On the Variability of Antarctic Circumpolar Current Fronts Inferred from 1992-2011 Altimetry

Antarctic Circumpolar Current (ACC) fronts, defined as water mass boundaries, have been known to respond to large-scale atmospheric variabilities, especially the Southern Hemisphere annular mode (SAM) and El Nino-Southern Oscillation (ENSO). Distinct patterns of localized variability in meridional front displacements during 1992-2011 are derived from the analysis of satellite sea surface height data. Major basin-scale differences are found between the southeast Pacific (150 degrees-90 degrees W) and the southeast Indian (75 degrees-150 degrees E) sectors of the ACC. Frontal positions in the southeast Pacific show large year-to-year meridional fluctuations, attributed mostly to ENSO and in part SAM, and no apparent seasonal cycles or long-term trends. In contrast, summer (winter) frontal locations in the southeast Indian extend farther to the south (north) of their long-term mean distribution. A southward drift of ACC fronts is indicated over the Indian sector during the past two decades. This long-term shift is not directly related to the atmospheric variabilities, but this is most likely in response to changes in large-scale ocean circulation, in particular to the poleward expansion of the Indian subtropical gyre. The existence of these localized, contrasting variability patterns suggests that a circumpolar-averaging analysis could possibly smooth out a local climate signal, with an emphasis on a basin-scale investigation for climate studies in the Southern Ocean.