Tropopause Characteristics and Variability from 11 yr of SHADOZ Observations in the Southern Tropics and Subtropics

Overview & Synopsis

In this paper, tropopause characteristics observed from tropical to subtropical Southern Hemisphere stations using Southern Hemisphere Additional Ozonesonde (SHADOZ) data are presented for the 11-yr period of 1998-2008. Three different definitions of tropopause--cold-point tropopause (CPT), lapse-rate tropopause (LRT), and ozone tropopause (OT)--are determined, and their variability for nine different SHADOZ sites is studied for the purpose of evaluating their usefulness as indicators of possible tropopause trends. For each station, the OT is uniquely defined by the ozone gradient and is found to be more variable than either LRT or CPT. The OT roughly coincides with the upper boundary of the region of most active convective mixing over the western Pacific Ocean and with the lower boundary of the transition region from the troposphere to the lower stratosphere that is generally referred to as the tropical tropopause layer. The monthly and year-to-year variations in the tropopause are examined, and the annual cycle in OT, the dominant signal, is described. The distance of separation of the OT from the CPT or LRT is smaller for the tropics (stations at 0°-15°S) than for the subtropics (15°-25°S). The decadal trend in tropopause heights is measured using a statistical model that accounts for natural variations expressed in El Niño-Southern Oscillation, the quasi-biennial oscillation, and the Indian Ocean dipole. The decadal trend estimation shows no statistically significant trend for the CPT and LRT in the tropics, in contrast to other studies. A decrease in altitude for the OT is significant. In the subtropics, the CPT and LRT decline significantly, by −240 and −190 m (10 yr)−1, respectively, but the OT increases.