Role of surface CO2 sources and wind transport on atmospheric CO2 variability over the Bay of Bengal during Southwest Monsoon - using mixing ratio, carbon isotope ratio and model simulated wind field

Abstract The present study examines the role of surface-CO2 sources and wind transport on atmospheric CO2 observations over the Bay of Bengal. The ship-based observations of mixing ratio and stable carbon isotope ratio (δ13C) of air CO2 was carried out at coastal and open ocean location during 17 July 2009 until 18 August 2009, covering the peak months of the Southwest Monsoon season. The average mixing ratio was higher for the coastal sites (407 ± 17 µmol mol− 1) than the open ocean sites (394 ± 9 µmol mol− 1), whereas the average δ13C value was lower (more depleted in 13C) (-8.75 ± 0.53‰) for the coastal sites than the open ocean sites (-8.48 ± 0.04‰). The observations were further used to identify the isotopic ratio of the source CO2 using the Keeling and the Miller-Tans plot. The presence of two different sources were confirmed from the observations. The coastal observations were affected by continental sources of CO2 (-19.95 ± 1.7‰), whereas for open ocean sites, oceanic sources (-10.31 ± 0.15‰) played a major role. Although there was a transport of continental air to open ocean, as observed from the WRF model simulated wind, the land-ocean contrast was maintained. It was evident from the Carbon Tracker simulation during the study period. Despite continental air transport, the open ocean observation was influenced by oceanic sources of CO2. To explore the reason, the predominant wind pattern for the sampling sites were determined from the WRF model simulated winds. A k-means clustering technique was used on the WRF model simulated hourly wind to understand the different atmospheric transport patterns during different days of sampling in July and August. Most of the sampling sites were resided within the clusters whose mean wind direction indicates south-westerly/southerly wind. Thus, irrespective of continental transport, the sampling sites were influenced by oceanic wind during July and August. The influence of oceanic wind allowed the identification of oceanic source CO2 over the open ocean sites despite the transport from the continent.