Seasonal controls on surface pCO₂ in the central and eastern Arabian Sea

Abstract

The variability in partial pressure of carbon dioxide (pCO₂) and its control by biological and physical processes in the mixed layer (ML) of the central and eastern Arabian Sea during inter-monsoon, northeast monsoon, and southwest monsoon seasons were studied. The ML varied from 80-120 m during NE monsoon, 60-80 m and 20-30 m during SW- and inter-monsoon seasons, respectively, and the variability resulted from different physical processes. Significant seasonal variability was found in pCO₂ levels. During SW monsoon, coastal waters contain two contrasting regimes; (a) pCO₂ levels of 520-685 µatm were observed in the SW coast of India, the highest found so far from this region, driven by intense upwelling and (b) low levels of pCO₂ (266 µatm) were found associated with monsoonal fresh water influx. It varied in ranges of 416-527 µatm and 375-446 µatm during inter- and NE monsoon, respectively, in coastal waters with higher values occurring in the north. The central Arabian Sea pCO₂ levels were 351-433, 379-475 and 385-432 µatm during NE-inter and SW monsoon seasons, respectively. The mixed layer pCO₂ relations with temperature, oxygen, chlorophylla and primary production revealed that the former is largely regulated by physical processes during SW- and NE monsoon whereas both physical and biological processes are important in inter-monsoon. Application of Louanchiet al (1996) model revealed that the mixing effect is the dominant during monsoons, however, the biological effect is equally significant during SW monsoon whereas thermodynamics and fluxes influence during inter-monsoons.