Monsoon variability and its physical mechanisms under different paleo- and future climate conditions
Final Report Abstract
A comprehensive model-proxy-comparison study is conducted to identify robust signals of Holocene monsoon variability over India at a high spatiotemporal resolution. The potential large-scale drivers of monsoon variability are studied by using a global climate model (COSMOS), as well as a high-resolution regional climate model (COSMO-CLM). Prior to investigating interesting time slices during the Holocene (severe monsoon phases), the regional climate model is adapted to a domain covering the Indian subcontinent. The model output is evaluated against different observation-based and reanalysis datasets. The results indicate that COSMO-CLM is able to simulate the main rainfall patterns during the summer monsoon season (JJAS). High rainfall rates over the Western Ghats, Central India and at the Himalayan foothills are well captured. Our results suggest that the model is able to simulate the observed daily variability, but has problems in simulating the observed location of rainfall events. We conclude that the model has positive skill in simulating observed dry and wet events with a similar magnitude and can be used to simulate paleo-climate conditions as well as extreme events during the Holocene. The annual moisture signals simulated by the atmosphere only global climate simulations during Little Ice Age and the Medieval Climate Anomaly agree over Himalaya and in Central India. The numerous proxies in the Himalaya region exhibit a high consistency with the model data in describing past moisture changes. In Central India, where less proxy data are available to describe the moisture distribution in the heterogeneous topography, the agreement is only prominent in summer (Lonar, Narmanda and Danak lake records are summer signals). Based on the model simulations, we differentiated three physical mechanisms which are leading to the moisture anomalities: (i) the western and central Himalaya is influenced by extra-tropical Westerlies during winter, (ii) the eastern Himalaya is affected by summer variations of temperature gradient between Bay of Bengal and Indian subcontinent and by a zonal band of intensified Indian – East Asian monsoon link north of 25°N and (iii) Central India is dominated by summer sea surface temperature anomalities in northern Arabian Sea which have an effect on the large-scale advection of moist air masses. The temperatures of the Arabian Sea are linked with the Indo Pacific Warm Pool, which has been found modulating the Indian monsoon strength. In order to analyse long-term monsoon variability over India and its physical drivers, a fully-coupled GCM simulation covering the period from 6ka BP until 1950 AD provided by the MPI-M, Hamburg, Germany, was used to analyse Indian monsoon rainfall. The results showed that monsoon rainfall decreased from the mid-Holocene until present. This can be explained by changes in the solar insolation due to varying orbital parameters. Finally, to investigate possible changes in Indian monsoon climate due to the freshwater inflow of Lake Agassiz around 8.2ka BP a fully-coupled AOGCM simulation has been carried out. This simulation starts 9ka BP and includes changes of orbital parameters, greenhouse gases, land-ice cover and orography. The first results show comparable temperature responses in comparison to other studies.
Publications
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2013. Simulation of the Indian Monsoon and its Variability during the last Millennium. Climate of the Past Discussion 9, 703-740
Polanski, S., B. Fallah, S. Prasad, and U. Cubasch
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Regional moisture change over India during the past Millennium: A comparison of multi-proxy reconstructions and climate model simulation. Global and Planetary Change, Volume 122, November 2014, Pages 176-185
Polanski, S., Fallah, B., Befort, D.J., Prasad, S., Cubasch, U.
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Intraseasonal variability of the Indian Summer Monsoon: Wet and Dry Events in COSMO-CLM. Climate Dynamics, October 2016, Volume 47, Issue 7–8, pp 2635–2651
Befort, D. J, G.C. Leckebusch and U. Cubasch
