Abstract
Observed long-term warming trends in the Southern Ocean have been interpreted as a sign of increased poleward eddy heat transport or of a poleward displacement of the entire Antarctic Circumpolar Current (ACC) frontal system. The two-decade-long record from satellite altimetry is an important source of information for evaluating the mechanisms governing these trends. While several recent studies have used sea surface height contours to index ACC frontal displacements, here altimeter data are instead used to track the latitude of mean ACC transport. Altimetric height contours indicate a poleward trend, regardless of whether they are associated with ACC fronts. The zonally averaged transport latitude index shows no long-term trend, implying that ACC meridional shifts determined from sea surface height might be associated with large-scale changes in sea surface height more than with localized shifts in frontal positions. The transport latitude index is weakly sensitive to the Southern Annular Mode, but is uncorrelated with El Niño/Southern Oscillation.
Highlights
The Southern Ocean, the winds that force it, and the cryosphere to its south have experienced distinct patterns of change over the past five to six decades
Previous studies have typically used the latitudinal positions of fixed sea surface height (SSH) contours h as a measure of Antarctic Circumpolar Current (ACC) frontal variability [21,50,51,52] to be compared with Southern Annular Mode (SAM)- or El Niño/Southern Oscillation (ENSO)-related climate fluctuations
The largest effects for all SSH contours are associated with the annual cycle in steric expansion of the ocean: a sinusoidal annual cycle explains between 27% and 52% of the variance, depending on the height contour
Summary
The Southern Ocean, the winds that force it, and the cryosphere to its south have experienced distinct patterns of change over the past five to six decades. Evidence from historic hydrographic data and newer temperature profiles indicates rising temperatures over a broad range of depths extending well below the ocean surface [1,2,3,4,5]. Southern Ocean warming is concentrated in the circumpolar band corresponding to the Antarctic Circumpolar Current (ACC). In the top 1000 m of the ocean, approximately 80% of the net heat content increase in the Southern Hemisphere rsta.royalsocietypublishing.org Phil.
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