Advection induced short period anomalies and seasonal features in aerosol optical depth over Bay of Bengal in the W-ICARB region B. Spandana, K. Niranjan * , T. Anjana Devi Department of Physics, Andhra University, Visakhapatnam 530 003, India article info Article history: Received 1 November 2011 Received in revised form 8 January 2012 Accepted 11 January 2012 Keywords: Aerosols Bay of Bengal Advection abstract Advection of aerosols from adjoining continental regions into the Bay of Bengal (BoB) is considered important in view of its potential aerosol radiative impacts on Indian summer monsoon and the mesoscale weather. Though several reports are available on the features of Aerosol Optical Depth (AOD) over BoB region based on cruise as well as satellite data, day to day variability over the oceanic regions due to day to day variability at the aerosol source regions and advection pathways is yet incomplete. In this article results on a systematic investigation carried out on the AOD vis-a-vis the advection pathways indicate that the day to day variability in AOD could possibly be reflected as spatial variation when short period cruise measurements are made. A statistical analysis of the short period variations using W-ICARB cruise data and the MODIS satellite data for the whole winter 2008e09 show that in 65% of the case, the airmass pathways have the potential of inducing large day to day variability in AOD with 79% of the cases over eastern BoB affected by east Asian airmass, 98% of the cases over western BoB affected by airmass pathways from the Indian region, while the southern BoB is mostly affected by the east Asian aerosol transport, which is significant in altering the mean AOD features over the regions. Long term assessment using MODIS data for the period 2005e2010 indicate that there is a steady increase in AOD over the western BoB during 2005e2010, while it is more or less stable over southern BoB. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The diversity in the source regions of aerosols inducing large spatio-temporal variability along with the relatively short residence times makes our understanding of the aerosol radiative effects on Earth’s climate a challenging task (IPCC, 2007). The study of aerosols over the Oceans is of paramount importance as the oceans cover most of the Earth’s surface and act as major contrib- utor of natural aerosols. Secondly aerosol optical properties over the oceans determine the sea surface state that drives the atmo- spheric general circulation. Even though the oceanic environment are less heterogeneous compared to land due to less diverse sour- ces, several studies reveal that the aerosol properties over oceanic regions show significant variations associated with aerosol trans- port from adjoining continents (Moorthy and Satheesh, 2000; Smirnov et al., 2002). During the past two decades, importance of campaign mode shipborne measurements as well as satellite observations coupled with airmass back trajectory analysis is realised to identify possible source regions that contribute to aerosol loading over coastal and remote oceanic region. Analysis of AVHRR and TOMS Aerosol Optical Depth (AOD) data over the Indian Ocean and the adjoining regions during 1981e2004 suggested that the mean value of AOD in these regions decreased from 1986 to 1990 to 1995e1999 (George et al., 2008). The Arabian Sea and Bay of Bengal (BoB) show increase thereafter whereas in the equatorial Indian Ocean, it decreased further, during 2000e2004. Rasch et al. (2001) reported that three points of entry are found for the anthropogenic aerosol to the INDOEX region; a strong near-surface southward flow near Mumbai, a deeper plume flowing south and east off Kolkata coast and a westward flow originating from south East Asia and entering BoB. Properties of aerosols in the surrounding landmass play a crucial role in determining the optical state of the atmosphere over the oceans (Moorthy et al., 2009; Kaskaoutis et al., 2010). Several studies have reported long range transport of dust from Sahara to faraway places such as the Mediterranean Sea (Kaskaoutis et al., 2008), the Arabian Sea (Badarinath et al., 2010) and even the North Atlantic Ocean (Dayon et al., 1991). * Corresponding author. Tel.: þ91 891 2713935; fax: þ91 891 2525611. E-mail address: niranjankandula@hotmail.com (K. Niranjan). Contents lists available at SciVerse ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2012.01.031 Atmospheric Environment 51 (2012) 175e186