The physics of random shearing by zonal flows and the consequent reduction of scalar field transport are studied. In contrast to mean shear flows, zonal flows have a finite autocorrelation time and can exhibit complex spatial structure. A random zonal flow with a finite correlation time τZFdecorrelates two nearby fluid elements less efficiently than a mean shear flow does. The decorrelation time is τD=(τη/τZFΩ2rms)1/2(τnis the turbulent scattering time, and Ωrmsis the rms shear), leading to larger scalar field amplitude with a slightly different scaling (∝τD/Ωrms), as compared to the case of coherent shearing. In the strong shear limit, the flux scales as ∝Ωrms-1. © 2004 American Institute of Physics.