equator is a cold current. In the tropics and subtropics the warm currents run along the east coasts of the continents and the cold ones along the west coasts.
Definition
The rule and the exception:
- Warm currents flow away from the equator: the Gulf Stream, North Atlantic Drift, Kuroshio, Brazil, Agulhas and the Equatorial currents.
- Cold currents flow towards the equator or out of polar seas: the California, Humboldt (Peru), Canary, Benguela, Labrador, Falkland and Oyashio currents and the West Wind Drift.
- El Niño: every few years a warm current appears off the coast of Peru, in place of the cold Humboldt current, around Christmas. NCERT lists its effects: it distorts the equatorial atmospheric circulation, makes the evaporation of sea water irregular, and reduces plankton, so there are fewer fish. It is also used to forecast India's monsoon.
| Current | Where it flows | Warm or cold |
|---|---|---|
| Gulf Stream | East coast of the USA, North Atlantic | Warm |
| North Atlantic Drift | Towards north-west Europe | WarmQ CDS 2020 (I): the North Atlantic Drift and the Brazil current are the warm pair. |
| Brazil Current | East coast of South America | Warm |
| Kuroshio | East coast of Japan, North Pacific | Warm |
| Agulhas Current | South-east coast of Africa (South Africa) | WarmQ CDS 2016 (II): Agulhas South Africa, Humboldt Peru, Labrador Canada, Falkland Argentina. |
| California Current | West coast of North America | ColdQ CDS 2021 (I): the California current is the cold one. |
| Humboldt (Peru) Current | West coast of South America (Peru) | Cold |
| Canary Current | North-west coast of Africa | Cold |
| Benguela Current | South-west coast of Africa | Cold |
| Labrador Current | East coast of Canada | Cold |
| Falkland Current | East coast of Argentina | Cold |
| El Niño | Off Peru and Ecuador, replacing the Humboldt | WarmQ CDS 2022 (I): the south-east trades blowing towards the Indian Ocean is the normal state, not a result of El Niño. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2020 · CDS (I) 2020 — General Knowledge · Q66Moderate
Example 3 · Oceanography · Tides and Ocean Currents
Which of the following are warm ocean currents?
Pairs in the options mix one warm with one cold
Kuroshio is warm but California is cold; West Wind Drift and Falkland are both cold; Canary and Benguela are both cold. Check each current in a pair against the rule before choosing.
El Niño weakens the trades, it does not cause them
In a normal year the south-east trades push warm water west across the Pacific. In an El Niño year they weaken and warm water spreads east to Peru. Trades blowing west are the normal state, not an El Niño effect.
Concept 4 of 4: Gyres and the Sargasso Sea
Push the surface of an ocean with the trade winds near the equator and the westerlies further out, and let the Coriolis force bend the flow, and the water goes round in a huge loop: a gyre. In the North Atlantic the calm middle of that loop is the Sargasso Sea, a sea with currents for its shores.
Definition
How the loops work:
- Gyres are huge circular loops of surface currents, one in each ocean basin on each side of the equator. They are driven by the surface winds.
- They turn clockwise in the Northern Hemisphere and anticlockwise in the Southern.
- The equatorial currents carry warm water westward; at the continents they turn poleward along the east coasts as warm currents (Gulf Stream, Kuroshio), then come back towards the equator along the west coasts as cold currents.
The Sargasso Sea sits inside the North Atlantic gyre:
- It is the only sea with no land boundary; currents ring it on every side.
- Its water is calm and covered with floating Sargassum seaweed, which gives it its name.
- With strong evaporation and no rivers, its water is saltier than the average ocean.
| Side of the Sargasso Sea | Current that bounds it |
|---|---|
| West | Gulf StreamQ CDS 2019 (I): the sea lies east of the Gulf Stream and west of the Canary current, not the other way round. |
| North | North Atlantic Drift |
| East | Canary Current |
| South | North Equatorial CurrentQ CDS 2026 (I): it is the one sea without a land boundary; it is salty, not the least salty part of the Atlantic. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2024 · CDS (II) 2024 — General Knowledge · Q79Moderate
Example 4 · Oceanography · Tides and Ocean Currents
Consider the following statements :
1. Ocean currents are dominated by huge surface gyres that are driven by the global surface wind pattern.
2. Equatorial currents move cold water westward and then poleward along the east coasts of continents.
With regard to the statements given above, which of the following is correct ?
