Evidence Of The Role Of Aerosols In Altering Regional Weather Patterns

A new Geophysical Research Letters paper has appeared which provides additional documentation on the role of aerosols in altering regional diabatic patterns, which results in significant changes in regional weather. Professor Dev Niyogi has discussed this subject on Climate Science in the posting “The Interlinked Role Of Changes In Radiative Forcings And Hydrology In The Climate System“, and alerted me to the new paper that is introduced below.

The paper by K.-M. Lau and K.-M Kim is titled
“Observational relationships between aerosol and Asian monsoon rainfall, and circulation”. (subscription required).

The abstract reads,

“Preliminary observational evidences are presented showing that the Indian subcontinent and surrounding regions are subject to heavy loading of absorbing aerosols, i.e., dust and black carbon, which possess spatial and temporal variability that are closely linked to those of the Asian monsoon water cycle. Consistent with the Elevated Heat Pump hypothesis, we find that increased loading of absorbing aerosols over the Indo-Gangetic Plain in the pre-monsoon season is associated with a) increased heating of the upper troposphere, with the formation of a warm-core upper level anticyclone over the Tibetan Plateau in April-May, b) an advance of the monsoon rainy season in northern India in May, and c) subsequent increased rainfall over the Indian subcontinent, and decreased rainfall over East Asia in June-July.”

The conclusion reads,

“Our observation results are consistent with the key features of the EHP {Elevated Heat Pump”] effect proposed by Lau et al. [2006] in showing that the anomalous high concentration of absorbing aerosol during the pre-monsoon season is associated with a) anomalous warming associated with the development of a large-scale anticyclone in the upper troposphere over the Tibetan Plateau in May and June, b) an advance of the monsoon season, with increased rainfall coming to northern India during May, and c) subsequent enhancement of the monsoon rain over India in June–July. Although the present analysis is focused on seasonal-to-interannual time scale, based on ongoing work, the relationships shown may also hold on decadal to climate change time scales reflecting the increased loading of the black carbon from anthropogenic sources in the IGB [Indo-Gangetic Basin]. Our results will provide guidance and new avenues for exploring monsoon variability and predictability. Aerosol effects on the monsoon water cycle may be important in years when influence from other controlling factors (sea surface temperature, land surface processes, and internal dynamics) are relative small, or incoherent so that they cancel out. Additionally, aerosol may have amplifying or damping effects when interacting with these factors. Therefore aerosol-monsoon water relationships have to be explored in conjunction with SST, snow cover and other agents of change in future studies.”

We discussed the importance of this climate forcing (including its relevance to the radiative forcing of the human input of CO2) in our Geophysical Research Letters paper “Measurement-based estimation of the spatial gradient of aerosol radiative forcing“. While not cited in the Lau and Kim paper, their work provides observational documentation of the importance of spatial gradients of aerosol radiative forcing.

An important issue with the new IPCC Report is whether the role of human climate forcing in regional diabatic heating of the troposphere is adequately discussed and communicated to policymakers. It was not an important issue in previous IPCC Reports.

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