Laura Herraiz-Borreguero

Variability of Subantarctic Mode Water and Antarctic Intermediate Water in the Australian sector of the Southern Ocean

Supervisors: Prof. Richard Coleman (UTAS), Dr Steve Rintoul (CMAR)

Scientific background: The Southern Ocean plays a key role in the global-scale overturning (or "conveyor belt") circulation. The overturning circulation is the primary means by which heat is carried around the Earth and so exerts a strong control on regional and global climate. Recent studies have demonstrated that water mass transformations taking place in the Southern Ocean connect the upper (warm) and lower (cold) limbs of the ocean conveyor belt, in contrast to the traditional view that the upper and lower layers of the ocean were linked by vertical mixing at lower latitudes (eg Sloyan and Rintoul, 2000; Rintoul et al., 2001). A key link in the Southern Ocean overturning is the conversion of deep water upwelling at high latitude to less dense Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW).

When the SAMW and AAIW sink from the sea surface, they carry carbon dioxide into the ocean. As a result, the subantarctic zone is the largest zonally-integrated ocean store of anthropogenic carbon dioxide. The SAMW and AAIW are relatively fresh, and their export to lower latitudes closes the hydrological cycle by returning the excess precipitation falling at high latitudes. For each of these reasons - the overturning circulation, carbon storage, and the hydrological cycle - the SAMW and AAIW are critical components of the Earth's climate system.

Despite the importance of the SAMW and AAIW, our understanding of how these water masses are formed remains primitive. Historically, few observations have been available to describe the variability of SAMW and AAIW in time and space. The rapidly-increasing archive of profiling float observations allows the variability of SAMW and AAIW to be looked at in detail for the first time. By developing an understanding of how SAMW and AAIW have varied in the past, the project will provide insights into how the upper branch of the Southern Ocean overturning is likely to change in the future as a result of climate change.

Project Outline and Objectives:

1. To quantify the variability of Subantarctic Mode Water and Antarctic Intermediate Water in the Australian sector of the Southern Ocean.
2. To quantify the relative contribution of air-sea fluxes, Ekman transport and circulation to the variability in properties of the SAMW and AAIW.

1. To use the understanding of water mass formation developed in the project to evaluate how well state-of-the-art ocean circulation and coupled models reproduce the formation, circulation and variability of SAMW and AAIW, and to assess the likelihood of future changes to these water masses.

The project will use Argo float observations and repeat sections (CTD, XBT, and thermosalinograph) from the Southern Ocean to quantify the temporal and spatial variability of SAMW and AAIW properties. Changes in upper ocean stratification and in water properties like oxygen and chlorofluorocarbons (CFCs) will provide an indication of changes in ventilation with time. Measurements from a detailed survey of a meander of the Subantarctic Front will be used to assess the role of cross-front exchange in setting the properties of these water masses. Satellite altimetry will be used to assess variability in circulation pathways. Air-sea fluxes from atmospheric reanalyses will be used to relate the changes in water mass properties to changes in forcing and to estimate the contribution due to Ekman transport. Output from a variety of ocean GCMs and coupled climate models will be compared to the observations.