April 2024 Sea Ice

The melt season begins

Arctic

Arctic-wide sea ice extent in April averaged 14.31 million km². This was about 1.4 percent below the 1991-2020 average and the highest April average extent since 2013. Figure 1 shows the April average ice concentration. Only the Sea of Okhotsk had notably less extensive ice than usual for this time of year.

Fig. 1 April 2024 average sea ice concentration. Image courtesy National Snow and Ice Data Center

The decrease in Arctic-wide sea ice extent from March was a bit over 700,000 km², which is close to the average March to April change.

---

Alaska and vicinity

Sea ice around Alaska was fairly typical for the past decade (excepting 2018 and 2019 sea ice crash years). The median daily sea ice concentration from the AMSR2 satellite derived data is shown in Fig. 2.

Fig. 2 April 2024 median daily sea ice concentration using the high resolution AMSR2 passive microwave data. AMSR2 data courtesy University of Bremen.

North of the ice edge, lower concentration ice is noticeable in eastern and southern Norton Sound and on the south sides of St. Lawrence Island and the Chukotka Peninsula. These are all favored areas for early season open water due to winds and currents both in the spring and earlier in the winter. North of the Bering Strait, ice concentrations (at this scale) were much closer to 100 percent, though areas of lower median concentration ice were found just west of Pt. Barrow, Alaska and in the southeastern Beaufort Sea offshore of the Northwest Territories coast.

Figure 3 shows the April average extent each year since 1979. This year, the Bering Sea April average ice extent was 623,000 km², which is very close to the averages in April 2014-16 and 2021-23, but a little lower than the average April extent prior to 2014.

Fig. 3 April average sea ice extent in the Bering Sea 1979-2024. Data courtesy National Snow and Ice Data Center.

---

End of April Sea Ice

Conveniently, the last day of April 2024 brought clear skies to most of the northern Bering and southern Chukchi Seas region. I’ve annotated the Suomi NPP satellite’s truecolor image, which is roughly what you would see if you were riding around on the satellite 518 miles have the ground, with a pair of high powered binoculars, in Fig. 3. There’s an immense amount of detail visible, from areas of mostly snow-free ground starting to appear to different kinds of sea ice as well areas of open water.

Fig. 4 April 30, 2024 314pm AKDT Suomi NPP truecolor image with annotations. Image courtesy of the Geographic Information Network of Alaska (GINA), University of Alaska Fairbanks.

As the melt season kicks in each Spring, the southern edge of the pack ice moves north as ice melts and patches of open water develop farther north in response to local winds and currents. Just where these areas develop and where ice persists varies significantly from year-to-year, and of course the ice is quite mobile as it’s pushed around by currents and winds, so any given area may see ice come and go multiple times before the ice melts out completely.

This interannual variability is nicely illustrated by Fig. 5, a “postage stamp” collage of the April 30th sea ice concentration for the past nine years as analyzed by the National Weather Service Alaska Sea Ice Program.

Fig. 5 Sea ice concentration analysis on April 30 each year 2016-2024 from the National Weather Service Sea Ice Program. Images extracted from Alaska Ocean Observing System (AOOS) website.

The winters of 2017-18 and 2018-19 were the historically low ice seasons in the Bering Sea and this was still evident at the end of April.

---

Technical details and underlying data sources

The NSIDC Sea Ice Index is based on the comparatively low resolution passive microwave data (nominal 25km² by 25km² but in practice is not that good) and given as the 5-day trailing average, so slightly lags changes in sea ice. At regional to Arctic-wide spatial scales this is usually not significant. Details on the Sea Ice Index are here. The regional daily data is available as an Excel spreadsheet here.

AMSR2 microwave data is from the Advanced Microwave Scanning Radiometer 2 on JAXA's Global Climate Observation Mission - Water 1 (GCOM-W1) satellite at 10km² by 10km² horizontal resolution. The University of Bremen AMSR2 landing page with links to near real-time data in a variety of formats is here.

A wide array of Alaska and polar related satellite derived products are available from the Geographic Information Network of Alaska (GINA) at the University of Alaska Fairbanks, here.

The National Weather Service Alaska Region Sea Ice Program produces daily, multi-sensor based and expert interpretation ice analysis products which are available here.

The Alaska Ocean Observing System has a vast array of ocean related data available, including archives of otherwise hard (or impossible) to find information. The near-daily archive of NWS Alaska Region sea ice concentration analyses starting in March 2010 is available here.