Tides and Ocean Currents
~10 min read
- Tides: Periodic rise/fall of sea level due to gravitational pull of Moon (chief) and Sun. Spring tide (full/new moon) = highest. Neap tide (quarter moon) = lowest.
- Currents: Continuous flows of ocean water. Warm (equator→poles) and cold (poles→equator).
- El Nino / La Nina: Periodic Pacific warming (El Nino) or cooling (La Nina) that disrupts global weather, including the Indian monsoon.
Tides are driven by gravity. Currents are driven by winds, density and Earth's rotation. NDA tests spring vs neap tides, named currents (Gulf Stream, Humboldt, Kuroshio), and El Nino/La Nina.
Tides
Caused mainly by the Moon's gravity (Sun contributes ~46% of the Moon's effect). Two high tides and two low tides occur per day at most coasts.
| Type | Cause | Effect |
|---|---|---|
| Spring tide | Sun, Moon, Earth in line (full or new moon) | Highest high tides, lowest low tides |
| Neap tide | Sun and Moon at right angles (quarter moon) | Smallest tidal range |
Highest tidal range on Earth: Bay of Fundy, Canada (~16 m). India: Gulf of Khambhat, Sundarbans (~11 m).
Major Ocean Currents
| Ocean | Warm currents | Cold currents |
|---|---|---|
| Atlantic | Gulf Stream, Brazil, Guinea | Labrador, Canary, Benguela |
| Pacific | Kuroshio, North Pacific, East Australian | California, Peru/Humboldt, Oyashio |
| Indian | Mozambique, Agulhas, monsoon currents | West Australian, Somali (seasonal) |
Gulf Stream warms NW Europe — London and Stockholm have milder winters than Quebec at the same latitude.
El Nino, La Nina, and the Indian Monsoon
El Nino: Periodic warming of central/east Pacific surface waters (every 3-7 years). Disrupts trade winds and global rainfall.
- India: usually weakens the monsoon → drought risk.
- Peru: heavy rain, fishery collapse (loss of cold Humboldt upwelling).
- Australia/Indonesia: drought, bushfires.
La Nina: opposite — east Pacific cools. Indian monsoon usually strengthens. La Nina year = good monsoon for India.
ENSO (El Nino Southern Oscillation) couples the warming with atmospheric pressure changes (Southern Oscillation Index).
NDA PYQ Examples
Q: Spring tides occur when:
(a) Sun and Moon are at right angles (b) Sun, Earth and Moon are in line (c) The Moon is at perigee (d) The Moon is at apogee
Answer: (b) Sun, Earth and Moon in line — full or new moon.
Q: The Gulf Stream is a:
(a) Cold current (b) Warm current (c) Tidal current (d) Subsurface current
Answer: (b) Warm current — flows from Gulf of Mexico across the Atlantic.
Q: El Nino typically:
(a) Strengthens the Indian monsoon (b) Weakens the Indian monsoon (c) Has no effect (d) Causes extreme cold
Answer: (b) Weakens the Indian monsoon — drought risk in India.
Drill Tides and Ocean Currents for NDA
NDA-pattern items on Tides and Ocean Currents with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why are there two high tides per day if there is only one Moon?
One bulge faces the Moon (gravity), the other is on the opposite side (inertia — the Earth is pulled slightly more than the water on the far side). Earth rotates through both bulges in 24 hours, giving two high tides.
What is the Coriolis effect on currents?
Earth's rotation deflects currents to the right in the Northern Hemisphere and to the left in the Southern. This creates the great clockwise gyres in N Pacific and N Atlantic, and counter-clockwise gyres in S oceans.
Why does the Peruvian fishery collapse during El Nino?
Normally, cold Humboldt Current upwelling brings nutrient-rich water that feeds anchovies. During El Nino, warm water blocks upwelling — fish migrate or die, and the local fishing industry collapses.
What is the difference between a tidal current and an ocean current?
Tidal currents reverse twice daily as tides flood and ebb. Ocean currents flow continuously in one direction, driven by wind and density.