Sea ice covers millions of square kilometers of the Earths ocean surface. Therefore it significantly regulates the surface fluxes of water, heat and momentum between the ocean and the atmosphere. Moreover, sea ice is important for the climate on Earth because: It hampers gas exchange between ocean and atmosphere, it reflects a large portion of sunlight and itcontributes to the formation of deep and bottom waters which are part of the global ocean circulation. Examining the changes of sea ice has thus become an important field in Earth System Science.Thickness and extent are the two main characteristics of a sea ice cover and are important indicators of climatic changes. Sea ice extent is measured with microwave sensors from satellites since 1979 and shows a large-scale retreat of Arctic sea ice. Also the ice thickness in the Arctic reduced, as shown by upward sonar measurements from submarines since 1953. With the rapid decline of Arctic sea ice, also the Antarctic sea ice cover has attracted morescientific interest. The extent of Antarctic sea ice shows a small but significant positive trend for the period since satellite measurements began. But contrary to the Arctic, our knowledge about the long-term development of Southern Ocean sea ice thickness is still very limited. There are two main reasons for this lack of information: (1) The thickness of sea ice is still not routinely measured from space with sufficient accuracy and (2) there are no submarinemeasurements of ice draft for the Antarctic. But various airborne and in-situ techniques -like electromagnetic induction sounding, laser altimetry, ship-based observations and drilling - have been successfully applied in different regions of the Southern Ocean. However, the data gained by these methods are often biased towards thin ice and provide only shortsnapshots of the ice thickness.To date, the only way of monitoring the long-term variations of the sea ice thickness in the Southern Ocean are moored upward looking sonars (ULSs). These instruments are attached to the upper end of a mooring rope and can measure over periods of up to two years. The basic principle of a ULS draft measurement is transmitting ultrasonic sound pulses towards the surface and measuring the travel time of the reflected sound signal. Knowing thesound velocity, the travel times can be converted into distances. With the precise knowledge of the instrument depth, the detected time intervals can be used to calculate the thickness of the subsurface portion (draft) of the sea ice. The Alfred Wegener Institute (AWI) maintains an array of 13 ULSs in the Atlantic sector of the Southern Ocean since 1990, which provides a unique dataset of Antarctic sea ice thickness.This presentation introduces the ULS-dataset and shows first results of the variability of sea ice thickness in the Weddell Sea. One goal of this project was to assimilate all available ULS-data that have been processed by different methods since 1990. The obtained dataset shows, that the monthly mean sea ice thickness at the tip of the Antarctic Peninsula decreasedby almost two meters since 1990. Contrary, the ice thickness near the Fimbul Ice Shelf in the southeastern Weddell Sea shows a positive trend for the period 2000-2008. As there were still gaps in the thickness record due to instrument failure or loss of moorings, the missing data for the eastern Weddell Sea were filled by an iterative method based on multichannel singular spectrum analysis (M-SSA). The resulting time series span a period of 12 years and enable the assessment of interannual variability. Whereas thickness changes in the eastern Weddell Sea show no distinct trend, significant changes occurr close to the Antarctic, in the region of the Antarctic coastal current.