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The interannual variability of the sea ice in the Southern Ocean and its evolution projected for the end of the 21st century are investigated using observations and different types of models. First of all, none of the known atmospheric modes of variability are able to explain much of the total sea ice extent variability in the Southern Ocean. However, they have large influences on the local and regional scales. In particular, the response of the sea ice to the Southern Annular Mode is characterized by a dipole between the Ross Sea and the region around the Antarctic Peninsula caused by a low pressure anomaly in the Amundsen Sea in high SAM-index years. Secondly, the sea ice extent in the different regions seems to be mostly uncorrelated, showing that the total sea ice cover cannot be seen as a single entity, but merely as a combination of regional covers. Finally, it is shown why the projected distribution of sea ice is not a simple extrapolation of the current sea ice trends. The mechanisms responsible for the regional variability of the future sea-ice extents are discussed.

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Approved for public release; distribution is unlimited. / The unique properties of the temperature and salinity profiles for polar oceans are critical for high-latitude mixed layer thermodynamics. In the Polar regions the water column is coldest and freshest at the surface where ice may be present. This density structure often leads to entrainment and affects both the mixed layer depth and the ice thickness. Thermobaricity, the combined dependence of seawater thermal expansion on temperature and pressure, magnifies the buoyancy flux associated with mixed layer convection. When thermobaricity amplifies entrainment so that the heat into the mixed layer is greater than the heat leaving the water column, the mixed layer warms and any existing ice begins to melt. Similarly, if the heat entrained is less than the heat leaving the column, the mixed layer cools and freezing occurs at the surface. In the former situation a polynya, or region of no ice surrounded by ice coverage, may form. A one-dimensional vertical model is built, and trial cases are run to show the intricate relationships that govern the heat and salt fluxes and subsequent ice thickness. The model shows the importance of thermobaricity to the air-sea-ice interactions. It also offers significant insight into how relatively constant atmospheric forcing can lead to polynya-like conditions. / Ensign, United States Navy

A criterion is defined to compare seasonal ice coverage in the Greenland Sea for the years 1900-57, and the areal coverage is graphed using the 1898-1913 average as a standard. The factors wielding possible influence on short-term variations of the ice cover are examined individually and their relative importance established. The influence of ocean currents is evaluated by analysis of hydrosections across the East Greenland Current at 74°-76° N and across the North Atlantic Current in the Faeroe-Shetland channel. Data from the latter area are used for numerical analysis of heat imported to the Greenland Sea by the North Atlantic Current in the 1927-52 period. Details about the Irminger Current's behavior are derived from station data from Denmark Strait and from surface temperatures at Selvogsbanki south of Iceland. Year-to-year variations are found to exist in the flow volumes of all three currents, and correlations with seasonal ice coverage in the Greenland Sea are shown. Above-average precipitation in conjunction with below-average storm activity is found to have negligible influence on the ice regime, and no significant correlation is found. The possible effect of evaporation is computed to be far below the threshold of detectability. Air temperatures in the Norwegian-Greenland Sea region display a trend of increase throughout the period studied, in harmony with a concurrent trend of decreasing ice cover; but no causal relationship is in evidence. The effects of strengths and directions of predominant winds are examined, and good correlations are shown between ice cover fluctuations and easterly wind components at Norwegian coastal stations. At the points of major currents' entrances to and exits from the Greenland Sea the wind effects are complex and cannot be fully evaluated on the basis of existing data. The fluctuations of ocean currents entering and leaving the Greenland Sea and of water movements within the Greenland Sea remain as the apparent determinant of year-to-year variations of the ice cover. / Graduation date: 1968

The interannual variability of the sea ice in the Southern Ocean and its evolution projected for the end of the 21st century are investigated using observations and different types of models. First of all, none of the known atmospheric modes of variability are able to explain much of the total sea ice extent variability in the Southern Ocean. However, they have large influences on the local and regional scales. In particular, the response of the sea ice to the Southern Annular Mode is characterized by a dipole between the Ross Sea and the region around the Antarctic Peninsula caused by a low pressure anomaly in the Amundsen Sea in high SAM-index years. Secondly, the sea ice extent in the different regions seems to be mostly uncorrelated, showing that the total sea ice cover cannot be seen as a single entity, but merely as a combination of regional covers. Finally, it is shown why the projected distribution of sea ice is not a simple extrapolation of the current sea ice trends. The mechanisms responsible for the regional variability of the future sea-ice extents are discussed.

Thesis (M.S. in Physical Oceanography)--Naval Postgraduate School, June 2003. / Thesis advisor(s): Roland W. Garwood, Arlene Guest. Includes bibliographical references (p. 59-61). Also available online.

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