Final answer:
Deep-ocean water from the poles gradually warms as it reaches the equator by going through a process called upwelling and mixing.
Step-by-step explanation:
When deep-ocean water from the poles gradually warms as it reaches the equator, it goes through a process called upwelling and mixing. This happens at the Subtropical Convergence, where the cold, dense Southern Ocean water meets the warmer waters of the Pacific, Atlantic, and Indian Oceans. The cold water from the south sinks below the warm waters from the north, creating a zone of upwelling that supports high levels of productivity for organisms like phytoplankton and krill.