Eddy momentum flux

Eddy momentum flux is the net transport of angular momentum by turbulent eddies in a rotating fluid, a quantity that determines the strength and structure of the mean atmospheric circulation. In the mid-latitude atmosphere, Rossby waves propagate meridionally, carrying eastward momentum into the jet stream core and depositing it through wave breaking and dissipation. This eddy momentum convergence maintains the jet stream against friction and drives the indirect circulation of the Ferrel cell. Without eddy momentum flux, the atmosphere would not support the strong westerlies that define the mid-latitudes; the circulation would collapse into a weak, thermally direct flow. The flux is not a passive byproduct of weather. It is an active dynamical agent that shapes the large-scale climate.

The atmosphere's mean flow is not the driver of its weather. Its weather is the driver of its mean flow. Eddy momentum flux is the mathematical signature of this reversal.