The Breakup of the World’s Largest Iceberg, “A23a”: The Trajectory of Iceberg A23a Observed by “GCOM-W”, “ALOS-2”, “ALOS-4” and the Latest Satellite, “GOSAT-GW”

The world’s largest iceberg, A23a, which originated from the Antarctic ice sheet and had been drifting northward, has been confirmed to have moved to the northeast of South Georgia Island in the South Atlantic Ocean and broken apart. This massive iceberg has been closely monitored because the movement and condition of A23a affect the island’s wildlife and shipping routes. So far, JAXA has monitored A23a using GCOM-W, ALOS-2, and ALOS-4, which observe microwave radiation and backscatter from the Earth’s surface^{*1, 2}. The Advanced Microwave Scanning Radiometer 2 (AMSR2) on board JAXA’s GCOM-W has a relatively low resolution of several kilometers to several tens of kilometers, but its wide observable range and high revisit frequency allow it to provide a temporally continuous view of the location and approximate shape of a large iceberg, such as A23a. In contrast, the phased array type L-band synthetic aperture radars (PALSAR-2, PALSAR-3) on board ALOS-2 and ALOS-4 observe relatively less revisit frequently than AMSR2 but have a high resolution of several meters to several tens of meters, allowing detailed observations of iceberg features such as area changes and surface cracks. Since these microwave-based sensors can observe the Earth’s surface globally regardless of cloud cover or darkness, they are suitable tools for continuously monitoring the dynamics of iceberg A23a. Following up on the previous article^*2 on iceberg A23a, this article extends the analysis of iceberg A23a’s trajectory and condition through October 2025 and introduces the latest observations of iceberg A23a from a new microwave radiometer, AMSR3^*3 on board GOSAT-GW launched on June 29, 2025.
Iceberg A23a broke away from the Filchner Ice Shelf in 1986 and remained stranded on the ocean floor of the Weddell Sea for over 30 years. However, it detached from the seabed around 2020 and began to drift. Figure 1(a) shows the results of monthly tracking of iceberg A23a since January 15, 2022, using data from GCOM-W/AMSR2. For about two years, the iceberg traveled northwest across Weddell Sea along the Antarctic continental shelf slope and then turned northeast under the influence of the Antarctic Circumpolar Current. Around February 27, 2025, it ran aground again on the continental shelf off South Georgia Island. Iceberg A23a remained in the same location for about three months, but due to ongoing collapse and melting during this period, it detached from the continental shelf around mid-May and began moving east again (Fig. 1b). After that, it moved counterclockwise around South Georgia Island. By September, when it had made its way to the northern part of the island, A23a’s area had shrunk by about half due to a series of large collapses that had occurred in the meantime.
Figure 1(c) shows the latest GOSAT-GW/AMSR3 brightness temperature (TB) map capturing iceberg A23a. As of October 10, 2025, iceberg A23a has broken into several large pieces, and the progression of its disintegration is clearly visible. As evident from changes in its surface area, the iceberg is releasing large amounts of cold, fresh water into the ocean. This raises concerns about the impact on marine ecosystems.

Figure 1. (a, b) Tracking results of iceberg A23a derived from AMSR2, and (c) an AMSR3 observation map (R: 36 GHz V-polarized TB, GB: 36 GHz H-polarized TB) acquired around 3:30 a.m. on October 10, 2025. Panel (a) shows monthly tracking results from January 2022 to March 2025, and Panel (b) shows biweekly tracking results from June to September 2025. The background color represents seafloor topography from ETOPO (https://www.ncei.noaa.gov/products/etopo-global-relief-model). The yellow box in (c) indicates the extent of ALOS-4 image shown in Figure 2. Note that this figure uses AMSR3 resolution enhanced TB with the grid spacing changed from approximately 10 km to a 5 km grid. The red star shows the location where the iceberg was grounded from the time it broke away from the Filchner ice shelf in 1986 until around 2020. The yellow star shows the location where the iceberg grounded again off South Georgia in March 2025.

Next, we observed more detailed iceberg conditions using images from the ALOS-2 and ALOS-4 with higher spatial resolution. Figure 2 (a) shows an image observed in the ALOS-2 ScanSAR mode (spatial resolution: 100 m), and Figure 2 (b) shows an image observed in the ALOS-4 Stripmap mode (spatial resolution: 10 m). The iceberg that had run aground in the Weddell Sea measured approximately 4,050 km² on August 29, 2015 (Fig. 2a), and had shrunk to about 1,350 km² by October 9, 2025 (Fig. 2b). The high-resolution ALOS series data allow us to observe the condition of the iceberg in detail and see that it is gradually collapsing from the edge. As spring arrives in the Southern Hemisphere and sea temperatures rise, the iceberg will melt and break up more quickly and it is expected to disappear soon. ^*4

Figure 2. Enlarged view of observation images from ALOS-2 and ALOS-4. (a) shows an image observed by ALOS-2 on August 29, 2015, and (b) shows an image observed by ALOS-4 on October 9, 2025. Both (a) and (b) were composited by assigning red and green to the HV polarization and blue to the HH polarization.

By utilizing the strengths of GCOM-W, GOSAT-GW, ALOS-2, and ALOS-4, which use microwaves, we can observe changes in snow and ice in the Antarctic region, where in-situ observations are extremely difficult. JAXA plans to continue observing the area with GCOM-W, GOSAT-GW, ALOS-2 and ALOS-4.

Reference：

*1. Results of observations of the drifting large iceberg “A23a” by ALOS-2 (in Japanese)
https://www.eorc.jaxa.jp/ALOS/jp/library/general-topics/20231205_nankyoku_j.htm.

*2. The world’s largest iceberg, A23a, may have run aground on the continental shelf of South Georgia: The trajectory of iceberg A23a observed by “GCOM-W”, “ALOS-2” and “ALOS-4”
https://earth.jaxa.jp/en/earthview/2025/02/28/8535/index.html

*3. The AMSR3 sensor continues more than 23 years of observations of the Earth’s waters from the AMSR series. This series began with the AMSR-E on board the U.S. Aqua satellite, which was launched in 2002. The AMSR series is Japan’s microwave radiometer series. It boasts the world’s highest performance thanks to its high observation frequency, large-diameter antenna that achieves high spatial resolution, and observation channels that cover low to high frequencies. By observing the faint microwaves emitted or scattered by natural sources, such as the Earth’s surface and atmosphere, it is possible to measure sea surface temperature, wind speed over the ocean, precipitation, soil moisture, snow depth, and sea ice concentration. These data are utilized not only for understanding climate change phenomena such as the decline in polar sea ice extent, but also in fields including weather forecasting, fisheries (aquaculture), and ship navigation support. The AMSR series contributes to a wide range of fields. For more information on AMSR3 and the AMSR series, please refer to the JAXA Satellite Navigation “Observing Earth’s Water: AMSR.”
https://www.satnavi.jaxa.jp/ja/news/2025/06/17/11087/index.html

*4. CNN Climate, World’s biggest iceberg, A23a, has broken up
https://edition.cnn.com/2025/09/03/climate/worlds-biggest-iceberg-a23a-broken-scli-intl