email: rsummers@tric.nric.gov.au
Introduction
There is increasing evidence of the relationship between variability in the Antarctic Circumpolar Current (ACC), which forms the core of the Southern Ocean, and the El Niño-Southern Oscillation (EN-SO) phenomenon. EN-SO is the most significant periodic climatic event in the Southern Hemisphere which has been linked to drought cycles in Australia and may have global effects. It is believed that variability in the ACC can be monitored by using sea level as an indicator of the dynamics. Inter-annual variability of the ACC can be best monitored at places where the ACC is constricted in some way. There are three such places. Moving eastwards they are: Drake Passage, Kerguelen Plateau and Macquarie Ridge. Since the EN-SO phenomenon occurs in the Pacific Ocean and the Macquarie Ridge is the last obstacle to the ACC before it enters the Pacific Ocean, it could be argued that variability in the ACC can be best monitored at the last stage before entering the ocean where the effect the greatest.
Instrumentation to measure sea level in the Southern Ocean
In 1991, the Australian Antarctic Division, in collaboration with the National Tidal Facility of the Flinders University of South Australia and the Australian Surveying and Land Information Group, began a new long term project to monitor sea level in the Southern Ocean and where possible to monitor the ACC. The Antarctic Division has deployed a tide gauge on each of the sub-Antarctic islands of Heard Island and Macquarie Island and at four locations on the coast of the Antarctic continent. Four of these tide gauges are part of the GLOSS network; these are Mawson, Davis, Casey and Macquarie Island. Between Mawson at 62 E and Macquarie Island at 159 E lies 97 of longitude of the Southern Ocean.
The Southern Ocean must be one of the worst places in the world for the survival of oceanographic instrumentation. In the latitudes of Heard and Macquarie Islands, “the Furious Fifties” the coastal environment is subjected to extremes of wind and wave energy. In Antarctic waters, the presence of sea ice and icebergs can be fatal to instrumentation. In both cases the design of the instrumentation is critical to the survival of the instrumentation in what is intended to be a long-term project, i.e. a minimum of 20 years. While these conditions are a threat to instrumentation, they are also an opportunity to re-evaluate how these parameters should be measured and to what degree of accuracy. A map showing the locations of these tide gauges and instrumentation designs can be found by connecting to http://www.antdiv.gov.au/aad/sci/human/tgn/tgn.html.
Initial Results
Antarctic tide gauges
The Mawson tide gauge has now been operational for two and a half years with continuous data. The Davis tide gauge has been operational for a similar period but a problem has developed with turbid sea water which has prevented access to the tide gauge and thus the data for the past six months. The second generation tide gauge at Casey has still not been fully deployed and the tide gauge at Commonwealth Bay, which can log two years of data on board, is not expected to be accessed until December 1995 at the earliest.
Sub-Antarctic tide gauges
The tide gauge at Heard Island which has no data telemetry but which can log up to four years of data on board, will not be visited until January 1996 at the earliest. There is considerable interest in the data from Heard Island which, like Kerguelen and Amsterdam Island, lies on the Kerguelen Plateau, one of the optimal sites for monitoring variability in the ACC. Heard Island is also very remote, it is 4000 km west of Perth in Western Australia, and the logistics of maintaining a tide gauge there are very difficult. If it is a crucial site for monitoring the ACC and its consequent potential links with EN-SO then a second generation tide gauge which can both withstand the oceanographic environment and provide high quality near real time data will have to be built.
The tide gauge at Macquarie Island, which is the eighth tide gauge to have been deployed on the island, has been operational since December 1993, although the early records contained some gaps. Since December 1994, however, the tidal record is continuous. Analysis of the data to date has concentrated on the sea surface gradient between Macquarie Island and Spring Bay in southern Tasmania. It is believed that variations in the sea surface gradient reflect changes in the ACC and this may be the first stage in detecting variability.
A temporary tide gauge was set up for about six weeks in March/April 1995 at Hurd Point at the southern end of the island in order to assess whether the main installation at Garden Cove, which is on the east coast of the island and relatively sheltered, was recording a true tidal signal. An analysis of the data shows that while the tidal regime is the same, there are variations in sea level between the two locations. The instrumentation is identical at both places and they are only 30 kilometres apart. It is believed (R. Williams personal communication) that the sub-Antarctic Front which normally lies well to the south of Macquarie Island may move north periodically and wash the shores of the island. Questions arise as to what is the relationship between variations in the movement of the sub-Antarctic Front and inter-annual variability of the ACC?
What next?
The importance of monitoring inter-annual variability of the ACC to attempt to detect links with EN-SO has overtaken global sea level rise in response to the “Greenhouse Effect” as the predominant scientific issue. The importance of the Macquarie Ridge in modifying the ACC will continue to be assessed and, for this reason, the Hurd Point tide gauge at the southern end of Macquarie Island will be reinstated for a period of a year to examine the effects seen during the temporary deployment in more detail. Hurd Point faces the full force of the Southern Ocean so this will not be any easy task. Sea level is currently being monitored at the southern end of the ACC at a similar longitude with a tide gauge in Commonwealth Bay. Variations across a complete transect from Antarctica to Spring Bay, Tasmania in the north, may give some insights into the behaviour of the ACC over a 12 month period. Heard Island will also be assessed for similar reasons, as will Mawson and Davis which are at the southern end of the Kerguelen Plateau.
The Australian Antarctic and Southern Ocean Sea Level Network is poised to play an exciting part in unravelling the connections between the Southern Ocean and El Ni¤o.
References
Summerson, R.M.V.. and Handsworth, R.J. 1995. Instrumentation for Sea Level Measurements in Antarctica and the Southern Ocean. In O. Bellwood, H. Choat and N. Saxena, Eds. Recent Advances in Marine Science and Technology ‘94. PACON International and James Cook University of North Queensland, July 1995.