The spatiotemporal structure of diabatic processes governing the evolution of Subantarctic Mode Water in the Southern Ocean

A coupled ice-ocean eddy-permitting Southern Ocean State Estimate (SOSE) for 2005-2010 is used to describe and quantify the processes forming and destroying water in the Subantarctic Mode Water (SAMW) density range (σ_θ=26.7 – 27.2 kg m^-3). All the terms in the temperature and salinity equations have been diagnosed to obtain a three-dimensional and time varying volume budget for individual isopycnal layers. We find that air-sea buoyancy fluxes, diapycnal mixing, advection, and storage are all important to the SAMW volume budget. The formation and evolution processes are highly variable with strong cancellations in both space and time (seasonal and interannual). These processes act over a large latitude range due to the seasonal migration of the outcrop window.

The strongest formation is by wintertime surface ocean heat loss occurring equatorward of the Subantarctic Front. Spring and summertime formation occur in the polar gyres by freshening of water with σ_θ > 27.2 kg m^-3, with an important contribution from sea ice melt. The freshening by sea ice melt is more important in formation of water in the SAMW density range in in years 2008-2010 when the Southern Annular Mode (SAM) is stronger, than in 2005-2007, when the SAM is weaker. Consequently, the SAMW is colder and fresher in 2008-2010 than in 2005-2007, as revealed both in the SOSE and from Argo observations. In all the years analyzed, formation of water in the SAMW density range by diapycnal ocean mixing is as important as formation by air-sea buoyancy fluxes.