Volume 4, Number 3, Winter 1996
Antarctic Zooplankton: A look
into the future
Luiz F. L. Fernandes
Antarctic Slope Front Water: In the transition zone between the ocean
and shelf regions of the Antarctic
Seong Joong Kim
Ocean heat transport across
the Antarctic Circumpolar Current
A look into the future
Luiz F.L. Fernandes
Everything started on a trip to the Southern Ocean in the Austral Summer of 1992. There we went-John Wormuth, Marilyn Yeager and I-with a ton of equipment on board NOAA's R/V Surveyor as a part of the United States Antarctic Marine Living Resources Program. This program was designed to test whether or not the distribution of krill is affected by both physical and biological aspects of their habitats, and if krill predators respond to changes in the availability of their food.
One of our objectives within this program was to use our net system, the Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS), to obtain information not only on krill, but also on the other zooplankton in the Elephant Island region. The MOCNESS is a very complete sampling tool giving us nine nets in a single frame with conductivity-temperature-depth sensors so we can take multiple samples in a specific column of water at various depths. It provides the perfect opportunity to study interaction between biological and physical oceanography.
Our sampling site, Elephant Island, is a northern component of the South Shetlands Islands. Around it is a very dynamic water regime with several water masses present, making this area a unique environment. We took our samples mainly from two of these water masses, the Weddell-Scotia Confluence (WSC) and a transition water formed by the mixing of WSC with Drake Passage water. Within these two water masses, we also collected samples over the island's shelf, shelf-break, and slope at different times of day and night to draw a more clear picture of the environment I was about to study.
With this information in hand, I went back to the samples collected and looked at the dominant zooplankton. The most representative groups I found were the copepods, euphausiids, chaetognaths, amphipods, polychaetes, salps, and fish larvae. Within these seven groups, eleven species were selected for being the most abundant in the samples. Among them were the euphausiid Euphausia superba (Antarctic krill), the copepods Calanoides acutus and Calanus propinquus, the amphipod Themisto gaudichaudii, and the salp Salpa thompsoni.
Statistical analyses showed varying degrees of associations between abundances of the above mentioned species and groups and between the species and the different water masses, diel periods, geographical areas, and bottom topography. I found that over distances of one to ten kilometers, water masses were the most important physical factor influencing the abundance of most types of zooplankton, and bottom depth was locally important within these two water masses. Changes in abundance were also observed with the time of day, with E. superba and E. frigida more abundant at twilight, and the other common species generally more abundant at night. I also observed swarms of E. superba in the region but they were not correlated with the abundance of the other zooplankton, suggesting that krill swarms do not exclude the other zooplankton from their aggregations via food competition as previously thought.
Studies like this are extremely important for monitoring the environment, giving us a better understanding of changes over time and distance. Hopefully more research will be done in the future in this area, providing us with more information on this still pristine environment that serves as nursery and habitat for so many different species.
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Antarctic Slope Front Water
In the transition zone between the ocean and shelf regions of the Antarctic
Seong Joong Kim
People have made an effort to understand the significant role of the Southern Ocean in oceanography and climate studies due to its immense volume (about one fifth of the total world ocean) and free communication with other oceans. In particular, the water masses produced along the Antarctic continental margin contribute to the formation of bottom water, thereby influencing the properties of the global ocean. The importance of these processes has caused interest in analyzing the characteristic mixing of water masses in the Antarctic continental margin. The purpose of my study is to analyze the characteristics and describe the distribution of a specific water mass, called the Antarctic Slope Front Water, observed in the Antarctic continental margin.
The most common and well understood water mass in the Southern Ocean is warm, saline Lower Circumpolar Deep Water (LCDW), which rises up toward the Antarctic continental margin and meets cold, dense water, called Shelf Water, at the edge of the continental shelf, producing a third water mass in the transition zone between the oceanic and shelf regimes. This third water mass is called Antarctic Slope Front Water.
Antarctic Slope Front Water (ASFW) is defined as water with a temperature between about 0.2°C and about -1.7°C and salinities greater than about 34.4. Most ASFW is produced by horizontal mixing between LCDW and Shelf Water, but some dense ASFW seems to be produced by vertical mixing with dense Shelf Water.
ASFW is observed in three sectors along the Antarctic continental margin. In the Pacific sector, this water extends westward from about 160°W in the eastern Ross Sea to eastern Wilkes Land at about 125°E. In the Indian Ocean sector, ASFW is observed around the Amery Basin between 110°E and 60°E, but it is not observed in the western Indian sector except between 45°E and 50°E. This water mass is observed in a relatively large area west of 30°W in the Atlantic sector. ASFW extends past the tip of Antarctic Peninsula and disappears near 63°W.
From the open ocean to the shelf of the Antarctic continental margin, the surface layer is filled with Antarctic Surface Water. LCDW is observed commonly in the subsurface layer of the ocean around Antarctica. However, Shelf Water is not observed everywhere around Antarctica. Therefore, the existence of ASFW depends on the presence of Shelf Water.
Antarctic Slope Front Water seems to be significant in understanding the ventilation of Antarctic deep water and the formation of bottom water. This water is produced in and distributed around the regions where deep and bottom waters are produced, such as Weddell Sea, the Amery Basin, off Wilkes Land, and the Ross Sea. Furthermore, some Antarctic Slope Front Water s dense enough to sink to depth. As it descends to the intermediate or deep areas, Antarctic Slope Front Water seems to entrain LCDW and attain the properties of Antarctic bottom water.
Map of Antarctic water masses [~25K]
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Ocean heat transport across the Antarctic Circumpolar Current
Global circulation of the ocean-atmosphere system is driven by the sun's radiation. At high latitudes, the heat lost by long-wave radiation is greater than the heat gained by short-wave radiation and the earth loses heat. At low latitudes, the former is less than he latter and the earth gains heat from space. To maintain a balance of Earth's heat, called the heat budget, the ocean-atmosphere system transfers heat from the tropics to the polar regions. The role of oceans has been widely recognized since publications estimating ocean heat transport in the 1950s.
In the Southern Ocean, poleward ocean heat transport is a key element in the heat budget and the circulation around Antarctica. A better estimate for this transport will lead to improved understanding of the ocean's role in the global heat balance and the climate change. For my dissertation I estimated poleward heat transport across five zonally closed ocean sections corresponding to the three fronts in the Antarctic Circumpolar Current (ACC). Along each section, the vertically averaged temperature is approximately constant. I also estimated the net air-sea heat exchange between the sections.
I considered primary ocean processes contributing to the total heat transport across the ACC. These include density-driven flow, surface wind-driven flow (Ekman flow), and eddies. Meso-scale eddies are the main mechanisms that transport heat southward and compensate for heat lost by the ocean to the atmosphere in Antarctica.
In 1987 Arnold Gordon and W. Brechner Owens estimated the annual mean air-sea heat exchange south of the ACC. By comparing my estimate of poleward ocean heat transport with their air-sea heat exchange rate, I evaluated the importance of the above-mentioned ocean processes to the ACC dynamics.
Employing hydrographic data, climatological wind data, and output of high-resolution, ocean circulation models, I identified two significant processes for poleward ocean heat transport across the ACC, a large equatorward heat transport by the Ekman flow, and the largest poleward heat transport by eddies. My total heat flux estimate is consistent with the previous and recent estimates.
Heat transport graph [~31K]
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Last updated January 31, 1997