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How do pressure gradients cause 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.

Ocean currents are driven by horizontal differences in pressure that occur across a horizontal plane or geopotential surface. These pressure gradients arise primarily from horizontal variations in seawater density, which are caused by regional differences in temperature and salinity.1

In a homogeneous ocean, horizontal pressure differences can only exist if the sea surface is tilted, creating a "pressure head" that drives surface circulation. However, because the world's oceans are not homogeneous, horizontal pressure gradients also vary with depth, leading to complex current patterns.1

On a rotating Earth, these currents are further influenced by the Coriolis force. Away from boundary layers, currents often reach a geostrophic balance, where the speed and direction of the water flow are determined by an exact balance between the horizontal pressure-gradient force and the Coriolis force. This results in geostrophic currents that flow perpendicular to the pressure gradient, with high pressure to the right of the current in the Northern Hemisphere and to the left in the Southern Hemisphere.2