FRISP Report No. 16 (2005) Evolution of tabular iceberg A-38B, observation and simulation Daniela Jansen, Henner Sandhäger and Wolfgang Rack Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany Introduction Calving of Antarctic tabular icebergs is a decisive factor in the overall mass budget of the Antarctic ice sheet. With an ice loss of about 2000 Gt per year it is by far the largest negative term in the mass balance equation (surface ablation ~30 Gt yr -1 , basal melting ~550 Gt yr -1 . Jacobs et al., 1992). The processes triggering melting and decay of tabular icebergs are of great relevance within many domains. Being a source of cold freshwater during their drift and especially during their final decay, tabular icebergs have a significant impact on Southern Ocean water masses (Gladstone et al., 2001). Dust particles enclosed in the ice, released during melting processes, can have a fertilizing effect on algae in the upper water column. Thus, melting icebergs can also affect biology (Arrigo et al, 2002). Furthermore the decay processes of icebergs are of great interest to shipping, as fast disintegrating icebergs, releasing a huge amount of smaller icebergs in short time, can be a severe threat to shipping lines in the Southern Ocean. In spite of its apparent relevance, little is known about tabular iceberg evolution and decay at the moment. To learn more about the role of ice dynamics in these processes, we simulate the inherent dynamics and evolution of gigantic tabular icebergs, by applying the numerical iceberg model COMBATIS (Computer-based Tabular Iceberg Simulator) based on the fundamental equations of ice shelf dynamics. In the following we will depict the evolution of a typical Antarctic tabular iceberg and present results of a simulation with the model COMBATIS (Jansen et al., submitted). Iceberg A-38, observed evolution In October 1998 the tabular iceberg A-38 calved off the Ronne Ice Shelf east of Berkner Island. This area of the Ronne Ice Shelf was characterised by several pronounced inlets, slowly grown in decades, obviously caused by stresses due to the Hemmen Ice Rise. The inlets were filled with a mélange of sea ice, snow and small icebergs (Hartl et al., 1994; MacAyeal et al., 1998). The crack which lead to the calving of iceberg A-38 followed the connecting line between the tip of an inlet perpendicular to the ice shelf front and one front-parallel inlet at Hemmen Ice Rise, forming a tabular iceberg with an approximate size of 150 km by 50 km (Fig. 1). The calving probably occurred instantaneously, following a straight line except to a small bifurcation in the middle of the iceberg. The iceberg was first detected on October 13 th on radar data of an USA military weather satellite. The description of the further development of A-38 is also based on remote sensing data, which proved to be the ideal mean of observation, regarding the size and the position of the observed object. Shortly after the calving, the tabular iceberg split in two halves of about equal size: A-38A, the formally eastern part of the iceberg, still containing two major inlets of about 50 km length, and A-38B, the western part. The two parts then drifted along the Weddell Gyre in direction of the Antarctic Peninsula and then northwards. The drift velocity varied strongly with sea ice coverage and thickness, the icebergs moved much slower in winter. In February 2003 they reached the tip of the Antarctic Peninsula and proceeded October 27, 1998 A-38B A-38A Inlets HIR Ronne Ice Shelf 50 km Figure 1: Icebergs A-38A and A-38B shortly after breaking apart. HIR: Hemmen Ice Rise. Image courtesy: RADARSAT.