Seasonal and QBO variations of ascent rate in the tropical lower stratosphere as inferred from UARS HALOE trace gas data

Abstract

Seasonal and interannual variations in ascent rates are investigated as a function of latitude and height, using water vapor (H 2 O) and methane (CH 4 ) data from the stratospheric measurements of the Halogen Occultation Experiment (HALOE). The ascent rate is inferred from the ascending signal of variations in the entry value of [H 2 O] + 2[CH 4 ] (Ĥ). Within ±15° of the equator the derived ascent rate exhibits two kinds of dominant variations with a clear latitudinal structure, seasonal variation, and the quasi‐biennial oscillation (QBO). The seasonal cycle exhibits a vertically in‐phase variation, with a northern winter maximum of 0.2–0.4 mm s −1 and a summer minimum of ∼0.2 mm s −1 in the 20–60 hPa layer. The latitudinal structure is characterized by an early appearance of a subtropical summer maximum of the ascent rate and by double peaks at 10–15°N and S during the northern winter season. The QBO component of the ascent rate shows tropically confined anomalies with a rapid downward propagation, but mass attenuation anomalies estimated from the ascent rate show a much slower downward propagation. The descent anomalies exhibit a well‐structured and equatorially symmetric variation, while the ascent anomalies have a tendency to propagate latitudinally. This might be connected with the phase dependency of the QBO acceleration. An examination of the phase and amplitude of the ascent rate and temperature for both the seasonal and QBO components emphasizes that the radiative damping timescale is considerably long (40–100 days) below 40 hPa.