Correct Option (b):
The Equatorial Countercurrent is an eastward-flowing ocean current situated between the westward-flowing North and South Equatorial Currents. As these westward currents transport large volumes of water towards the western margins of ocean basins, water accumulates in these regions. This accumulation leads to a higher sea level and creates a significant pressure gradient, with elevated water levels in the west compared to the east. Consequently, the excess water flows back eastward along the equator, driven by this pressure gradient, forming the Equatorial Countercurrent. This mechanism is essential for balancing the mass transport within the equatorial ocean system.
Incorrect Options:
The Earth's rotation on its axis generates the Coriolis effect, which deflects moving objects, including ocean currents. This effect is crucial for the general circulation patterns of ocean currents, causing deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. However, the Coriolis effect is negligible at the equator and does not directly explain the eastward reversal of the Equatorial Countercurrent.
Differences in salinity affect the density of seawater, which is a primary driver of thermohaline circulation (density currents) in the deep ocean. While salinity variations can influence vertical water movements and deep-ocean currents, they are not the main mechanism responsible for the large-scale, eastward-flowing Equatorial Countercurrent, which is a surface current primarily driven by pressure gradients.
The belt of calm near the equator, also known as the Doldrums or Intertropical Convergence Zone (ITCZ), is characterized by low atmospheric pressure, light and variable winds, and strong convection. This atmospheric condition primarily results in vertical air movements and heavy rainfall. While it influences surface wind patterns, it does not directly explain the eastward flow of the Equatorial Countercurrent, which is an oceanic phenomenon driven by the accumulation of water by the westward equatorial currents.