Atmospheric Transport Processes in the Asian Monsoon System

The Asian monsoon system is characterised by robust seasonal circulations that transport heat, moisture and chemical species across vast regions. During the boreal summer, deep convection over the Indian subcontinent and Southeast Asia injects boundary-layer air into the upper troposphere and lower stratosphere (UTLS). Within the monsoon anticyclone, air masses can be trapped for days to weeks, promoting chemical transformations and the formation of the Asian Tropopause Aerosol Layer (ATAL). Convective upwelling drives rapid vertical transport, while anticyclonic circulation governs horizontal dispersion. The monsoon outflow channels pollutants, tracers and water vapour into the UTLS, influencing radiative forcing, cloud microphysics and stratosphere–troposphere exchange. Transport pathways include direct lofting through deep convection, isentropic advection and slower diabatic ascent. Seasonal and interannual variability in monsoon intensity modulates the strength and extent of these processes, with implications for regional air quality, global climate feedbacks and the distribution of greenhouse gases and aerosols.

Research from Nature Portfolio

Ozonesonde measurements in the UTLS over the Himalayas have been evaluated against multiple reanalyses and a chemistry–climate model. Observations reveal that both models and reanalyses tend to overestimate ozone mixing ratios in the lower troposphere and UTLS. Sensitivity simulations indicate that a 50% reduction in NOx emissions yields improved agreement with measured profiles, highlighting the influence of precursor emissions on ozone distribution. The study underscores the need for enhanced observational coverage of both ozone and its precursors to refine model representations of UTLS chemistry in monsoon regions.

Atmospheric Transport Processes in the Asian Monsoon System publication trend

The graph below shows the total number of articles in atmospheric transport processes in the asian monsoon system across all publications each year (not limited to Nature Index journals).

Technical terms

Anticyclone: A region of high atmospheric pressure characterised by a clockwise circulation that confines air masses.

Tropopause: The boundary between the troposphere and stratosphere, acting as a barrier to vertical transport.

Convective uplift: Vertical transport of air resulting from buoyancy and deep thunderstorm activity.

Upper Troposphere and Lower Stratosphere (UTLS): The transition zone where tropospheric air can enter the stratosphere.

Asian Tropopause Aerosol Layer (ATAL): A seasonally enhanced aerosol concentration layer near the tropopause over South Asia.

Cloud Condensation Nuclei (CCN): Particles that serve as the initial surface for cloud droplet formation.

References

1. Comparison of ozonesonde measurements in the upper troposphere and lower Stratosphere in Northern India with reanalysis and chemistry-climate-model data. Scientific Reports (2023).
2. The impact of ammonia on particle formation in the Asian Tropopause Aerosol Layer. npj Climate and Atmospheric Science (2024).
3. New evidence for CH4 enhancement in the upper troposphere associated with the Asian summer monsoon. Environmental Research Letters (2024).
4. NOx‐Induced Changes in Upper Tropospheric O3 During the Asian Summer Monsoon in Present‐Day and Future Climate. Geophysical Research Letters (2023).
5. Transport pathways from the Asian monsoon anticyclone to the stratosphere. Atmospheric Chemistry and Physics (2016).
6. Spatiotemporal and Vertical Distribution of Asian Tropopause Aerosol Layer Using Long-Term Multi-Source Data. Remote Sensing (2023).