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Polar fronts

Par olvac — Dernière modification 03/05/2015 16:03

Early work based on the SURVOSTRAL data allowed us to define the position and vertical extension of the main hydrological fronts in the Southern Ocean south of Tasmania, as well as their seasonal evolution (Rintoul et al., 1997). The Subtropical Front is centred between Tasmania and the « South Tasman Rise » around 48°S, the Polar Front is near 53°S. The strongest front south of Tasmania, and the most energetic, is the Subantarctic Front which is often divided into two branches: the northern part is associated with the 6-8°C isothermes, and is characterised by strong meridional gradients in temperature and salinity, and is found around 50°S-51°S. A southern branch, is associated with the 3-5°C isothermes, has weaker meridional gradients in temperature and salinity, and its position varies more in time.

Sokolov et Rintoul (2003, 2006) have determined 10 main fronts between Tasmania and Antarctica using a combination of altimetry (which gives the time-variable component) and a mean sea surface (steric height) obtained from the Olbers climatological atlas. Their satellite derived frontal positions have been validated with the WOCE SR3 CTD sections and the SURVOSTRAL XBT sections. The satellite front positions agree closely with positions inferred from these hydrographic sections using traditional water mass criteria. Their technique allows us to follow the temporal and spatial evolution of the different fronts, and gives a better interpretation of the frontal meanders and detached eddies from altimetry data. They find that the ACC does consist of circumpolar fronts, which coincide with criteria based on temperature or salinity distributions, but the structure is much richer than traditionally thought. Fronts split into multiple branches or merge to form "super-fronts" and the intensity of the fronts varies along the circumpolar path.

Polar Front

Annual average of front positions for 1993, overlaid on the annual mean SST gradient.
After Sokolov and Rintoul, 2006.

Sallee et al (2006) have also shown how the SAF and PF can be tracked in a circumpolar way using altimetry data. They find that the fronts are strongly controlled by bathymetry. Away from large bathymetric features, the fronts show considerable meridional meandering, which can be linked to the dominant climate modes: the Southern Annular Mode (SAM) and ENSO. However, the ocean response is not zonal, but shows large regional patterns of variability. If the South Pacific is dominated by ENSO, the southern Indian basins respond more directly to SAM forcing.

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