secondary circulation

{{short description|Circulation induced in a rotating system}}

{{One source|date=December 2019}}

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.{{Cite book|url=http://worldcat.org/oclc/1124306270|title=An introduction to dynamic meteorology.|last=Holton|first=James R.|date=2019 |publisher=Elsevier Science |isbn=9780128093290|oclc=1124306270}} 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.