Tropospheric carbon monoxide over the northern Indian Ocean during winter: influence of inter-continental transport

South Asian outflow over the adjoining marine regions is most pronounced during winter season, whereas the prevalence of strong winds aloft can transport the influences from inter-continental sources. Here, we investigate the tropospheric carbon monoxide (CO) distribution over the northern Indian Ocean (IO) combining shipborne measurements carried out during the Integrated Campaign for Aerosols, gases, and Radiation Budget (ICARB) campaign (January–February 2018), retrievals from Measurements of Pollution in the Troposphere (MOPITT), and Copernicus Atmosphere Monitoring Service (CAMS) model. Surface CO varied from ~ 50 to 365 ppbv (179 ± 67 ppbv) with higher levels over the coastal region of southeast Arabian Sea and lower levels over the equatorial IO. We observed lower CO levels (200 ± 43 ppbv) than those during the Indian Ocean Experiment-1999 (229 ± 40 ppbv) supporting the reported decreasing trend of tropospheric CO. In situ CO observations are found to be in a good agreement with the satellite retrievals (MOPITT-version 8) as well as the CAMS model results (r 2 = 0.60‒0.65). The upper-tropospheric CO (300‒200 hPa) over the equatorial IO is observed to be higher by up to 30% during February-2018 as compared to the decadal mean, coinciding with anomalous westerlies resulting from a disturbed Walker cell over the equatorial IO and deeper penetration of the sub-tropical jet. The influences of African forest fires are suggested to have enhanced the upper-tropospheric CO over the IO during February-2018. Our study highlights the importance of strong large-scale dynamics and global biomass-burning emissions in the wintertime pollution loading over the IO, besides the South Asian outflow.