Antarctic Iceberg Characterization and Variability from 60+ Years of Ship-Based Observations in the Southern Ocean

In an ongoing effort to improve characterization of icebergs in Antarctica, a joint program between Shirshov Institute of Oceanology of Russian Academy of Sciences (IO RAS) and Arctic and Antarctic Research Institute (AARI) is being undertaken to collect and bring together the results of in situ observations of icebergs and iceberg properties from ships traveling in the Southern Ocean. Within this program iceberg observation data are recovered from ship log journals, are obtained from published research cruise reports and are acquired from various internet resources. As a result we have developed one of the largest collections of in situ iceberg observations which incorporates more than 60, 000 records characterizing the iceberg concentration, size distribution and iceberg shapes. The core of the dataset comprises ∼45 000 iceberg observation reports delivered since the middle of the 1950s from research vessels of AARI. The dataset also incorporates the collection of Norwegian Polar Institute which incorporates the results of ∼4000 ship observations of Antarctic icebergs dating back to 1978-1987 and about ∼7000 iceberg observations conducted under the auspices of the Australian Antarctic Program (AAP) during the period from 1978 to 2001. Most observations in the dataset provide information solely on the iceberg concentration (or number count) in the vicinity of the ship. Observations collected by NPI and AAP also characterize the iceberg size distribution. About 4 000 records present the results of morphometric measurements of selected individual icebergs (including their shape, freeboard and length) and a small part of the dataset of about 2 000 records characterizes the frequency of iceberg occurrence by shape. The dataset is routinely updated with most recent observations and newly recovered historical data.

In the poster we provide details on the content of the developed dataset and summarize the results of the study of Antarctic icebergs mean distribution and variability based on the collected data. The focus of the work is on small and medium sized icebergs ranging from 15m to several km in length. Since most observations in the dataset have been collected in the south of the Atlantic and Indian Ocean, the available statistics on the iceberg occurrence in these parts of the Southern Ocean was much more extensive than in the Pacific sector. The analysis of the dataset revealed an overall highly uneven distribution of the icebergs across the Southern Ocean. The iceberg concentration generally increases towards the coast reaching 50-100 icebergs within a circle of 15 nautical miles radius (equivalent to one iceberg per 25 -50 km²) in the vicinity of the coastal line. The mean iceberg concentration below 50°S in the Atlantic and Indian Ocean sectors of the Southern Ocean and below 54°S in the Pacific sector amounts to about 8 icebergs observed within a circle of 15 nm radius (which is equivalent to one iceberg per ~ 311 km²). The estimated variability of the iceberg concentration amounts to about 100% of the mean iceberg concentration value at a particular location. There are three well pronounced clusters of elevated values of the iceberg concentration in the Atlantic Ocean within 0°–60° W, in the Indian Ocean within 40°–120° E and in the Southern Pacific within 90°–170°W (see the map of the mean iceberg concentration below). The area in the Atlantic Ocean corresponding to the Weddell Sea has the maximum northward extension with large numbers of icebergs north of 50°S. We explain the development of these clusters by the location and productivity of outlet glaciers and ice fields, prevailing ocean currents, specifics of the ocean floor topography and atmospheric circulation. It is shown that at least in the Atlantic and Indian Ocean the iceberg distribution pattern experiences long-term variations well-correlated with El Nino/Southern Oscillation (ENSO) events. The strongest effect is observed in the region east of the Drake Passage where the iceberg concentration increases by about 50% during years of the negative ENSO phase.

We further combined the iceberg concentration data combined with information on the iceberg size and shape distribution to estimate the area-wide cumulative characteristics of Antarctic icebergs. The instantaneous number of icebergs in the Southern Ocean was estimated as 132 269 with an uncertainty of 7%. The area and volume of icebergs were equal correspondingly to 55 805 km² and 16 893 km³ with uncertainties of 32–33%. Our estimates of the total number and area of Antarctic icebergs are generally within the range of values obtained by other authors; however the estimated total volume of ice in the icebergs is mostly larger than earlier estimates.

Mean concentration of Antarctic icebergs as the number of icebergs within a circle of 15 nm