Transport of aerosols into the UTLS and their impact on the Asian monsoon region as seen in a global model simulation

S Fadnavis,M G Schultz,S D Ghude,K Semeniuk,R Kakatkar,L Pozzoli,S Das

doi:10.5194/acp-13-8771-2013

Abstract

Abstract. An eight-member ensemble of ECHAM5-HAMMOZ simulations for a boreal summer season is analysed to study the transport of aerosols in the upper troposphere and lower stratosphere (UTLS) during the Asian summer monsoon (ASM). The simulations show persistent maxima in black carbon, organic carbon, sulfate, and mineral dust aerosols within the anticyclone in the UTLS throughout the ASM (period from July to September), when convective activity over the Indian subcontinent is highest, indicating that boundary layer aerosol pollution is the source of this UTLS aerosol layer. The simulations identify deep convection and the associated heat-driven circulation over the southern flanks of the Himalayas as the dominant transport pathway of aerosols and water vapour into the tropical tropopause layer (TTL). Comparison of model simulations with and without aerosols indicates that anthropogenic aerosols are central to the formation of this transport pathway. Aerosols act to increase cloud ice, water vapour, and temperature in the model UTLS. Evidence of ASM transport of aerosols into the stratosphere is also found, in agreement with aerosol extinction measurements from the Halogen Occultation Experiment (HALOE) and Stratospheric Aerosol and Gas Experiment (SAGE) II. As suggested by the observations, aerosols are transported into the Southern Hemisphere around the tropical tropopause by large-scale mixing processes. Aerosol-induced circulation changes also include a weakening of the main branch of the Hadley circulation and a reduction of monsoon precipitation over India.

Highlights

Port pathway of aerosols and water vapour into the tropi- portation, industry, agriculture, and urban land development cal tropopause layer (TTL)
The results presented here broadly agree with the transport picture presented by Fu et al (2006) in that the primary route for tracers into the tropical tropopause layer (TTL) in the Asian summer monsoon (ASM) region is via the convective zone on the southern flanks of the Himalayas
An eight-member ensemble of ECHAM5-HAMMOZ simulations of a boreal summer season is analysed to study the transport of aerosols during Asian summer monsoon in the upper troposphere and lower stratosphere (UTLS)

Summary

ECHAM5-HAMMOZ model simulation and experimental setup

The ECHAM5-HAMMOZ aerosol–chemistry–climate model used in the present study comprises the general circulation model ECHAM5 (Roeckner et al, 2003), the tropospheric chemistry module, MOZ (Horowitz et al, 2003), and the aerosol module, the Hamburg Aerosol Model (HAM) (Stier et al, 2005). The HAM module takes into account the major aerosol compounds, namely SO24−, BC, OC, sea salt, and mineral dust. It represents aerosols as internal and external mixtures with four soluble and three insoluble modes (Vignati et al, 2004). The chemical scheme used in the tropospheric chemistry module, MOZ, is identical to the MOZART-2 model with small modifications as described by Pozzoli et al (2008a). It includes 63 tracers and 168 reactions to represent Ox–NOx–hydrocarbon chemistry. The parameterizations for precipitation formation in warm clouds, cold clouds, and mixed-phase clouds are discussed in detail in Sect. 10.3.4 of the MPI-Report No 349 “The atmospheric general circulation model ECHAM5, Part I” (http://www.mpimet.mpg.de/ fileadmin/publikationen/Reports/max_scirep_349.pdf)

Satellite observations

Impact of aerosols on clouds

Impact of aerosols on precipitation

Findings

Conclusions