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Why do subtropical gyres rotate clockwise in the Northern Hemisphere?

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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.

Subtropical gyres in the Northern Hemisphere rotate clockwise primarily due to the Coriolis force, which arises from Earth's rotation.1

As wind blows across the ocean surface, it creates Ekman transport, where the net movement of the surface layer is directed 90° to the right of the wind direction in the Northern Hemisphere. In subtropical regions, this transport causes surface waters to converge toward the center of the gyre, creating a "hill" or elevation in sea level.2

This accumulation of water establishes a horizontal pressure gradient. The resulting geostrophic current—where the pressure gradient force is balanced by the Coriolis force—flows in a clockwise (anticyclonic) direction around this elevated water mass. Essentially, the combination of wind-driven convergence and the Earth's rotation forces the water to circulate in a clockwise pattern, a phenomenon that is a fundamental feature of ocean currents in the Northern Hemisphere.321