The sea ice thickness in the Atlantic sector of the Southern Ocean

The remoteness of the Antarctic sea ice field and the fact that there is still no reliable way to measure its thickness from satellites renders it one of the most difficult geophysical parameters to measure. In this study, variations in sea ice thickness were examined by means of upward looking sonar (ULS) measurements in the Weddell Sea. ULS instruments measure the subsurface portion (draft) of the ice as a proxy for sea ice thickness. Both quantities are linearly related. A consistent and extensive data set of sea ice draft was produced and its quality was investigated in detail. The obtained draft time series were studied in terms of variability, trends, oscillations and autocorrelation. A correlation analysis was performed to examine the geophysical driving mechanisms for changes in sea ice thickness and their relation to climate variability in the Southern Hemisphere. It was shown that sea ice draft can be measured with an accuracy of 5-12 cm, depending on the season. Furthermore, it was demonstrated that biases introduced by high ice concentration can be avoided by applying a simple sound speed model. Different significant oscillations with sub-annual periods and periods of up to 3-4 years were found in the draft time series. It was also shown that autocorrelation of sea ice thickness extends over up to eight years. Trends in the ice draft could be observed in three regions of the Weddell Sea: (1) A strong decline of about 2 m at the tip of the Antarctic Peninsula between the periods 1991-1992 and 2005-2007. (2) A statistically significant trend of -3 cm per year in the northeastern Weddell Sea during 2001-2005. (3) A statistically significant trend of 9 cm per year in the southeastern Weddell Sea during 2000-2004. The decrease in ice thickness in the peninsula region was found to be due to changing patterns of ice advection and is therefore not the result of temperature changes. The ice thickness, however, has become more sensitive to surface air temperature in the recent years. The trend in the northeastern Weddell Sea was explained by reduced ice advection from the central Weddell Sea. The upward trend in the southeastern Weddell Sea can be assumed to be the result of increasing advection of thicker ice from further south in combination with falling air temperatures due to increasing southerly winds. It was found that the measured sea ice draft responds to different large-scale climate modes and processes - such as El-Nino Southern Oscillation (ENSO) - due to: (1) advection of thinner or thicker ice from other regions over the ULS position, (2) varying surface air temperatures and (3) changing drift speed and ice deformation.