Antarctic iceberg distributions and calving rates

<p><strong>Antarctic iceberg distributions and calving rates</strong></p><p>We present the first results from a database of 34 695 individual ship observations of Antarctic icebergs that were collected by identical recording protocols in a programme established in 1981 through the Scientific Committee on Antarctic Research (SCAR). From the austral summers 1982/1983 to 1997/1998, icebergs were recorded by most research vessels cruising Antarctic waters. Data were collected until the 2009/2010 season, but from a decreasing number of ships. The database was compiled in 2021 as the SCAR International Iceberg Database, and can be downloaded at https://doi.org/10.21334/npolar.2021.e4b9a604. It contains positions of 377 169 icebergs, of which 300 763 icebergs were classified into five different length classes for 80% of the observations, and seven classes for 20%. The observations cover most parts of the waters surrounding Antarctica, but with large differences in data density. Collected before the era of polar orbiting satellites and on–board remote sensors, these data on Antarctic iceberg sizes and distributions are material upon which future remotely sensed iceberg data can be compared and tested. Approx. 90% of the observed icebergs are in sizes smaller than those recorded in past and current satellite-based iceberg databases, making the ship–based data particularly useful for discussions of calving and dissolution rates, and iceberg contributions to fresh water flux to the Southern Ocean.</p><p> </p><p>Icebergs ˃200 m account for 28% of the Antarctic iceberg population. From the largest size classes and down, the icebergs approximately double in numbers for each size step, implying that fracture is the dominant dissolution process for tabular icebergs. On the other hand, attrition by melting is the dominant process for the dissolution of icebergs < 200 m horizontal dimension. The iceberg frequencies vary considerably in the coastal waters. High densities in some sectors of East Antarctica indicate higher iceberg calving rates here than presently concluded from satellite observations, perhaps because smaller calvings are not detected. The ship observations also indicate higher calving rates than reported for glaciers calving into the eastern Weddell Sea. The off-shore waters show high iceberg frequencies in four exit zones. The largest of these extends north-eastwards from the Antarctic Peninsula. The highest off-shore iceberg frequencies are, however, seen north of the Amundsen Sea, suggesting high calving rates from Pine Island Glacier and Thwaites Ice Stream in the last two decades of the past century.</p><p> </p><p>Satellite data suggest that annually ~100 icebergs larger than 1 km² calve from the ice shelves exceeding 10 km² in area. These clearly represent the main part of calved iceberg masses. On the other hand, the ship observations indicate that altogether 100 000 icebergs ˃10 m calve annually from Antarctica. Around 120 000 icebergs are afloat in the austral summer, of which 40 000 are found in the zone within 200 km from the coast.</p><p> </p><p>Iceberg frequencies for different size categories demonstrate that larger icebergs veer more to the left, i.e. they “hug” the continent in the counter-clockwise current, while they take a more northerly route than small icebergs in the clockwise Antarctic Circumpolar Current.</p><p> </p><p> </p><p> </p>