Temperature fields in the tropical tropopause transition layer

There has been increasing recognition of the role of the tropical tropopause layer (TTL) in determining stratospheric water vapor concentrations-the TTL being a layer of transition between air showing tropospheric properties below and stratospheric properties above. This study investigates the spatial structure of temperatures in the TTL. A dehydration index based on the atmospheric region with temperatures colder than a specific reference temperature was defined to examine the TTL temperature structure and possible influences on stratospheric water vapor. The results indicate that dehydration regions with cold temperatures (e.g., <190 K) occur mainly over the western Pacific and are about 1.5-2.0 km in depth during Northern Hemisphere winter. The dehydration index is mainly dependent on the annual cycle of the TTL temperatures, but is strongly affected by interannual variations associated with the quasi-biennial oscillation (QBO) and El Niñio-Southern Oscillation (ENSO). Dehydration regions with extremely cold temperatures and large sizes occur when cold temperature anomalies associated with the QBO arrive at the TTL in wintertime while the TTL is at the coldest phase of the annual cycle and under La Niña conditions. La Niña events have a more dramatic influence on the dehydration index than El Niño events.