Seasonal and interannual variations of sea surface carbon dioxide in the subtropical Indian Ocean

Abstract

We study the seasonal and interannual variations of sea surface carbon dioxide fugacity (fCO2) in the subtropical region of the south-western Indian Ocean by using a one-dimensional model that includes the main processes governing the oceanic CO2 cycle. The model is constrained by monthly SST fields (period 1986–1994) and monthly climatological mixed-layer, wind and CZCS-ChlA. We first focus the results on the period 1991–1994 for which seasonal observations are available. Except for January 1994 when a strong anomaly occurred (with high fCO2 and low SST related to high wind conditions), the seasonal fCO2 cycle is well reproduced each year: near-equilibrium values in austral summer and undersaturation in winter. Although the oceanic CO2 cycle is believed to be mainly thermodynamically controlled in the subtropics, we obtained better results when mixing and biological processes are included in the model. Compared to observations, residuals of the complete model are about 10 μatm in austral summer and almost zero in winter months when the air–sea flux toward the ocean is large. The simulated yearly air–sea CO2 flux shows the same interannual variations as the observations: the Indian Ocean subtropical sink was twice in 1993 compared to 1991 and 1992. For the period 1986–1994, in response to atmospheric CO2 growth rate (1.4 μatm/yr), the modelled oceanic fCO2 increased, but at a lower rate (about 0.5 μatm/yr); the oceanic response appears to be faster in summer (0.75 μatm/yr) than in winter (0.4 μatm/yr). Due to the decreasing of ΔfCO2, the model suggests that the Indian Ocean subtropical sink increases progressively from 1986-1987 (0.1 to 0.2 mol/m2/yr) to 1993–1994 (0.5 mol/m2/yr), with a strong anomaly in 1989 (0.4 mol/m2/yr).