Recent widening of the tropical belt: Evidence from tropopause observations

Radiosonde measurements and reanalysis data are used to examine long-term changes in tropopause behavior in the subtropics. Tropopause heights in the subtropics exhibit a bimodal distribution, with maxima in occurrence frequency above 15 km (characteristic of the tropical tropopause) and below 13 km (typical of the extratropical tropopause). Both the radiosonde and reanalysis data show that the frequency of occurrence of high tropopause days in the subtropics of both hemispheres has systematically increased during the past few decades, so that tropical characteristics occur more frequently in recent years. This behavior is consistent with a widening of the tropical belt, and the data indicate an expansion of about 5 - 8° latitude during 1979 - 2005.