Equatorial Ocean Currents and the Equatorial Counter-Current

UPSC once asked about an odd ocean current. It flows east, against the winds around it.

What was the question?

The 2015 Civil Services Prelims, General Studies Paper I, asked what explains the eastward flow of the equatorial counter-current. This was Series A, Question 14. The four options were:

• (a) The Earth’s rotation on its axis
• (b) Convergence of the two equatorial currents
• (c) Difference in salinity of water
• (d) Occurrence of the belt of calm near the equator

What is the answer?

The official answer key gives (b) convergence of the two equatorial currents. Convergence means that flows come together and meet. The next cards show where the counter-current sits and why it flows east.

Why are the other options wrong?

• Ocean scientists explain this current by the spin of the winds, not by rotation alone.
• Sources explain this flow by winds, not by salinity.
• The belt of calm is the older 1940 idea. Modern sources stress the wind spin of the ITCZ instead.

Near the equator, the ocean has three surface currents side by side. Two flow west. One flows east.

Which currents flow west?

• The North Equatorial Current flows west, north of the counter-current.
• The South Equatorial Current flows west, south of the counter-current.
• Steady trade winds blow from east to west and drag the surface water along.

Which current flows east?

The North Equatorial Counter-Current flows east between the two. It is called a counter-current because it flows counter to the trade winds. In the Pacific and Atlantic, it lies between about 5°N and 10°N.

What does that look like on a map?

Picture three lanes of traffic. The outer two lanes move west. The middle lane moves east.

The trade winds blow west. So an eastward current looks strange at first. Ocean scientists have two ideas that help.

What do the trade winds do to the water?

At the equator, the easterly trade winds push surface water west. This water piles up gently in the western Pacific. The sea there is about half a metre higher than in the eastern Pacific.

What is the doldrums?

The doldrums are a belt of weak winds near the equator. Trade winds from both hemispheres meet there. Air rises, so there is often little surface wind. Scientists call this belt the Intertropical Convergence Zone, or ITCZ.

What is the older explanation?

In 1940, two scientists named Montgomery and Palmen offered an idea. In the doldrums, the wind stress drops a lot. The pressure gradient then keeps pushing and an eastward flow develops. This is the idea behind the “belt of calm” option.

What do scientists say now?

Oceanographers link the counter-current to the winds of the ITCZ. These winds have a spin, called a cyclonic wind stress curl. This curl drives the eastward current between 5°N and 10°N.

Is Coriolis force the cause?

Not by itself. Scientists explain this current by the wind stress curl, not by rotation alone.

The counter-current is not a fixed stripe. It shifts and changes with the seasons.

How far north does it reach?

Sources give slightly different limits. Most place it between 5°N and 10°N. In the eastern Atlantic it sits nearer the equator, at about 3°N to 6°N.

Does it flow all year?

Mostly, but not always. The wind pattern changes through the year.

• Both the South Equatorial and counter-currents are strongest in July and August.
• In the Pacific, the counter-current is weak in northern winter and spring.
• In the eastern Pacific, it may break up or vanish for a time.
• In the Atlantic, it disappears or reverses west of about 18°W from January to June.

What about El Niño?

During El Niño events, the counter-current strengthens in the central Pacific. So it is linked to wider climate changes.

Is there a southern counter-current?

A weaker South Equatorial Counter-Current exists at 5°S to 10°S in both oceans. It is not as well developed as the northern one.

The Indian Ocean does not copy the Pacific and the Atlantic. The monsoon winds reverse with the seasons.

Why do the winds matter so much?

In the Indian Ocean, the yearly average wind is weak. Its seasonal cycle is strong. So the equatorial currents behave very differently.

What happens on the equator?

The surface currents reverse four times a year. They flow west in winter. They flow weakly west in summer. They flow strongly east in spring and in autumn. These eastward surges are called Wyrtki Jets.

Where is the counter-current?

The Indian Ocean’s eastward counter-current lies south of the equator. It is called the South Equatorial Counter-Current. It runs at about 3°S to 6°S. It is best developed in February, in the northeast monsoon.

What about the north?

North of the equator, the flow follows the monsoon. The Southwest Monsoon Current south of Sri Lanka flows east in summer. The Northeast Monsoon Current flows west in winter. The Somali Current also reverses with the monsoon.

The Equatorial Undercurrent is a different current. Do not mix it up with the counter-current.

Where does it flow?

It flows east, just beneath the surface layer, right on the equator. It is centred at about 150 to 200 metres depth. It is only about 150 metres thick. Its speed can pass 100 cm per second.

Why does it exist?

The sea surface is higher in the west than in the east. This creates a pressure difference. It drives the flow below the surface eastward.

What does it do in the east?

The undercurrent shoals toward the east. This goes with the upwelling of cool water. In normal years, the upwelling forms the cold tongue of the eastern Pacific.

What about the Indian Ocean?

An eastward undercurrent is seen steadily only in spring there. Equatorial upwelling is absent in the eastern Indian Ocean. So scientists treat it as a short-lived wave effect.

These currents are linked to El Niño.

How is it linked to El Niño?

Trade winds push water west and pile it up there. When the trade winds weaken, the flow weakens too. Upwelling slows, and the eastern Pacific warms.

Why do fishers care?

NCERT notes that mixing of warm and cold currents replenishes oxygen. It helps plankton grow, and plankton feed fish. The best fishing grounds lie in such mixing zones.

What did the doldrums mean for sailors?

NOAA notes that the calm area can becalm sailing ships for weeks. That is where the name “doldrums” comes from.

Exams ask why the current flows east. They also test the causes of ocean currents.

Which questions have appeared?

• UPSC 2015: why the equatorial counter-current flows east.
• CDS I 2026, statement 1: is density the main cause of ocean currents?
• CDS I 2026, statement 2: does density depend on temperature, salinity and pressure?
• CDS I 2026, statement 3: does water move from higher to lower density?

Which facts should you hold?

• The counter-current flows east, against the trade winds.
• It lies at about 5°N to 10°N in the Pacific and Atlantic.
• The Indian Ocean’s flow changes with the monsoon.
• The Equatorial Undercurrent is deeper and sits on the equator.

What does NCERT list as primary forces?

NCERT lists four: heating by solar energy, wind, gravity and Coriolis force. Density differences also affect deep circulation.