University of Otago, NIWA, PhD

Seasonal variation in ocean density stratification during growth and decay of Antarctic sea ice

The aim of this project is to understand the fine-scale processes operating in the upper ocean beneath forming and melting sea ice which lead to multi-layered density stratification.  These have been the focus of several studies in the Arctic but complications arise when transferring this knowledge to coastal Antarctica.  This is mainly due to processes related to circulation within, and communication with, adjacent ice shelf cavities - phenomena that affect ~44% of the Antarctic coastline. The export of Ice Shelf Water (ISW), defined as being cooler than the surface freezing point of seawater (~ -1.9°), to the open ocean is one of the main differences caused by the presence of an adjacent ice shelf cavity.

The spatial and temporal variability in the supply of ISW in McMurdo Sound will be the focus of a 3-dimensional numerical modelling study of the seasonal circulation within the sound. For this purpose, one of the models currently in use for regional scale modelling will be further developed to incorporate features such as the flow of supercooled water, frazil ice formation and the dominance of horizontal velocities by tides. In addition, a number of simulations will be run using a separate 1-dimensional layered model in order to study the impact of overturning and mixing under various scenarios. It is hoped that by combining these two modelling approaches on the relatively small scale of McMurdo Sound, a better representation of sub-sea ice processes in regional climate models can be achieved.

Field Season 2007-08: At each site along 2 transects of McMurdo Sound single Conductivity-Temperature-Depth (CTD) profiles will be collected in order to investigate spatial variability in distribution of water masses, particularly ISW. In addition, repeat profiles at some of the sites will be used to investigate processes such as brine rejection and short-term convective events. Temporal variability will be investigated by collecting current meter data and repeated, high-resolution hydrographic profiles at a site near Scott Base over a number of days early and later-on in the season.

This suite of observations is designed to lead to a more detailed knowledge of ocean circulation in McMurdo Sound and an improved understanding of features of the ice-ocean interaction.  Some of these features are unique to McMurdo Sound, while others are representative of the sub-sea ice processes of the broader Ross Sea region. The observational data will also be vital for extending and validating the numerical modeling study.