SITASSO
The Sea Ice Thickness in the Atlantic Sector of the Southern Ocean
Sea-ice has a profound influence on the regional climate and the polar physical environment in both hemispheres. It also plays a major role in the global climate system, due to its ability of reflecting sunlight or its influence on the freshwater budget of the polar oceans. Examining the changes of sea-ice has thus become an important field in Earth System Science.
With the rapid decline of Arctic sea-ice, also the Antarctic sea-ice cover has attracted more scientific interest. Contrary to the sea-ice extent and concentration, the thickness of sea-ice is not routinely measured by satellites with sufficient accuracy. Moreover, submarine sonar measurements of ice thickness are not available for the Antarctic. The only way of monitoring the long-term variations of the sea-ice thickness in the Southern Ocean are moored Upward Looking Sonars (ULSs).
Fig.1: Mooring arrangement with ULS
The basic principle of a ULS measurement is transmitting ultrasonic sound pulses towards the surface and measuring the travel time of the reflected sound signal. Knowing the sound velocity, the travel times can be converted into distances. With the precise knowledge of the instrument depth, the detected time intervals can thus be used to calculate the thickness of the subsurface portion (draft) of the sea-ice (Fig.1). The draft can then be converted into total ice thickness using empirical relations obtained from drillings in the Weddell Sea.
Since 1990 the Alfred Wegener Institute (AWI) maintains an array of ULSs on 13 positions in the Atlantic sector of the Southern Ocean (Fig.2). The instruments measure the ice draft for 2-3 years and are recovered and redeployed by RV Polarstern.
Fig.2: The AWI ULS-array in the Atlantic sector of the Southern Ocean. The diagrams show the available ULS-data from 1990-2007. Six ULSs (red frames) are currently deployed
This project is focused on the analysis of ULS-data in terms of sea-ice thickness variations and possible long-term trends. The goal is to identify the forcing mechanisms of sea-ice thickness changes by finding possible correlations between the ice thickness and other oceanic and atmospheric properties (e.g. the Southern Annular Mode). The available time series are long enough to enable spectral analysis techniques.
Another goal of this project is to set up a consistent dataset of ULS draft measurements which can be used to validate satellite-derived ice thickness estimates and sea-ice models.