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Ann. Geophys., 24, 2075-2089, 2006
www.ann-geophys.net/24/2075/2006/
© European Geosciences Union 2006
Theoretical aspects of the onset of Indian summer monsoon from perturbed orography simulations in a GCM
A. Chakraborty1, R. S. Nanjundiah2, and J. Srinivasan2
1Department of Meteorology, Florida State University, Tallahassee, Florida, USA
2Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore 560012, India
Abstract. A theory is proposed to determine the onset of the Indian Summer
Monsoon (ISM) in an Atmospheric General Circulation Model (AGCM).
The onset of ISM is delayed substantially in the absence of global orography.
The impact of orography over different parts of the Earth on the onset of
ISM has also been investigated using five additional perturbed simulations.
The large difference in the date of onset of
ISM in these simulations has been explained by a new theory based on the
Surface Moist Static Energy (SMSE)
and vertical velocity at the mid-troposphere.
It is found that onset occurs only after SMSE crosses a threshold value and the
large-scale vertical motion in the middle troposphere becomes upward. This study
shows that both dynamics and thermodynamics play profound roles in the onset of the monsoon.
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