Seven year satellite observations of the mean structures and variabilities in the regional aerosol distribution over the oceanic areas around the Indian subcontinent

S K Nair,K Parameswaran,K Rajeev

doi:10.5194/angeo-23-2011-2005

S K Nair, K Parameswaran + Show 1 more

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https://doi.org/10.5194/angeo-23-2011-2005

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Journal: Annales Geophysicae	Publication Date: Sep 15, 2005
Citations: 31	License type: CC BY 3.0

Affiliation: Vikram Sarabhai Space Centre

Abstract

Abstract. Aerosol distribution over the oceanic regions around the Indian subcontinent and its seasonal and interannual variabilities are studied using the aerosol optical depth (AOD) derived from NOAA-14 and NOAA-16 AVHRR data for the period of November 1995–December 2003. The air-mass types over this region during the Asian summer monsoon season (June–September) are significantly different from those during the Asian dry season (November–April). Hence, the aerosol loading and its properties over these oceanic regions are also distinctly different in these two periods. During the Asian dry season, the Arabian Sea and Bay of Bengal are dominated by the transport of aerosols from Northern Hemispheric landmasses, mainly the Indian subcontinent, Southeast Asia and Arabia. This aerosol transport is rather weak in the early part of the dry season (November–January) compared to that in the later period (February–April). Large-scale transport of mineral dust from Arabia and the production of sea-salt aerosols, due to high surface wind speeds, contribute to the high aerosol loading over the Arabian Sea region during the summer monsoon season. As a result, the monthly mean AOD over the Arabian Sea shows a clear annual cycle with the highest values occurring in July. The AOD over the Bay of Bengal and the Southern Hemisphere Indian Ocean also displays an annual cycle with maxima during March and October, respectively. The amplitude of the annual variation is the largest in coastal Arabia and the least in the Southern Hemisphere Indian Ocean. The interannual variability in AOD is the largest over the Southeast Arabian Sea (seasonal mean AOD varies from 0.19 to 0.42) and the northern Bay of Bengal (seasonal mean AOD varies from 0.24 to 0.39) during the February–April period and is the least over the Southern Hemisphere Indian Ocean. This study also investigates the altitude regions and pathways of dominant aerosol transport by combining the AOD distribution with the atmospheric circulation. Keywords. Atmospheric composition and structure (Aerosols and particles) – Meteorology and atmospheric dynamics (Climatology) – Oceanography: physical (Ocean fog and aerosols)

Highlights

The effects of aerosols on the radiation budget of the Earthatmosphere system, cloud characteristics and the Earth’s climate are well recognised (Charlson et al, 1992; Hansen et al, 1997; Ramanathan et al, 2001a,b)
During the Asian dry season, the number of days in which aerosol optical depth (AOD) values could be derived for each 1◦×1◦ grid is more than 10 per month except between 0 to 20◦ S, where the number of days in which AOD could be derived for each grid is around 5 per month (Rajeev and Ramanathan, 2000)
At the western Arabian Sea close to the coast where the fractional cloudiness is less than 30% throughout the summer monsoon, the AOD values are available for more than 10 days per month

Summary

Introduction

The effects of aerosols on the radiation budget of the Earthatmosphere system, cloud characteristics and the Earth’s climate are well recognised (Charlson et al, 1992; Hansen et al, 1997; Ramanathan et al, 2001a,b). The prevailing lower tropospheric wind in this region during the Asian dry season has a dominant northerly component directed from the continents to the ocean This flow pattern enables transport of continental aerosols over to the Arabian Sea, Bay of Bengal and Indian Ocean, as revealed by several studies During the June–September period, due to the intense low pressure zone generated in the northwestern parts of the Indian subcontinent, the ITCZ migrates north and brings the massive Asian summer (southwest) monsoon over to this region During this season horizontal pressure gradient and lower tropospheric wind speeds over the Indian subcontinent and adjoining oceanic regions are significantly larger than those during the Asian dry period. Though this season is characterised by high rainfall in the region east of the central Arabian Sea, leading to increased aerosol removal

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