POTENTIAL CHALLENGES TO THE U.S. FLEET

Production, Distribution, and Drift of Icebergs in the Ross Sea, Antarctica, and Their Potential Impact on Sea-Ice Conditions

In spring 2000, several of the largest icebergs ever witnessed calved from the Ross and Ronne-Filchner ice shelves. These icebergs were named B15, A43, and A44 by the U.S. National Ice Center, and together they represent approximately 5,000 km3, or about 2.5 times the annual accumulation of ice on the entire Antarctic ice sheet. Despite the fact that the titanic size of these icebergs garnered a great deal of public attention, their creation was not glaciologically unusual or unexpected. The initial width of these icebergs, approximately 40 km, represents about 50 years of the forward flow of the ice shelves—floating glacial ice that is hundreds of meters thick—from which these icebergs calved; thus, their sudden appearance after 50 years of slow northward advance of the ice shelves represents the normal maintenance of Antarctica’s glacial ice coverage. In a steady state the various ice shelves, including the Ross Ice Shelf, must undergo calving of these behemoth icebergs about once every 50 years, simply because the Antarctic ice sheet is either in steady state, or very close to steady state. Aside from a small piece of the Ross Ice Shelf’s front located near 180 degrees longitude, the entire front of the ice shelf has calved back since B15 was released; this means that the next calving, all other effects being equal, is not expected until 2050 or so.

Following the release of B15 from the eastern half of the Ross Ice Shelf’s calving margin, the iceberg broke into several small pieces. Unlike the sibling pieces, B15A failed to take a course of drift that would eject it from the Ross Sea in a matter of a few months. Instead, B15A crashed into the ice front near Ross Island, spawning a smaller iceberg, C16, which quickly ran aground in Lewis Bay and finally settled itself into a “holding pattern” just north of Cape Crozier on the eastern end of Ross Island. B15A remained adrift in this holding pattern for the next four years (until November 2004, when it began to move away) constantly gyrating on the ocean’s diurnal tide.

The reason for B15A’s apparent attraction to the area just north of Ross Island is still a subject of research and debate. It is possible, for example, that the prevailing southerly winds in the region of McMurdo are blocked by the high topography of the island’s tall volcanic cones (Mt. Erebus and Mt. Terror), and this contributes to the protection of icebergs from the effects of fierce winds. Other factors contributing to the iceberg’s failure to flush from the area may include the inverse barometer effect (there is a persistent atmospheric low in the lee of Mt. Erebus and Mt. Terror), and general localized convergence of the ocean currents generated on the western side of the Ross Sea Polynya discussed below.

By April 2006, both C16 and B15A had left the area of Ross Island; the conditions that triggered this move are unclear. B15J continues to hover near Ross Island; however, this smaller, round-shaped iceberg tends to have less of an impact on sea-ice conditions because of its size and inability to move into shallower waters west of its current position (e.g., to run aground on the pinning point that held C16 for so long in a position that could “blockade” sea ice west of Cape Bird).

Ice Conditions in the Ross Sea and McMurdo Sound, Antarctica

With annual resupply of the U.S. Antarctic Program’s major bases, McMurdo and South Pole, currently dependent on ship access into McMurdo Sound, ice conditions in this region are a major factor in recommending a sound strategy in the context of this report. Large-scale ice conditions in the Ross Sea are characterized by the Ross Sea Gyre that transports water and ice in a clockwise fashion through the south-western Ross Sea off McMurdo Sound. The prevailing strong, offshore winds coming down the Ross Ice Shelf result in the development of the Ross Sea Polynya, a vast expanse of open water and thin ice maintained by wind-driven advection of ice to the north (Figure 7.1). This results in a sea-ice thickness gradient such that the thinnest ice in the Ross Sea is found in the very north along the marginal ice zone and the very south where young thin ice emerges from the polynya region (Jeffries et al., 2001). As a consequence of this distribution of thin ice, the ice cover in the Ross Sea typically recedes from both the northern and the southern edges during summer. In most years, however, some ice survives summer melt. This ice is typically confined to the eastern Ross Sea (see Figure 7.1) and occupies less than one-tenth of the total ice-covered area.

The seasonal ice retreat typically does not start until mid-November, with the summer minimum ice extent in the Ross Sea reached during mid-February. Thus, much of the icebreaking associated with resupply efforts (which, of course, are constrained by more than ice conditions, see Chapter 10) takes place one to three months before the climatological seasonal ice minimum in the Ross Sea sector. (This seasonal sea ice is relatively thin and does not pose an undue icebreaking burden.)

In the 1980s and 1990s, the Ross Sea sector of the Antarctic experienced an increase in maximum ice extent of about 9 percent per decade. At the same time, the ice cover of the neighboring Amundsen and Bellingshausen Seas declined by 10 percent per decade, suggesting that at least part of this increase is explained by advection of ice from the west. Despite the increases in ice extent, a freshening of the Ross Sea also points toward reduced ice production and hence overall thinner ice (Jacobs et al., 2002).

McMurdo Sound itself is characterized by a complex ice regime that depends strongly on the interplay of calving



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