Tropical Cold Point Tropopause Characteristics Derived from ECMWF Reanalyses and Soundings

Abstract

Tropical cold point tropopause (CPT) characteristics have been calculated using ECMWF reanalyses and high resolution radiosonde soundings obtained in TOGA COARE as well as operational sounding data. It is found that ECMWF reanalyses are suitable for investigating the morphology of the tropical CPT and the variabilities of the tropopause over the entire Tropics, despite an almost constant one-side warm bias. The daily locations of the tropical coldest CPT are clustered over the western Pacific warm pool region in January and spread out longitudinally as the year progresses. During the Indian summer monsoon, the locations of the coldest CPT are deflected northward. The influences of the quasi-biennial oscillation (QBO) and the El Niño?Southern Oscillation (ENSO) on the tropical CPT have been separated using bivariate regression. The stratospheric zonal wind shear at 50 mb leads the variation in the tropical CPT temperatures by about 6 months. The QBO signature in the tropical CPT is mainly zonally symmetric and is consistent with the downward propagating tropical stratospheric QBO meridional circulation. CPT temperatures and the sea surface temperature anomalies (SSTAs) in the Niño-3.4 region are simultaneously correlated. The fingerprints of the ENSO in the CPT show east?west dipole and north?south dumbbell features, which can be explained by changes of convection during ENSO events. The interference between the QBO and ENSO effects on the tropical CPT, and the effect of the quasi-biennial oscillations in SSTA are discussed. The low-frequency variabilities of the CPT related to the QBO and ENSO can be a cause of the observed interannual variability of stratospheric water vapor because they affect the entry values of the water vapor mixing ratio across the tropical CPT to the stratosphere.