Overall, it has been a rather warm fall season over West Antarctica. Time series of 2m temperature over Thwaites Eastern Ice Shelf show that the daily maximum temperature frequently exceeded 1 standard deviation above the mean, sometimes 2, and was frequently close to 0C in April and May.

With the atmospheric river, warm air advances towards the coast, with the 0C contour reaching Abbott Ice Shelf and Pine Island Glacier by May 18, and again on May 19-21. We can observe pockets of high temperature in the mountain lees up to Pine Island Glacier, indicating foehn conditions.

The first snowfall occurs on May 16, followed by a second, more intense wave that persists through May 19. By May 20, there is a noticeable eastward shift in the precipitation, but it produces one more wave of snowfall over Thwaites and surrounding regions from May 20-22.

What makes this event an atmospheric river is the combination of moisture and strong winds. The 850hPa northerly wind reaches 80kts over the Southern Ocean at 00Z on May 17, and 50kts at the Amundsen Sea Embayment coast by 09Z on May 18. These strong winds are sustained through May 22.

From May 19 - 22, the successive low-pressure systems merge and expand, producing a very stagnant system that continues to channel the flow of marine air towards the Amundsen Sea Embayment on its southeastern side. A strong ridge develops downstream, blocking flow across the Antarctic Peninsula.

From May 15 - 19, the cyclone's central pressure deepens (down to 940 hPa), and it moves towards the coast, initiating a direct north-south flow pattern onto Thwaites. The elongated upper level trough forms a cutoff low, supporting the development of successive low-pressure systems farther north.

At the start of this period, we see an oblong surface low-pressure system associated with a deep trough at 500hPa (blue dashed lines) offshore the eastern Ross region/Marie Byrd Land.

Many thanks to co-authors Andrew Winters, @catswx.bsky.social, Rudradutt Thaker, Léonard Barthelemy, Francis Codron, and @jonathanwille.bsky.social 🤩

‼️The bottom line: anthropogenic warming makes today's extremes far more common in the future, meaning future estimates of Antarctic contributions to sea level rise ought to take weather extremes into account‼️

This means that how we detect atmospheric rivers and attribute precipitation in the present-day, and how we adapt these methodologies to future climate states, will determine how we describe the importance of atmospheric rivers in the Antarctic climate system. 🌧️🐧

However, by raising the threshold for atmospheric river detection to account for increases in moisture, frequencies are comparable to the present-day, with small regional shifts aligned with changes in atmospheric circulation.

We find that atmospheric river frequencies are highly sensitive to increasing atmospheric moisture in the 21st century, leading to a doubling of frequencies and 2.5x increase in precipitation impacts under a medium-high emissions scenario. 👀📈🌨️