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Multiscale periodicities in aerosol optical depth over India

Aerosols exhibit periodic or cyclic variations depending on natural and anthropogenic sources over a region, which can get modulated by synoptic meteorological parameters such as winds, rainfall and relative humidity, and long-range transport. Information on periodicity and phase in aerosol properties assumes significance in prediction as well as to examine the radiative and climate effects of aerosols including its association with changes in cloud properties and rainfall. Periodicity in aerosol optical depth, which is a columnar measure of aerosol distribution, is determined using continuous wavelet transform over 35 locations in India. Continuous wavelet transform is used in the study because it is better suited to extract the periodic and local modulations present at various frequency ranges, as these features are invisible in conventional methods such as Fourier Transform. Monthly mean aerosol optical depths (AODs) from MODerate Resolution Imaging Spectroradiometer (MODIS) on board the Terra satellite from January 2001 to December 2012 are used. Annual and quasi-biennial oscillations (QBO) in AOD are evident in addition to the weak semi-annual and quasi-triennial. The semi-annual and annual oscillations are consistent with the seasonal and yearly cycle of variations in AODs. The 40-month periodicity indicates the presence of long term correlations in AOD. The observed periodicities in MODIS Terra AODs are also evident in the ground-based AOD measurements made over Kanpur in the Indo-Gangetic Plain. The phase of the periodicity in AOD is stable in the mid-frequency range, while local disturbances in the high-frequency range and long term changes in the atmospheric composition give rise to unstable phases in low-frequency range. That modulations in AOD over one location/region can influence the other is revealed by the presence of phase relation among different locations.