Western Intensification of Wind-Driven Gyres

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The reason every subtropical gyre on Earth has a fast, narrow current on its western side and nothing comparable on its eastern side. Henry Stommel showed in 1948 that if the Coriolis parameter is held constant the circulation is symmetric, and that letting it vary with latitude — the beta effect — crowds the return flow against the western boundary. The consequence is the Gulf Stream, the Kuroshio, the Agulhas and the Brazil Current: flows tens of kilometres wide moving at up to two metres per second, returning in a thin ribbon all the water the broad interior carries the other way.