The structures of temperature and velocity fields in the tropical tropopause layer (TTL) in boreal winter are investigated using an atmospheric general circulation model (AGCM). The model reveals strong upward motions in the lower part of the TTL over the maritime continent and the western tropical Pacific, corresponding to the stratospheric fountain region, and downward motions in the upper part of the TTL over Indonesia, representing the stratospheric drain. In the TTL, strong easterlies prevail, and the cold ascent region tilts eastward. A down-slope flow over the upward-bulging isentropic surface produces the downward p velocity over Indonesia. In addition, reduction of longwave heating over deep convection suppresses the upward motion. The model simulates the observed stratospheric drain signature well, without convective overshootings. A trajectory analysis using the AGCM-simulated three-dimensional wind and temperature is performed to clarify the entry process of air parcels from the tropical troposphere to the stratosphere and to investigate the dehydration process during passage through the TTL. Tropospheric air parcels are advected upward to the bottom of the TTL mainly from the stratospheric fountain region. A pair of anticyclonic circulations in the tropical western Pacific entrains air parcels, which then pass through the equatorial cold region several times during the slow ascent in the TTL. This slow spirally ascending motion brings about low humidity in the stratosphere, despite the local downward motion over Indonesia. In addition, transient disturbances, particularly low-frequency disturbances, produce intermittent upward motions over the fountain region, resulting in effective dehydration of the air. The spiral ascent and transient mechanisms are key factors in the dehydration process in the TTL. The interannual variation in the water vapor mixing ratio into the tropical lower stratosphere with the El Ni¿o/Southern Oscillation cycle is also estimated, and it is found that in La Ni¿a years, air is more dehydrated.