The influence of low-level inflow wind and its high-frequency variability on the spatial characteristics of the eastern margin of the South Pacific Convergence Zone (SPCZ) is examined. Compositing daily and 5-day mean low-level wind, precipitation, and tropospheric moisture data reveals a clear relationship between high-frequency zonal inflow variations and the eastern SPCZ margin, with relaxation of trade wind intensity associated with increased moisture near the mean eastern SPCZ margin and an eastward displacement of the convection. An idealized 2-dimensional model demonstrates that variations in dry air inflow cause tropospheric moisture and precipitation variations akin to those observed. In this prototype, factors affecting the extent of SPCZ variability are also important to the mean margin position. SPCZ margin shifts under natural variability thus offer an observational target against which to evaluate simulated interactions of inflow air mass characteristics with convection.