Sea–air CO2 fluxes in the Indian Ocean between 1990 and 2009

[摘要] The Indian Ocean (44° S–30° N) plays an important rolein the global carbon cycle, yet it remains one of the most poorly sampled oceanregions. Several approaches have been used to estimate net sea–air CO₂fluxes in this region: interpolated observations, ocean biogeochemicalmodels, atmospheric and ocean inversions. As part of the RECCAP (REgionalCarbon Cycle Assessment and Processes) project, we combine these differentapproaches to quantify and assess the magnitude and variability in IndianOcean sea–air CO₂ fluxes between 1990 and 2009. Using all of the modelsand inversions, the median annual mean sea–air CO₂ uptake of−0.37 ± 0.06 PgC yr⁻¹ is consistent with the −0.24 ± 0.12 PgC yr⁻¹calculated from observations. The fluxes from the southernIndian Ocean (18–44° S; −0.43 ± 0.07 PgC yr⁻¹ are similar in magnitude to the annual uptakefor the entire Indian Ocean. All models capture the observed pattern offluxes in the Indian Ocean with the following exceptions: underestimation ofupwelling fluxes in the northwestern region (off Oman and Somalia),overestimation in the northeastern region (Bay of Bengal) and underestimation ofthe CO₂ sink in the subtropical convergence zone. These differenceswere mainly driven by lack of atmospheric CO₂ data in atmosphericinversions, and poor simulation of monsoonal currents and freshwaterdischarge in ocean biogeochemical models. Overall, the models and inversionsdo capture the phase of the observed seasonality for the entire Indian Oceanbut overestimate the magnitude. The predicted sea–air CO₂ fluxes byocean biogeochemical models (OBGMs) respond to seasonal variability withstrong phase lags with reference to climatological CO₂ flux, whereas theatmospheric inversions predicted an order of magnitude higher seasonal fluxthan OBGMs. The simulated interannual variability by the OBGMs is weakerthan that found by atmospheric inversions. Prediction of such weakinterannual variability in CO₂ fluxes by atmospheric inversions wasmainly caused by a lack of atmospheric data in the Indian Ocean. The OBGMmodels suggest a small strengthening of the sink over the period 1990–2009of −0.01 PgC decade⁻¹. This is inconsistent with the observations in thesouthwestern Indian Ocean that shows the growth rate of oceanic pCO₂ wasfaster than the observed atmospheric CO₂ growth, a finding attributedto the trend of the Southern Annular Mode (SAM) during the 1990s.

[学科分类] 地球化学与岩石