Kaleschke, Lars
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Item type:Publication, The sea ice thickness in the Atlantic sector of the Southern Ocean(2013-05-28); ; ; 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.doctoral thesis369 168 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Improvements and analyzes of sea ice drift and deformation retrievals from SAR images(2020-10-26); ; ; Sea ice is a key component of the climate system since it regulates the exchange of heat, the moisture, and the salinity in the polar oceans. The sea ice cover is set into motion and drifts, deforms and fractures under the action of external forces and respective boundary conditions. A positive trend in sea ice mobility, differential kinematics and deformation has been observed in the last decades. This increasing dynamics can increase the ice-albedo feedback loop, one of the most important feedback mechanisms related to climate change. The kinematics of sea ice is observed from space by synthetic aperture radar (SAR) and quantified using drift detection algorithms. The requirements that the applied algorithm has to meet regarding resolution, reliability and computing time vary with the type of application. Sea ice kinematics and associated deformation processes occur across a wide range of spatial scales, from meters to thousands of kilometers. It is important to understand how large-scale behavior monitored by satellite relates to and depends on small scale processes of sea ice. Thus, the major objective of this study was to review and validate recent research results of sea ice drift and deformation characteristics obtained at larger scales by extending the analyzes towards smaller scales and examine emerging difficulties and challenges.doctoral thesis371 179