Heat Transport and Long-Term Change in the Southern Ocean: Assessing the Role of Eddies

Abstract:

Long-term change in the Southern Ocean can be difficult to evaluate because of both the paucity of historic observations and the magnitude of eddy variability. The low stratification of the Southern Ocean means that eddies detected by altimetry at the ocean surface extend through the top 2 km of the ocean. Sea surface height anomalies are more strongly correlated with sub-surface variability at depths between about 600 and 1400 dbars than they are with variability in the upper 200 dbars. Altimetric variability can thus be used to remove eddy-related anomalies from individual Argo profiles, resulting in a smoother estimate of mean temperature and salinity. This “eddy-free” mean field serves as a benchmark against which to assess decadal-scale changes in the Southern Ocean, and we use historic hydrographic data to evaluate temperature and salinity changes through the second half of the 20^th century. We also evaluate the behavior of Southern Ocean eddies themselves: Although in most parts of the ocean closed oceanic eddies appear to result in thermally indirect heat transport, eddies that are carried eastward by the ACC tend to propagate in the opposite direction, resulting in thermally direct, poleward heat transport across the ACC. Evidence suggests that this cell is maintained by the effective eastward propagation of eddies relative to the mean flow at deep levels.