In fluid dynamics, a secondary circulation or secondary flow is a weak circulation that plays a key maintenance role in sustaining a stronger primary circulation that contains most of the kinetic energy and momentum of a flow.
[1] For example, a tropical cyclone's primary winds are tangential (horizontally swirling), but its evolution and maintenance against friction involves an in-up-out secondary circulation flow that is also important to its clouds and rain.
On a planetary scale, Earth's winds are mostly east–west or zonal, but that flow is maintained against friction by the Coriolis force acting on a small north–south or meridional secondary circulation.
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