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How are upwelling and downwelling related to ocean currents?

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World map showing major ocean current systems, with arrows indicating the direction and speed of warm and cold currents in the Atlantic, Pacific, Indian, Southern, and Arctic Oceans. Warm currents are marked in red and cold currents in blue, with currents labeled by name and symbols denoting those that reverse direction seasonally. A legend explains current speed categories and color codes. — ocean current Major ocean current systems of the world.

Upwelling and downwelling are vertical movements of seawater that are fundamentally linked to the horizontal flow patterns of ocean currents. Because seawater is nearly incompressible, these vertical processes occur in response to regions of convergence and divergence within the horizontal circulation.1

Mechanisms of Vertical Movement

These movements are primarily driven by Ekman transport, where wind stress causes surface water to move at an angle to the wind direction. Variations in this transport create zones of convergence and divergence:23

Downwelling: Occurs in regions of convergence, where surface waters accumulate and are forced downward. This process often creates a "hill" in sea level, which drives anticyclonic geostrophic currents extending deep below the surface.23
Upwelling: Occurs in regions of divergence, where surface water is pulled away, drawing deeper, denser, and nutrient-rich water to the surface. This creates a depression in sea level, inducing cyclonic geostrophic currents.23

Beyond wind-driven effects, vertical movement is also a key component of the thermohaline circulation, where dense, cold water sinks in polar regions and eventually upwells elsewhere to maintain the global balance of the ocean's water masses.4