The distribution, circulation and variability of subantarctic mode water

The temporal and spatial variability of Subantarctic Mode Water (SAMW) is investigated using a compilation of several observational data sets: hydrography, XBTs, Argo profiles, satellite altimetry, and a climatology. The subduction and export of SAMW as part of the overturning circulation play an important role in global heat, freshwater, carbon and nutrient budgets. Determining the formation, circulation, and variability of SAMW is therefore an important step towards understandmg the Southern Ocean's role in climate variability and change. The spatial distribution of SAMW properties and export paths is investigated. SAMW is identified by a dynamical tracer: a minimum in potential vorticity. SAMW consists of several modes with distinct properties rather than a continuous water mass with properties that gradually evolve towards the eastern region of the Southern Ocean as previously believed. (Modified) Montgomery streamlines on the density surfaces corresponding with potential vorticity (PV) minima are used to determine the circulation paths of SAMW. The different SAMW modes are injected into the ocean interior at "hot spots" north of the Subantarctic Front (SAF), and then follow narrow export pathways into the subtropical gyres. We have also investigated the temporal variability of SAMW in the formation region, south of Australia. SAMW properties show high intra- and inter-annual variability, but no evidence of a trend over the 17-yr mean temperature record studied (1991-2007). Abrupt changes in temperature and salmity at the SAMW core are related to cross-frontal advection of cool and fresh surface water through eddies and meanders from the SAF, and warm and salty water originating from the East Australian Current in the north. In addition, the SAMW pool on a single section often consists of two or more modes with distinct temperature, salinity and vertical homogeneity characteristics, but similar density. The main features of the regional circulation and the physical drivers of the east/west contrast in biomass concentration in the Subantarctic Zone (SAZ) south of Tasmania have been identified. Enhanced input of subtropical water high in micronutrients (such as iron) in the east likely supports the higher surface biomass observed there. The physical processes shown to be responsible for maintaining the east/west contrast south of Tasmama (e.g., regional circulation, eddies and subsurface salinity intrusions) are likely to drive variability in physical and biogeochemical properties of SAMW, Antarctic Intermediate Water and the SAZ elsewhere m the Southern Ocean. These results have revealed the spatial and temporal variability of SAMW in greater detail than was possible due to lack of data. Different modes of SAMW are formed in each ocean basin. Ventilation "windows" revealed by the PV distribution show hot spots where SAMW subducts. The export of warmer, saltier modes in these "hot spots" contnbutes to the circumpolar evolution of mode water properties towards cooler, fresher and denser modes in the east. We also found that mesoscale features strongly influence SAMW variability south of Tasmania, and are likely to be important in settmg SAMW properties in other regions with high eddy kinetic energy.