Abstract:

The variability of the Subantarctic Front (SAF) and Polar Front (PF) in austral summer (January–March) during 1980–2015 was investigated using monthly mean potential temperature and salinity reanalysis data, and by applying criteria based on physical phenomena and fixed property values to frontal detection. The results derived from two types of criterion were consistent with each other regarding the long-term trend of frontal position during the 36–year period. In terms of circumpolar average, no significant trend of frontal movement was found between 1980 and 2015, although frontal variability appeared as fluctuation during these years. The migration amplitude, stability and long-term trend of the fronts showed regional differences. Minimal displacement and high stability were found in regions with large topographic obstacles, whereas the fronts move significantly and frequently in deep basins and downstream of obstacles. The regions with the highest stability and the smallest long-term movement of the SAF and PF were found in the Indian and Atlantic sectors, respectively. In the eastern Pacific sector, the SAF showed significant long-term movement with both northward and southward shifts, whereas southward displacement of the PF dominated the Indian sector. On decadal and interannual timescales, evident differences were found between the results of frontal detection derived from the two criteria; thus, the applicability of both criteria was discussed accordingly. For the PF, the criterion based on the temperature phenomenon was found more stable than the isotherm criterion.

Key words: Subantarctic Front, Polar Front, long-term variability, Southern Ocean