Why is the equatorial countercurrent in Atlantic ocean so poorly defined?

The Equatorial Countercurrent (ECC) in the Atlantic Ocean is better defined in the eastern sector of the basin mainly during January–March. Over the rest of the year, and in much of its width in the central-west to eastern basins, the subsurface transport in the equatorial sector consists, more or less, in one broad velocity core at around 30–50 m depth without showing a significant eastward surface flow separating the North and South Equatorial Currents, i.e., a poorly defined ECC