The natural event that weakens the western surface ocean currents in the equatorial Pacific ocean is the EL NINO EVENT.
Step-by-step explanation:
During this event, the easterly trade winds which converge which converge across the equatorial Pacific Ocean, weakens. Due to this reason the ocean current that draws the surface water away from the western coast of South America slows down. This results in precipitation in the southern part America while the northern part remains dry.