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

Abstract

SUMMARYA 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.

Stratospheric dryness: model simulations and satellite observations

“…This air is relatively humid owing to the deep penetration of monsoon convection, and it plays a key role in moistening the stratosphere (Bannister et al, 2004;Gettelman et al, 2004a;. In Sect.…”

“…More recently, trajectory studies based on meteorological analyses have elucidated the transport pathways through the tropical tropopause region during which the air masses are dehydrated (Bannister et al, 2004;Bonazzola and Haynes, 2004;Fueglistaler et al, 2004. It was shown that the upper troposphere over the western Pacific Ocean is a major ascent region, and that the upper level monsoon circulation plays an important role in the transport deeper into the stratosphere.…”

Aura-MLS Observations of Water Vapor Entering the Stratosphere over the Northern Bay of Bengal and East Equatorial Indian Ocean

“…Newell and Gould-Stewart (1981) explained that air enters the stratosphere only where the tropopause temperature is coldest. Earlier observations also showed that a cold tropopause temperature is observed over Bay of Bengal and the Indian tropical region during the pre-monsoon (March -May) and monsoon (June -August) months suggesting that the Indian tropical region may be participating in the stratospheric dehydration (Newell and Gould-Stewart 1981;Bannister et al 2004;Jain et al 2006Jain et al , 2010. Highwood and Hoskins (1998) also observed that during June -August the ASM region plays a role in the stratospheric dehydration.…”

“…During the ASM period ~6 -7 ppmv of water vapor is observed in the VOT and around 5 -6 ppmv is observed in the lower stratosphere. The water vapor transport through observations (e.g., Newell and Gould-Stewart 1981;Park et al 2007), modeling (e.g., Bannister et al 2004;Lelieveld et al 2007) and the Lagrangian analyses (e.g., Fueglistaler et al 2005;Wright et al 2011) are well-documented. Gettelman et al (2002) observed through simulations that convection over the southern hemisphere moistens the TTL significantly throughout the depth and in deep convection the water vapor concentration may also increase in the stratosphere.…”

A Case Study of Mass Transport during the East-West Oscillation of the Asian Summer Monsoon Anticyclone

“…The Asian summer monsoon anticyclone (South Asia High) is one of the dominant features of the Asian summer monsoon [1,[18][19][20][21][22][23][24][25][26]. It plays an important role in STE because of persistent deep convections and the impact on mass transport, especially on chemical distribution in the Upper Troposphere and Lower Stratosphere (UTLS) region [9,19,20,[27][28][29]. The Asian summer monsoon anticyclone is bounded by the westerly jet to the north and easterly jet to the south which leads to independent feature of air inside of the anticyclone [23,24].…”