Observations of Upwelling and Downwelling Around Antarctica Mediated by Sea Ice

Abstract

We infer circumpolar maps of stress imparted to the ocean by the wind, mediated by sea-ice, in and around the Seasonal Ice Zone (SIZ) of Antarctica. In the open ocean we compute the wind stress using surface winds from daily atmospheric reanalyses and applying bulk formulae. In the presence of sea ice, the stress imparted to the underlying ocean is computed from satellite observations of daily ice concentration and drift velocity assuming, first, that the ocean geostrophic currents beneath are negligible, and then including surface geostrophic ocean currents inferred from satellite altimetry. In this way maps of surface ocean stress in the SIZ are obtained. The maps are discussed and interpreted, and their importance in setting the circulation emphasised. Just as in parallel observational studies in the Arctic, we find that ocean currents significantly modify the stress field, the sense of the surface ageostrophic flow and thus pathways of exchange across the SIZ. Maps of Ekman pumping reveal broad patterns of upwelling within the SIZ enhanced near the sea ice edge, which are offset by strong narrow downwelling regions adjacent to the Antarctic continent.

Southern Ocean Ice‐Covered Eddy Properties From Satellite Altimetry

“…Consequently, the n ‐MIZ is a region favorable for a strong horizontal gradient in surface stress, between ice‐free, open‐ocean regions and partially ice‐covered ocean regions. This change in sea ice concentration can lead to strong Ekman pumping velocities (Manucharyan et al., 2022; Ramadhan et al., 2022).…”

“…Finally, we compute the Ekman pumping w e as in Ramadhan et al. (2022): $$${w}_{e}=\nabla \times \left(\frac{\vec{{\tau }_{\mathit{ocn}}}}{{\rho }_{w}f}\right).$$$ where f is the Coriolis parameter.…”

“…Here, ρ a and C dao are the air density and air-ocean drag coefficients, respectively, and are taken from Tsamados et al (2014). The ocean density is ρ w = 1,028 kg m −3 , and the ocean-ice drag coefficient is C dw = 5.50 × 10 −3 , as used in Garabato et al (2019) Finally, we compute the Ekman pumping w e as in Ramadhan et al (2022):…”

Column-Compound Extremes in the Global Ocean

“…Local parcels of water carrying the MHW signal may be disconnected from the surface, and later reconnected again through mixing and surface heating. Meanwhile, anomalous intensification of winds drive the upwelling of CO 2 -rich waters within the ACC and Antarctic convergence zone (Negrete-García et al, 2019;Ramadhan et al, 2022), heaving the thermocline and hence also waters with higher acidity, inducing OAX in the subsurface. The events of cluster 4 occur along the continent of Antarctica, with co-located MHW and OAX occurring largely beneath the surface between 100 −300 m. Here, MHW-OAX events have the highest frequency of 21 days per year, the longest mean duration of 19 days, and have the highest intensity index of 3.3.…”

“…Strengthened zonal westerlies driving the Antarctic Circumpolar Current (ACC) could lead to increased upwelling of Circumpolar Deep Water (CDW) at the northern edge of the sea-ice zone (Morrison et al, 2015;Wilson et al, 2019;Ramadhan et al, 2022). This results in vertical entrainment of deep waters which tend to be warmer and more acidic (Gordon, 1981;Gordon & Huber, 1990;Pellichero et al, 2017).…”