Diagnostics from ECMWF analyses and a GCM integration indicate that the dominant factor in driving the annual cycle of temperatures near the tropical tropopause is breaking by synoptic-scale waves on the equatorial side of the subtropical jets. This wave breaking extends further into the tropics during northern hemisphere winter than northern hemisphere summer where it is displaced northward by the Asian monsoon. Hence there is greater mean ascent through the tropical tropopuse in northern hemisphere winter and colder temperatures. Longitude-latitude plots show that transient wavebreaking of synoptic-scale waves into the "westerly duct" regions of both the Atlantic and the Pacific are important in the seasonal variation of wave driving.