Increasing poleward intrusion of the Circumpolar Deep Water and its impact on the changing ocean-biogeochemistry of the Southern Ocean

A study published in Nature Geoscience (Bronselaer et al., 2020) analyzed new observations from autonomous floats with ocean-biogeochemical sensors (Bio-Argo) along with historical shipboard ocean measurements to document changes in Southern Ocean physical and ocean-biogeochemical variables (i.e., temperature, salinity, pH, concentrations of nitrate, dissolved inorganic carbon and oxygen) during the last two decades. Due to the poleward shift and strengthening of the Southern Hemisphere (SH) Westerlies (as a result of increasing GHGs and its impact on the SH polar vortex) during the recent decades, the warm and salty Circumpolar Deep Water (CDW) has intruded deeper into the Antarctic continental shelf regions. This in turn increases the basal melting of the Antarctic ice sheet. The increased melt water around the Antarctica in turn enhances ocean stratification near the surface and thereby insulates CDW from the atmosphere. Since CDW is largely formed in the high-latitude North Atlantic through deep ocean convection, and carried slowly southward into the Southern Ocean at depth, it is oxygen-poor, carbon- and nutrient-rich water. Therefore, the supply of nutrients to the surface, which is vital for biological production, is limited; thus this may lead to a decrease in overall biological productivity in the Southern Ocean.

Figure 1 from Silvano et al. (2020): The Southern Ocean in a changing climate. a, Divergence between westerly and easterly winds drives upwelling of Circumpolar Deep Water, which upon interaction with atmosphere is transformed into either lighter intermediate and mode waters (upper overturning) or denser Antarctic Bottom Water (lower overturning). b, The strengthening and poleward shift of westerly winds, concurrent with a weakening of easterly winds (see dashed arrows), cause isopycnals to shoal near the Antarctic coast and deepen further offshore (see dashed line). Increased meltwater discharge from the Antarctic ice sheet enhances ocean stratification near the surface (see hatched area). Bronselaer et al. (2020) show how these mechanisms can help explain observed decadal changes in physical and biogeochemical properties of the Southern Ocean.

Silvano, A. Changes in the Southern Ocean. Nat. Geosci. 13, 4–5 (2020). https://doi.org/10.1038/s41561-019-0516-2. https://www.nature.com/articles/s41561-019-0516-2.

Bronselaer, B., Russell, J.L., Winton, M. et al. Importance of wind and meltwater for observed chemical and physical changes in the Southern Ocean. Nat. Geosci. 13, 35–42 (2020). https://doi.org/10.1038/s41561-019-0502-8. https://www.nature.com/articles/s41561-019-0502-8.