Near-surface pollution over the northern Indian Ocean during the winter monsoon is subject to large variations between different months and different years. We study this variability using global chemistry-climate model simulations of carbon monoxide (CO) during February and March of 1996 to 1999. Our results suggest that three processes largely determine the CO variability: the monsoon cycle, the El Ni¿o/Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO). During the winter monsoon, northeasterlies advect polluted air masses in the boundary layer from the Asian continent to the InterTropical Convergence Zone (ITCZ). Variations in the strengths of the Asian continental high pressure and the ITCZ strongly influence pollution levels over the continent and the northern Indian Ocean. Strong continental outflow and a strong ITCZ moderate pollution concentrations through efficient transport. Strong continental outflow combined with a weak ITCZ, on the other hand, prevents efficient removal of pollution from the marine boundary layer, hence CO accumulates. During ENSO the Asian continental high pressure reduces, which weakens advection of polluted air masses, increases CO buildup in the polluted continental boundary layer and decreases CO over the northern Indian Ocean. Variations in the ITCZ strength are associated with active (strong) and passive (weak) phases of the MJO. During active phases the ventilation of pollution from the marine boundary layer reduces CO concentrations.