SASKOS (2005/2006)

Summer Atmosphere Study at Kohnen Station

The Summer Atmosphere Study at Kohnen Station, Antarctica (SASKOS) was carried out as a 2.5 months measurement program in austral summer 2005/2006 at the drilling site of the European Project for Ice Coring in Antarctica (EPICA) on the plateau of Dronning Maud Land (Fig. 1).

To study the influence of the local wind regime on the formation of small-scale accumulation patterns like snow dunes (Fig. 2), synoptic observations were combined with a glaciological field program. At the same time, forecasts of the Antarctic Mesoscale Prediction System and satellite images were used to study the large-scale and regional synoptic situation.

Analyses of shallow cores obtained at the EPICA drilling site Kohnen Station (75°00’ S, 00°04’ E; 2892 m a.s.l.) reveal the presence of conserved snow dunes in the firn. The snow dunes reduce air circulation due to their high density. If the layering propagates downwards to the firn-ice transition, the occurrence of snow dunes could affect the air entrapment. The

high-density layers serve as a lid, isolating the air from the atmosphere in lower depths than is expected in non-stratified firn. It follows that the age difference between enclosed air and the surrounding ice at a location like Kohnen Station becomes smaller than at sites with similar snow accumulation but without dune formation.

In situ observations

In situ observations during three dune formation events in the 2005/06 austral summer show that these periods were characterized by a phase of 2 or 3 days with snowdrift prior to dune formation which only occurred during high wind speeds of > 10 m s-1 at 2 m height caused by the influence of a low-pressure system. The dune surface coverage after a formation event varied between 5% and 15%, with a typical dune size of (4±2) m x (8±3) m and a maximum height of 0.2 ± 0.1 m. The mean density within a snow dune varied between 380 and 500 kg m-3 , whereas the mean density at the surrounding surface was 330 ± 5 kg m-3 . The firn cores reveal that approximately three to eight events per year occurred, during which snow dunes had been formed and were preserved in the firn.

Categorisation of strong-wind events at Kohnen Station

Based on an analysis for the 7 year period 1998-2000/2002-05, we conclude, that 80% of all strong-wind events at Kohnen Station were caused by synoptic evolutions, which belong to the following four categories (Fig. 3):

Category I: Lows which first move from the Antarctic Peninsula/ northern Weddell Sea southeastwards to a position close to Neumayer Station (70°39’ S,  08°15’ W, ~600 km north of Kohnen Station) and then continue to move to the east or northeast. (42% of events).

Category II: Lows initially situated off the coast east of the Greenwich meridian, which move to the west (retrograde movement) along the ice-shelf edge. (14% of events).

Category III: Lows which move from the Antarctic Peninsula/ northern Weddell Sea southeastwards to a position close to Neumayer Station and become stationary there. (12% of events).

Category IV: Lows which first move from the Antarctic Peninsula/ northern Weddell Sea southeastwards, then turn southwest and fill up close to the Filchner Ice Shelf. (12% of events).

The remaining 20% of events show different synoptic evolutions, but their number is too small to further categorize them (see  Birnbaum et al., 2010).

Fig. 3: Synoptic developments causing strong-wind events at Kohnen Station: pattern for category I [a], category II [b], category III [c] and category IV [d]. The red character L indicates the center of a

low-pressure system. The black straight lines and arrows show the track of a low-pressure system in a simplified way (Illustration: Gerit Birnbaum, AWI)