Modulation of the Southern Annular Mode on Surface Acidification rates in the Southern Ocean

Although ocean acidification is due primarily to increasing atmospheric CO₂, it may be enhanced by other processes such as upwelling and eutrophication. Using observational data from south of Tasmania, here we show how the Southern Annular Mode (SAM), a dominant climate variability mode in the South Hemisphere, modulate surface acidification rates in the Southern Ocean. We find that during a period with positive SAM trends, surface water pH and aragonite saturation state (Ω_arag) at 60°–55° S (Antarctic Zone) decrease in austral summer at rates faster than those predicted from atmospheric CO₂ increase alone, whereas an opposite pattern is observed at 50°–45° S (Subantarctic Zone). While during a period with insignificant SAM trend, pH and Ω_arag at 50°–45° S both showed enhanced rates of decrease relative to the atmospheric CO₂ based prediction. Additionally, after removing the effects of atmospheric CO₂ increase, the rates of pH and Ω_arag change are negatively correlated with change rates of zonal wind speed, indicating important influences of wind on acidification rates. Taking the case at high-latitude (60°–55° S) during the 1991-2000 positive SAM trend as an example and conducting a mass balance model analysis, we quantified the contribution of Ekman transport, vertical mixing, air-sea CO₂ flux and biological activity to acidification rates. We conclude that the enhanced acidification at 60°–55° S may be attributed to increased westerly winds that bring in more “acidified” waters from the higher latitudes via enhanced meridional Ekman transport and from the subsurface via increased vertical mixing. Our observations improve the understanding of the Southern Ocean acidification mechanism, and support climatic modulation of ocean acidification superimposed on the effect of increasing atmospheric CO₂.