Abstract
While the effects of the Southern Annular Mode (SAM), a dominant climate variability mode in the Southern Ocean, on ocean acidification have been examined using models, no consensus has been reached. Using observational data from south of Tasmania, we show that during a period with positive SAM trends, surface water pH and aragonite saturation state 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). Together with other processes, 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 support climatic modulation of ocean acidification superimposed on the effect of increasing atmospheric CO2.
Highlights
While the effects of the Southern Annular Mode (SAM), a dominant climate variability mode in the Southern Ocean, on ocean acidification have been examined using models, no consensus has been reached
A faster fCO2 increase occurred during the pre-2000 positive SAM trend period in the high-latitude zone (60°–55° S), and a slower increase in the mid-latitude zone (50°–45° S) compared to the atmospheric increase (Fig. 2)
To put our findings in a broad context, we explore the possible influence of the SAM on pH and Ωarag changes in other regions of the Southern Ocean by examining CCMP (Cross-Calibrated Multi-Platform) January zonal wind trends in the Southern Ocean basin (Fig. 1b, c)
Summary
While the effects of the Southern Annular Mode (SAM), a dominant climate variability mode in the Southern Ocean, on ocean acidification have been examined using models, no consensus has been reached. Using observational data from south of Tasmania, we show that during a period with positive SAM trends, surface water pH and aragonite saturation state 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). Global OA is due primarily to increasing atmospheric CO2 by fossil fuel combustion and land use changes since the Industrial Revolution[2,9], it may be enhanced by other processes such as upwelling, eutrophication, sea ice melt, and anomalous ocean circulation[10,11,12,13,14,15,16,17] Such rapid acidification challenges the evolutionary adaptation capacity of organisms[18]. Given that the region south of Tasmania is perhaps the only region where there is continuous observational CO2 data since
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