The role of the south‐east Asian monsoon and other seasonal features in creating the ‘tape‐recorder’ signal in the Unified Model

Abstract

AbstractA multi‐year simulation with an atmospheric general‐circulation model (AGCM), the Unified Model, is shown to simulate the main features of seasonal variations in the concentrations of water vapour in the stratosphere—the so‐called tape‐recorder signal. An off‐line transport model, utilizing winds from the AGCM, is used to synthesize the signal from local contributions. During June–July–August, the most significant localized contribution to the moist phase of the signal comes from an air stream emanating from the South‐East Asian monsoon. The moist air does not enter the stratosphere immediately above the monsoon in a localized ‘fountain’. Rather, the air stream moves southward, via the monsoon's upper level anticyclone, into the tropical stratosphere while moving steadily upwards across isentropic surfaces in a field of radiative heating in the tropical tropopause layer (TTL). As a result of this steady ascent during equatorward movement, not all the airstream is freeze dried in the cold cap of low temperatures which exists in the TTL above the monsoon. The water vapour mixing ratios of air entering the stratospheric tape‐recorder are therefore not entirely set by the minimum temperatures near the equator, but in part by physical conditions outside the inner tropical region used to define the tape‐recorder signal. During December–January–February, the flow near the tropopause is simpler. Dry air enters the stratosphere by slow upglide through the localized temperature minimum near the tropical tropopause over the western Pacific. The mixing ratios during the dry phase are set largely by freeze drying in this region. The simple tape‐recorder model, which envisages that mixing ratios are set by the minimum temperature near the tropical tropopause is therefore an oversimplification. Copyright © 2004 Royal Meteorological Society