Changes in upper tropospheric humidity (UTH) are important in understanding the climatic response to increased anthropogenic forcing. This paper explores the asymmetry and non-linearity in the response of subtropical UTH to tropical intraseasonal forcing. During northern winter, subtropical UTH anomalies, varying on intraseasonal timescales, propagate eastwards from northern Africa to the central Pacific. There is a discontinuity in the propagation of moist anomalies over the Tibetan Plateau, whilst dry anomalies show reduced propagation in the lee. This is consistent with the expected evolution of cyclones and anticyclones incident on high topography. Correlations of five-day mean UTH show no significant linear correlation between moist anomalies over Indonesia and subtropical anomalies to the north. This appears to be explained by the limitation of dry anomalies in the region of low ambient vorticity south of the East Asian Jet Stream.