What explains the eastward flow of the equatorial counter-current?

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The Earth's rotation on its axis

Convergence of the two equatorial currents

Difference in salinity of water

Occurrence of the belt of calm near the equator

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This question was previously asked in

UPSC IAS – 2015

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The eastward flow of the equatorial counter-current is primarily explained by the pressure gradient created by the ‘pile-up’ of water in the western equatorial ocean, which is allowed to flow back eastward in the belt of calm near the equator.

The persistent westward-blowing trade winds in the tropics push surface water westward, causing it to accumulate in the western part of the ocean basin (e.g., western Pacific, western Atlantic). This creates a higher sea level and a pressure gradient sloping downwards towards the east. In the doldrums (Intertropical Convergence Zone – ITCZ), a belt of low pressure and calm or weak winds near the equator, the wind stress is minimal. With reduced opposition from wind, the water flows eastward down this pressure gradient, forming the Equatorial Counter-Current.

While other factors influence ocean currents, the primary driving mechanism for the eastward equatorial counter-current is the interplay between the westward-flowing trade winds, the resulting pressure gradient caused by water accumulation in the west, and the presence of the doldrums where this eastward flow is not inhibited by strong winds. The Earth’s rotation (Coriolis effect) causes deflection, but not the primary eastward flow. Salinity differences mainly influence deeper thermohaline circulation.

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