Variability of aerosol optical thickness in the tropical Indian Ocean and South China Sea during spring intermonsoon season

ABSTRACT

The variability of aerosol optical thickness (AOT) in the tropical Indian Ocean (IO) and South China Sea (SCS) during the intermonsoon (February–May) is investigated using shipboard and satellite data from 2011 to 2014, in order to understand the mechanism controlling AOT production and transport. Overall AOT in tropical IO is significantly smaller than those in SCS, and a strong intraseasonal variability of AOT, along with significant dependence on wind speed (especially when <8 m s⁻¹), is observed in both basins. Our analysis showed that in tropical IO, aerosols are mostly of natural marine production in spring, and the AOT shows greater dependence on the higher wind speeds in May, while in SCS, AOT dependence on wind speed increases when the prevailing wind turns from offshore (February, March) to onshore (April, May), due to less transport of terrestrial dust and anthropogenic aerosols from land. A better agreement between shipboard and once-daily satellite AOT is observed in the Equatorial belt (5° S–5° N, i.e. remote areas of IO) than in the Non-Equatorial area (5° N–25° N), and can be explained by the overall lower AOT values and the smaller diurnal variation in the Equatorial belt. Changes of wind regime during the monsoon evolution and the consequent transport of land-based aerosols reduce AOT dependence on wind speed and are the major drivers for the AOT variability.

Acknowledgments

The shipboard AOT data in this study are available on the Maritime Aerosol Network website (http://aeronet.gsfc.nasa.gov/new_web/maritime_aerosol_network.html). The satellite AOT data used in this work are available from NASA Giovanni (http://giovanni.gsfc.nasa.gov) and NASA’s Ocean Colour Web (http://oceancolor.gsfc.nasa.gov/cgi/l3), and wind data of ASCAT from the Remote Sensing Systems (http://www.remss.com/). We appreciate Dr Alexander Smirnov for his kind help with data processing and transfer calibration, and Dr Naoto Ebuchi (Hokkaido University) for his useful advice on the interpretation of data. We thank the science party and crew of the R/V Shiyan1 (SCSIO, CAS) for their support and technical assistance, especially Jingrou Lin, Jing Sun and YongJun Song.

The present work was supported by State Key Laboratory of Tropical Oceanography and Guangdong Key Laboratory of Ocean remote Sensing (LORS), South China Sea Institute of Oceanology Chinese Academy of Sciences (Project No. LTO1604 and LORS1701), and funding from South China Sea Branch of State Oceanic Administration (1606), National Natural Sciences Foundation of China (41430968, 41106105), Research Project of Collaborative Innovation Center for 21st-Century Maritime Silk Road Studies (2015HS05), and China Petroleum & Chemical Corporation (313099) awarded to DL Tang, and US National Science Foundation (ATM - 0741832), and Chinese Academy of Sciences program for visiting professorships for senior international scientists (Grant 2013T1Z0048 at South China Sea Institute of Oceanology) awarded to G. Levy.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Chinese Academy of Sciences program for visiting professorships for senior international scientists [2013T1Z0048]; US National Science Foundation [ATM – 0741832]; Funding of South China Sea Branch of State Oceanic Administration [1606]; National Natural Sciences Foundation of China [41106105, 41430968]; Open Research Program: State Key Laboratory of Tropical Oceanography and Guangdong Key Laboratory of Ocean remote Sensing (LORS); South China Sea Institute of Oceanology Chinese Academy of Sciences [LTO1604 and LORS1701]; Research Project of Collaborative Innovation Center for 21st-Century Maritime Silk Road Studies [2015HS05], and China Petroleum & Chemical Corporation [313099].