Tracking changes in the area, thickness, and volume of the Thwaites tabular iceberg “B30” using satellite altimetry and imagery

Abstract

Abstract. Icebergs account for half of all ice loss from Antarctica and, once released, present a hazard to maritime operations. Their melting leads to a redistribution of cold fresh water around the Southern Ocean which, in turn, influences water circulation, promotes sea ice formation, and fosters primary production. In this study, we combine CryoSat-2 satellite altimetry with MODIS and Sentinel-1 satellite imagery and meteorological data to track changes in the area, freeboard, thickness, and volume of the B30 tabular iceberg between 2012 and 2018. We track the iceberg elevation when it was attached to Thwaites Glacier and on a further 106 occasions after it calved using Level 1b CryoSat data, which ensures that measurements recorded in different acquisition modes and within different geographical zones are consistently processed. From these data, we map the iceberg's freeboard and estimate its thickness taking snowfall and changes in snow and ice density into account. We compute changes in freeboard and thickness relative to the initial average for each overpass and compare these to estimates from precisely located tracks using the satellite imagery. This comparison shows good agreement (correlation coefficient 0.87) and suggests that colocation reduces the freeboard uncertainty by 1.6 m. We also demonstrate that the snow layer has a significant impact on iceberg thickness change. Changes in the iceberg area are measured by tracing its perimeter, and we show that alternative estimates based on arc lengths recorded in satellite altimetry profiles and on measurements of the semi-major and semi-minor axes also capture the trend, though with a 48 % overestimate and a 15 % underestimate, respectively. Since it calved, the area of B30 has decreased from 1500±60 to 426±27 km2, its mean freeboard has fallen from 49.0±4.6 to 38.8±2.2 m, and its mean thickness has reduced from 315±36 to 198±14 m. The combined loss amounts to an 80 %±16 % reduction in volume, two thirds (69 %±14 %) of which is due to fragmentation and the remainder (31 %±11 %) of which is due to basal melting.

Giant icebergs increase mixing and stratification in upper-ocean layers

“…Using the glider estimated depth mean flow rate gives 1.4 * 10 9 m 3 day −1 . Quantifying the freeboard change over time [29,38] attributed 1.7 * 10 9 m 3 day −1 to basal melting, meaning satellite estimates of melt rate are between 1.25 and 2.49 times our in-situ estimates. The satellite and in-situ estimates of basal melt are thus in broad agreement, especially considering the assumptions and inherent differences in measurement.…”

“…These included a uniform melt rate around the circumference of A-68A, an assumption that the iceberg is flat-bottomed without pockets of meltwater stored in crevasses beneath, and that all melting emanates from the base. The ice density at the base of the iceberg is assumed to be 915kgm −3 [38]. Given the observation of intrusions up to 3 km from the edge of the berg, we assume this band is the "influence area"; integrating this and the mean meltwater content in the profiles yields an estimated basal meltwater contribution of 1.9 * 10 8 m 3 .…”

Grounding Event of Iceberg D28 and Its Interactions with Seabed Topography

“…The melting rate of iceberg A38B varied from 1.2 m per year to 10.8 m per year with its trajectory from Ronne Ice Shelf to the warm sea water near the Antarctic Peninsula [56]. Iceberg B30 had an average thinning rate of 17.3 ± 1.8 m per year estimated with satellite altimetry data in 6.5 years [23]. Melting rates of icebergs between 60 and 150 • E were 11-18 m per year from ship-based observations over 15 years [57].…”

“…Satellite altimeters are used to estimate the iceberg freeboard and thickness, especially on large icebergs. Studies of tabular icebergs, such as iceberg A68, C28, and B30, are carried out with data from the European Space Agency (ESA) Envisat Advanced Synthetic Aperture Radar (ASAR), Sentinel-1 SAR, and Cryosat-2 SAR Interferometric Radar Altimeter (SIRAL) to establish changes in area, freeboard, thickness, and volume over years [19,22,23]. Luckman identified several grounded icebergs by averaging time-series SAR images and found a submarine ridge in the western Weddell Sea [18].…”

Impact of Giant Iceberg A68A on the Physical Conditions of the Surface South Atlantic, Derived Using Remote Sensing

“…Satellite data are better able to measure an iceberg's long‐term evolution. Space‐borne instruments are commonly used to observe the physical decay of giant icebergs, as volume lost from the iceberg base and sidewalls over time (Braakmann‐Folgmann et al., 2021; Jansen et al., 2007; Li et al., 2018). Others have examined the impact of the meltwater on the surface ocean.…”