Gulf Stream west, Canary east
On a map the Gulf Stream runs up the American side, so it is on the Sargasso Sea's west. The Canary current runs down the African side, on its east.
No land boundary does not mean fresh
The Sargasso Sea has no coast and no rivers, so nothing dilutes it, and strong evaporation makes it salty. A statement calling it the least salty part of the Atlantic is false.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (4)
Tides and their kinds5 rows
| Kind of tide | Position | Effect |
|---|---|---|
| Spring tide | Sun, Moon and Earth in a line (new and full moon) | Largest range: very high high tides |
| Neap tide | Sun and Moon at right angles to the Earth (quadrature) | Smallest range |
| Perigean tide | Moon nearest the Earth | Higher than usual |
| Apogean tide | Moon farthest from the Earth | Lower than usual |
| Equinoctial spring tide | Spring tide near an equinox | Recurs about every six monthsQ CDS 2026 (I): spring 4, neap 1, apogean 2, equinoctial spring 3. |
What drives ocean currents6 rows
| Force | What it does to sea water |
|---|---|
| Heating by the Sun | Expands water near the equator, making a gentle slope |
| Wind | Drags the surface water along: the main push behind surface currents |
| Gravity of the Earth | Pulls heaped-up water down the slope |
| Coriolis force | Turns currents right in the north, left in the south |
| Differences in density | Make cold, salty water sink and drive the deep circulation |
| Pull of the Sun and Moon | Raises tides; not a cause of currentsQ CDS 2022 (II): the gravitational pull of the Sun and Moon is the one that does not influence currents. |
Warm and cold currents, and El Niño12 rows
| Current | Where it flows | Warm or cold |
|---|---|---|
| Gulf Stream | East coast of the USA, North Atlantic | Warm |
| North Atlantic Drift | Towards north-west Europe | WarmQ CDS 2020 (I): the North Atlantic Drift and the Brazil current are the warm pair. |
| Brazil Current | East coast of South America | Warm |
| Kuroshio | East coast of Japan, North Pacific | Warm |
| Agulhas Current | South-east coast of Africa (South Africa) | WarmQ CDS 2016 (II): Agulhas South Africa, Humboldt Peru, Labrador Canada, Falkland Argentina. |
| California Current | West coast of North America | ColdQ CDS 2021 (I): the California current is the cold one. |
| Humboldt (Peru) Current | West coast of South America (Peru) | Cold |
| Canary Current | North-west coast of Africa | Cold |
| Benguela Current | South-west coast of Africa | Cold |
| Labrador Current | East coast of Canada | Cold |
| Falkland Current | East coast of Argentina | Cold |
| El Niño | Off Peru and Ecuador, replacing the Humboldt | WarmQ CDS 2022 (I): the south-east trades blowing towards the Indian Ocean is the normal state, not a result of El Niño. |
Gyres and the Sargasso Sea4 rows
| Side of the Sargasso Sea | Current that bounds it |
|---|---|
| West | Gulf StreamQ CDS 2019 (I): the sea lies east of the Gulf Stream and west of the Canary current, not the other way round. |
| North | North Atlantic Drift |
| East | Canary Current |
| South | North Equatorial CurrentQ CDS 2026 (I): it is the one sea without a land boundary; it is salty, not the least salty part of the Atlantic. |
Watch out for (7)
- Spring tides are not about the season→ Tides and their kinds
- The direction of the Earth's spin does not explain tides→ Tides and their kinds
- Gravity is on the list, the Moon's pull is not→ What drives ocean currents
- Pairs in the options mix one warm with one cold→ Warm and cold currents, and El Niño
- El Niño weakens the trades, it does not cause them→ Warm and cold currents, and El Niño
- Gulf Stream west, Canary east→ Gyres and the Sargasso Sea
- No land boundary does not mean fresh→ Gyres and the Sargasso Sea
Test yourself on Oceanography
10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.