Short period variations of the aerosol mass concentrations over Bay of Bengal: association with quasi-periodic variations in the marine atmospheric boundary layer parameters and fluxes

Abstract

Analysis of the time-series of daily mean total aerosol mass concentrations (M_T), measured within the Marine Atmospheric Boundary Layer (MABL) over the Bay of Bengal (BoB) revealed the presence of short-period modulations with periods of 4–6 days, 8–10 days as well as quasi 16-day (14–20 days). These were found to be distinctively associated with similar oscillations in the concurrently measured MABL parameters such as air temperature (AT), pressure (P), relative humidity (RH), wind speed (WS), sea surface temperature (SST) and derived parameters such as Momentum Flux (MF), Latent Heat Flux (LHF) and Sensible Heat Flux (SHF). Examination of the phase relations revealed that the 4–6 days and quasi 16-day periodicities in AT, P, RH and SST maintained a nearly in-phase (very small phase difference <±20°) variation between themselves and also with similar periodicities in the aerosol mass concentration MT. On the other hand, the periodicities in WS, SHF, LHF and MF, though were nearly in-phase among themselves, exhibited an out-of-phase (phase difference 180±20°) relation with that of M_T. Interestingly, the 8–10 day periodicities revealed a different phase relationship; the variables AT, P, SST, WS and M_T> were in-phase and these variables were out-of-phase with similar periodicities in RH and the fluxes. It was also observed that all the three waves represented westward propagating Rossby waves. An easterly phase in the wind was found to result in advection of particles from the East Asia to BoB, as evident from the out-of-phase relationship between the periodicities of 4–6 days and quasi 16-day in M_T and zonal wind. The meridional component, that was stronger than its zonal amplitude in the 8–10 day periodicity, resulted in enhanced advection of particles from the southern part of Bay of Bengal in comparison with that from the Eastern region, leading to an in-phase relationship between M_T and zonal wind.