Mechanisms for the intraseasonal variability of tropospheric ozone
over the Indian Ocean during the winter monsoon

R. B. Chatfield,¹ H. Guan,^1,2 A. M. Thompson,³ and H. G. J. Smit⁴

¹Earth Science Division, NASA Ames Research Center, Moffett Field, California, USA.
²Bay Area Environmental Research Institute, Sonoma, California, USA.
³Department of Meteorology, Pennsylvania State University, University Park, Pennsylvania, USA.
⁴Forschungszentrum Ju¨lich, Ju¨lich, Germany.

Received 29 March 2006; revised 21 December 2006; accepted 29 January 2007; published 17 May 2007.

ABSTRACT We synthesize daily sonde (vertical) information and daily satellite (horizontal) information to provide an empirical description of ozone origins over the northern Indian Ocean during the INDOEX (Indian Ocean Experiment) field campaign (February–March 1999). This area is shown to be a significant portion of the ‘‘high-ozone tropics’’. East-west O₃ features and their flow are identified, and ozone origins are compared to other tropical regions, using water vapor as a second tracer. In the study period, multiple processes contribute to O₃ column enhancements, their importance varying strongly by latitude: (1) Low-altitude O₃ pollution over the northern Indian Ocean mainly originates from the Indian subcontinent and is traceable to high emission areas. Convective activity south of Sri Lanka helps direct ozone outflow from the northern Indian subcontinent. (2) Middle tropospheric O₃ maxima over the northern Indian Ocean originate from various sources, often transitioning within a few hours. Convective venting of Asian pollutants can add 20–30 ppbv to the middle troposphere at 5^oN–10^oN, alternating with stratospheric influence. (3) A number of cases suggest that strong mixing-in of stratospheric air along the subtropical jet raised tropospheric O₃ in early March by ~40–50 ppbv, especially poleward of ~10^oN. (4) Influences of lightning and large-scale biomass burning were not strong during this period, in contrast to the situation in Africa and the South Atlantic or locally in Southeast Asia. This work illustrates successes and limitations in approaches to synthesizing disparate information on trace-gas distributions taken from satellite retrieval products and ozonesondes.