Ocean processes over the southern Weddell Sea shelf using seal tags

Lead Research Organisation: NERC British Antarctic Survey
Department Name: Science Programmes


Antarctic Bottom Water (AABW) is an important water mass in the cooling and ventilation of the World's deep ocean. One of the principal sources of AABW has its roots in the production of cold, dense water that results from wintertime sea-ice production over the continental shelf of the southwestern Weddell Sea. However, there remains great uncertainty about the processes controlling the initial import of the source waters onto the continental shelf, and the export of the dense waters from the shelf regime. The uncertainty results from the extremely challenging sea ice conditions existing in the southern Weddell Sea, especially during winter. Conditions in the area of interest during winter are exceptionally difficult for any ship-based work, and instruments deployed in the ocean during the rather less difficult summer months, and which are left to monitor the water properties over the Winter period, are vulnerable to dredging by passing icebergs. We will tackle the problem using a technology that has recently come of age. We propose to attach conductivity-temperature-deph (CTD) tags, miniaturised oceanographic instruments, to Weddell seals (Leptonychotes weddellii). The tags have a satellite transmitter that relays the oceanographic data collected during the seals' dives, together with the dive location. The tag is glued harmlessly to the animal's fur using standard marine two-component epoxy and comes off again during the annual moult about eleven months later. A pilot study undertaken by the British Antarctic Survey involved the tagging of four seals, three of which supplied over-winter datasets. Although the coverage was impressive from only three tags, emphatically confirming the practicality of the technique, the region of interest is nearly 500,000 km^2 in area and a comprehensive dataset requires substantially more tagged animals. We will tag 20 Weddell seals at the eastern end of the shelfbreak north of the Filchner-Ronne Ice Shelf during the late Austral summer of 2010/2011. The resulting dataset resulting from the animals' dives during the winter will give the most comprehensive picture to date of the ocean conditions over the southern Weddell Sea continental shelf. By mapping the temperature of the water near the sea floor we will determine the locations where dense waters leave the shelf, and the processes involved: either a direct flow down the slope under gravity, or initially mixing at the shelf edge with waters from off the shelf before descending down the slope. We will also be able to determine where the source waters come on to the shelf. Weddell seals are very accomplished divers, diving repeatedly for long periods and to depths regularly reaching the on-shelf seafloor. Among Antarctic seals, Weddell seals also inhabit the southernmost waters, and remain within the pack-ice in winter when the ice expands northward. These characteristics make Weddell seals ideally suited for the proposed study. Although primarily an oceanographic project, the movements and diving behaviour of Weddell seals is of great interest to seal biologists who wish to understand differing behaviours in different parts of Antarctica. These variations were ably demonstrated by the extraordinarily diverse behaviour of the animals tagged during the pilot, and by comparisons with previous tracking of this species in other parts of the Antarctic. The long-ranging movements displayed by some of the seals tracked during the pilot study are untypical for the species, at least at other Antarctic locations, and may be related to the local oceanographic conditions. It is widely recognised that multidisciplinary studies such as proposed here will provide us with the tools to better predict how the distribution, behaviour and ultimately population status may be affected by changing ocean and climate conditions.
Description Seal-derived, wintertime CTD data reveal strong seasonality in shelf conditions north of Filchner Ice Shelf Import of warm waters onto the Antarctic continental shelf has a major influence on the heat and salt budget of the coastal ocean, and is a potential heat source for ice-shelf basal melt. Summertime observations indicate a southward flow of modified Warm Deep Water (MWDW) on the eastern side of Filchner Depression. However, the lack of wintertime observations has meant that it has not yet been possible to establish whether the inflow exhibits a significant seasonality. Results from this project revealed the seasonality of the flow of MWDW onto the Filchner continental shelf using data from 19 Weddell seals (Leptonychotes weddellii) tagged with miniaturized conductivity-temperature-depth sensors in February 2011. During the following eight months the instruments yielded about 9,000 temperature-salinity profiles from an area for which there had hitherto been no wintertime data. The data show a pronounced decrease in warm water inflow from summer to winter. This result was further supported by an almost three-year long time series from a shelf-break mooring. The reduced wintertime inflow seems to be related to seasonal changes in wind forcing; stronger easterly winds during winter drives the off-shelf core of MWDW downward below the shelf break. Intrusions of warm water and its sensitivity to winds imply that the oceanographic regime of the southern Weddell Sea is susceptible to change on a variety of timescales.

A numerical model simulating the flow of water into the cavity beneath Filchner-Ronne Ice Shelf has demonstrated two different flow patterns, that depend on the amount of sea ice being formed over the continental shelf north of the ice front. One flow regime seems to be associated with a weaker forcing, which is likely to be the way the climate is presently shifting. The higher forcing scenario is the situation we seem to be in at present, with a relatively vigorous circulation, taking a different route around the ice shelf cavity.
Exploitation Route These results have been published in the scientific literature in two papers so far. The dataset itself is available via BODC so that it can be used by colleagues from elsewhere. We anticipate additional papers using the data once the numerical model of the southern Weddell Sea is running. A study using the seal-derived data to discuss the behaviour of Weddell Seals in the southern Weddell Sea has been published by collaborators at University of St Andrews. At least two different Norwegian studies have made use of the dataset for studying oceanographic processes to the east of the FRIS.
Sectors Environment

Description Our findings are contributing to improving our understanding of processes over the Filchner Depression. That understanding is essential if we are to determine the likely response of the region to future changes in climate. In addition, as the seal-derived ocean data have contributed to a large international depository of sea-mammal data, they been used in several publications. They are particularly valuable as they remain the only data from a key region of the southern Weddell Sea.
First Year Of Impact 2011
Sector Environment