Left‐hand rule for synoptic eddy feedback on low‐frequency flow

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.