In this study, scale interaction between synoptic eddies and low-frequency flow is investigated. The synoptic eddy feedback is a key process in sustaining the low-frequency flow. We show clear evidence using NCEP reanalysis data that there is a general underlying rule—the "left-hand rule", governing the synoptic eddy feedback onto low-frequency flow. This rule states that eddy-vorticity fluxes are directed preferentially about 90 degrees toward their left-hand side, so that they converge into cyclonic flow and diverge from anticyclonic flow. Therefore, the eddy vorticity flux plays a positive role in reinforcing low-frequency flow.
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