The eastward flow of the equatorial countercurrent is primarily driven by the convergence of the North and South Equatorial Currents, which flow westward. This convergence occurs as both currents pile up warm water in the western Pacific, creating a gradient that results in a subsequent eastward flow of water at the surface, known as the equatorial countercurrent. This phenomenon is essential in maintaining the thermohaline circulation and influencing global climate patterns, underlining the interconnectedness of oceanic currents in regulating weather and climate systems, critical knowledge in the context of UPSC Prelims GS Paper I.