The ocean that moves upward

The missing direction on a flat map

A map of ocean currents often invites your eyes to travel sideways. Arrows cross a basin, bend around a coast, or join a broad loop. That is a useful picture, but the ocean has depth as well as width. Some movements connect different levels of the water column, and their consequences can reach far beyond the patch of water where they begin.

NOAA describes currents driven by several influences, including wind and density differences associated with temperature and salinity. Surface and deep circulation are connected parts of a moving ocean. The familiar line of waves seen from a beach is therefore not a complete inventory of motion, and a calm-looking surface is not proof that the water beneath it is still.

A gyre is a large system of rotating currents. An eddy is a circular current, often a smaller feature that can form within or separate from a larger flow. Keeping those terms distinct helps avoid imagining every curved arrow as the same kind of giant whirlpool. Scale matters when deciding what a diagram is actually showing.

Now add the missing vertical direction. If surface water moves away from an area, water from below can rise to replace it. This process, upwelling, is not a wave crest briefly lifting the surface. It is a route by which subsurface water enters a different part of the ocean, carrying properties that can change conditions there.