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Why is the Gulf Stream a strong western boundary current?

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Map of the North Atlantic Ocean showing major ocean currents, with warm currents like the Gulf Stream and North Atlantic Drift highlighted in pink and cold currents in blue. Arrows indicate current directions and relative speeds, connecting the eastern coasts of North America, the western coasts of Europe and Africa, and the Arctic region. — Gulf Stream Major warm and cold currents of the North Atlantic Ocean.

The Gulf Stream is a strong western boundary current primarily due to a phenomenon known as westward intensification.1

This process occurs because the horizontal Coriolis force increases with latitude. As wind-driven surface waters converge within a subtropical gyre, they form a "hill" of water that creates a horizontal pressure gradient. The interaction between this pressure gradient and the Coriolis force results in a geostrophic current.123

Because the Coriolis effect varies with latitude—a concept often referred to as the beta effect—the circulation of the gyre is pushed toward the western side of the ocean basin. This causes the poleward-flowing current on the western boundary to become compressed and intensified into a narrow, jetlike flow. Consequently, the Gulf Stream attains high velocities, reaching speeds of 2 to 4 metres per second, as it transports warm water from low latitudes toward the poles.21