Circumpolar structure and distribution of the Antarctic Circumpolar Current fronts: 2. Variability and relationship to sea surface height

Abstract

In Part 1 of this study, we showed that the Antarctic Circumpolar Current (ACC) consisted of multiple fronts, each of which was consistently associated with a particular contour of sea surface height (SSH) or approximate streamline. In Part 2 we have used maps of SSH to examine the variability of the ACC fronts between 1992 and 2007. The SSH label associated with each frontal branch is nearly constant around the circumpolar belt. The front labels are also nearly constant in time: the bands of enhanced SSH gradient (i.e., fronts) occur along the same streamlines throughout the 15 year period of observations. Both short‐ and long‐period changes of the SSH frontal labels of the ACC are small. Based on a tight relationship between dynamic height and cumulative baroclinic transport of the ACC, the baroclinic transport variability of the individual branches of the ACC is also expected to be small. The major change in the total ACC baroclinic transport occurs in the Drake Passage. The streamline associated with the northern branch of the SAF (SAF‐N) does not pass through Drake Passage and the waters carried by this branch are not observed there. Instead, the transport of the SAF‐N turns north in the Pacific to supply the export of water to the Indian Ocean north and south of Australia. Strong eddy activity in the southeast Pacific acts to dissipate the hydrographic signature of the SAF‐N there. In the Atlantic, the SAF‐N reappears as the same streamline is again associated with enhanced SSH gradients to the east of the Brazil – Malvinas confluence zone. While the large changes in SSH have occurred in the Southern Ocean between 1992 and 2007, there are strong regional differences. Because the ACC fronts are robustly associated with particular SSH contours, the changes in SSH reflect shifts in the position of the ACC fronts. In the circumpolar average, each of the ACC fronts has shifted to the south by about 60 km. The changes in SSH in the Southern Ocean are largely due to changes in ocean circulation, rather than warming and freshening by atmospheric fluxes. Much larger changes in SSH are observed in some locations of the Southern Ocean, particularly where the fronts interact with large‐scale topography. The northern branch of the PF (PF‐N) near the Kerguelen Plateau is an extreme example, where the PF‐N followed a path around the northern end of Kerguelen Plateau between 1992 and 1997, passed through the Fawn Trough after 2003, and oscillated between the two paths between 1997 and 2003.

Using kinetic energy measurements from altimetry to detect shifts in the positions of fronts in the Southern Ocean

“…This is a possibility, but if both fronts have moved south as some have argued (e.g., Sokolov and Rintoul, 2009b), then the CKE envelope should also shift, regardless of whether it includes one or two fronts. If the exact frontal location was known at any time, one could judge how well the CKE envelope (or half-center) point was associated with just one front.…”

“…So far, the results are mixed. Using the contour method and tracking how the dynamic topography contours associated with a front position shift in time, Sokolov and Rintoul (2009b) found that the SAF and PF had both moved south by approximately 60 km over 15 years between 1993 and 2008. Kim and Orsi (2014) recently updated this analysis and found that while the average frontal position across the Southern Ocean indicates a strong southward shift, this is due primarily to substantial shifts only in the Indian Ocean sector.…”

Projected poleward shift of king penguins' ( Aptenodytes patagonicus ) foraging range at the Crozet Islands, southern Indian Ocean

“…Kerguelen and Heard islands, located in Kerguelen plateau (498 S and 538 S, respectively), hold about 350 000 breeding pairs [23]. In contrast to Crozet Islands, bathymetric features of Kerguelen plateau constrain polar front position and reduce interannual variability in Antarctic circumpolar currents [49,54]. Such features may help these populations overcome future environmental changes.…”

Aspects of the Antarctic Circumpolar Current dynamics investigated with drifter data

“…In this region, the polar front (PF) is subject to particularly strong meridional shift (Orsi et al, 1995;Belkin and Gordon, 1996;Gille, 1999;Moore et al, 1999;Dong et al, 2006;Sokolov and Rintoul, 2009b). We believe that the variability revealed in Figs.…”