Abstract
Abstract. The "elevated heat pump" (EHP) hypothesis has been a topic of intensive research and controversy. It postulates that aerosol-induced anomalous mid- and upper-tropospheric warming in the Himalayan foothills and above the Tibetan Plateau leads to an early onset and intensification of Asian monsoon rainfall. This finding is primarily based on results from a NASA finite-volume general circulation model run with and without radiative forcing from different types of aerosols. In particular, black carbon emissions from sources in northern India and dust from Western China, Afghanistan, Pakistan, the Thar Desert, and the Arabian Peninsula drive the modeled anomalous heating. Since the initial discussion of the EHP hypothesis in 2006, the aerosol–monsoon relationship has been investigated using various modeling and observational techniques. The current study takes a novel observational approach to detect signatures of the "elevated heat pump" effect on convection, precipitation, and temperature for contrasting aerosol content years during the period of 2000–2012. The analysis benefits from unique high-resolution convection information inferred from Meteosat-5 observations as available through 2005. Additional data sources include temperature data from the NCEP/NCAR Reanalysis and the European Reanalysis (ERA-Interim) precipitation data from the Global Precipitation Climatology Project (GPCP), aerosol optical depth from the Multi-angle Imaging Spectroradiometer (MISR) and the Moderate Resolution Imaging Spectroradiometer (MODIS), and aerosol optical properties from the Modern-Era Retrospective Analysis for Research and Applications (MERRA) aerosol reanalysis. Anomalous upper-tropospheric warming and the early onset and intensification of the Indian monsoon were not consistently observed during the years with high loads of absorbing aerosols. Possibly, model assumptions and/or unaccounted semi-direct aerosol effects caused the disagreement between observed and hypothesized behavior.
Highlights
The role of the Tibetan Plateau as an elevated heat source has long been recognized as one of the driving mechanisms of the Asian monsoon (Flohn, 1968; Yeh, 1981; Murakami, 1987; Ueda and Yasunari, 1998). Li and Yanai (1996) observed that the reversal of the meridional temperature gradient due to intense heating over the Tibetan Plateau in springtime coincides with the onset of the monsoon
As part of individual studies and numerous national programs (e.g., Indian Middle Atmospheric Program (IMAP) and the Indian Space Research Organisation – Geosphere Biosphere Programme (ISRO–GBP)), a wealth of information on important aerosol properties such as size, mass concentration, optical depth, and scattering and absorption coefficients have been collected with special attention to black carbon (BC)
These studies showed the persistence of high-aerosol optical depth and BC concentrations near the surface (Pant et al, 2006; Niranjan et al, 2006, 2007; Moorthy et al, 2005; Ganguly et al, 2005; Nair et al, 2006)
Summary
The role of the Tibetan Plateau as an elevated heat source has long been recognized as one of the driving mechanisms of the Asian monsoon (Flohn, 1968; Yeh, 1981; Murakami, 1987; Ueda and Yasunari, 1998). Li and Yanai (1996) observed that the reversal of the meridional temperature gradient due to intense heating over the Tibetan Plateau in springtime coincides with the onset of the monsoon. The basic premise of the hypothesis is that in the pre-monsoon season (March through May), absorbing aerosols such as black carbon from northern India and dust from the deserts of western China, Afghanistan, Pakistan, the Thar Desert, and the Arabian Peninsula stack up against the foothills of the Himalayas in the Indo–Gangetic Basin (IGB) (Fig. 1) and cause anomalous upper-tropospheric warming in the Tibetan Plateau region. It is argued that the warming by aerosol absorption causes the air to rise and act as an “elevated heat pump”, drawing in moist air from the Indian Ocean and causing an early onset of the monsoon and intensification of monsoon rainfall. An anomalous sinking motion forms in the southern part of the Indian subcontinent, causing this region to experience dryer-than-normal conditions in the early part of the peak-monsoon season. The potential impact of the absorbing aerosols is magnified in this situation since the mass of the atmosphere above the plateau is roughly
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