Drivers of Interannual Variability in Southeast Pacific Subantarctic Mode Water

Subantarctic mode water (SAMW) forms seasonally on the northern edge of the Antarctic Circumpolar Current. The formation is critical to the subducting portion of the Southern Ocean overturning circulation, which is vital to the transfer of carbon and heat from the atmosphere into the deep ocean. In recent decades, there has been an observed freshening of SAMW, linked to changes in the concentration and northward transport of sea ice, but variability in these mechanisms is not well understood.

Using the output from the Southern Ocean State Estimate (SOSE) and ECCO version 4 release 3, we approximate the mixed layer heat and salt budgets for the Southern Ocean. The results are used to determine the processes responsible for changes in the temperature and salinity of the mixed layer, and by focusing on the southeast Pacific SAMW formation region, the subsequent changes in properties of the subducted SAMW.

In recent years, the influence of both entrainment and surface fluxes on the properties of the mixed layer has increased. Vertical entrainment through the base of the mixed layer acts to increase salinity by up to 1.13 psu yr^-1 each austral winter, while surface freshwater fluxes and horizontal advection cause a salinity decrease in summer by a maximum of 0.44 and 0.35 psu yr^-1, respectively. Thus, surface buoyancy fluxes, entrainment and advection are the predominant drivers of variability in mixed layer temperature and salinity within the SAMW formation region.