Interannual variability of the tropical tropopause derived from radiosondedata and NCEP reanalyses

Interannual variability of the tropical tropopause is studied using long ti me series of radiosonde data, together with global tropopause analyses from the National Centers for Environmental Prediction (NCEP) reanalyses over 1 957-1997. Comparisons for the period 1979-1997 show the NCEP tropopause tem perature is too warm by similar to 3-5 Ii and too high in pressure by simil ar to 2-6 mbar. However, these biases are approximately constant in time, s o that seasonal and interannual variability is reasonably well captured by the NCEP data. Systematic differences in NCEP tropopause statistics are obs erved between the presatellite (1957-1978) and postsatellite (1979-1997) pe riods, precluding the use of the reanalyses for the study of multidecadal v ariability. Interannual anomalies in tropical average radiosonde and NCEP d ata show variations of order +/-1-2 Ii over the period 1979-1997, but there can be differences between these two estimates which are of similar magnit ude. These differences impact estimates of decadal trends: During 1979-1997 , negative trends in tropopause temperature of order -0.5 K/decade are obse rved in radiosonde data but are not found in NCEP reanalyses. The space-tim e patterns of several coherent signals are identified in both sets of tropo pause statistics. The volcanic eruption of El Chichon (1982) warmed the tro pical tropopause by similar to 1-2 ii and lowered its altitude by similar t o 200 m for approximately 1-2 years. Smaller tropopause variations are obse rved following Mount Pinatubo (1991), particularly in radiosonde data. The signatures of the quasi biennial oscillation (QBO) and El-Nino/Southern Osc illation (ENSO) events are strong in tropopause statistics. QBO variations are primarily zonal mean in character, while ENSO events exhibit dipole pat terns over Indonesia and the central Pacific Ocean, with small signals for zonal averages.