In addition to regions of time-mean easterly winds, the tropical atmosphere contains substantial areas of men westerlies in both upper and lower troposphere. Their existence is thought to be related to the large-scale atmospheric response to regional convective heating and, ultimately, to the latent heating and sea surface temperature distribution in the tropics. It has been hypothesized that regions of upper tropospheric equatorial westerlies act as efficient ducts or corridors for transient and stationary extratropical modes deep into the tropics and even to the other hemisphere. Furthermore, it has been argued that the degree of efficiency depended upon the strength of the local equatorial westerlies, thus inducing a probable distinct seasonal and interannual variability in interactions between the hemispheres and between the extratropics and low latitudes. These hypotheses are tested in the present study using an 11-year National Meteorological Center (NMC) climate data set. Large positive pattern correlations are found between the time-mean zonal wind component (ū) and perturbation kinetic energy (PKE). The PKE increases substantially as ū increases from small negative to large and positive values. Values of PKE measured within the mean westerlies of the central and eastern Pacific ocean are at least a factor of 2 or 3 larger than the PKE found in the moderate and strong equatorial easterlies. If the strength of the easterlies is greater than about −5 m s−1, the PKE assumes a near constant value of well less than 40 m2 s−2,. Pattern correlations with the time-mean meridional velocity component (v̄) are substantially smaller and do not appear to be significant. To test the robustness of the statistical relationship between ū and PKE, individual mean months and seasons and the same months of different years are examined. In all cases, the distribution of PKE appears to be determined by the local ū. As arguments can be established that relate changes in the boundary forcing to changes in the mean equatorial zonal wind distributions, causal relationships can also be proposed for the variability in interannual and intraannual interhemispheric interaction.