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Analysis of 6 years of spectral aerosol optical depths (AOD), obtained during boreal winter at the tropical semi-arid location Anantapur, India, revealed significant modulations (to the seasonal mean AOD) by planetary scale atmospheric waves. Most significant contributions came from 30 to 50 day, and quasi-16 day periodicities; each contributing similar to 10% to 24%, and jointly up to 45% to the seasonal mean AOD; the contribution varying linearly with zonal wind speed. Analysis of the upper air wind fields revealed these to be associated with Madden Julian Oscillations and quasi-16 day planetary waves. The spectral dependence of AOD also responded to these waves, becoming flatter with the strengthening of the MJO and steeper with the quasi-16 oscillations. The possible implications of these to the long-range transport of different aerosol types to the measurement region are discussed. The findings signify the role of natural atmospheric motions in producing significant changes in spectral AOD, which will have strong implications to regional climate. Citation: Beegum, S. N., K. Krishna Moorthy, S. S. Babu, R. R. Reddy, and K. R. Gopal (2009), Large scale modulations of spectral aerosol optical depths by atmospheric planetary waves, Geophys. Res. Lett., 36, L03810, doi: 10.1029/2008GL036509.