How does the Southern Annular Mode control surface melt in East Antarctica?

New research shows that the Southern Annular Mode’s influence on surface melt varies across East Antarctica due to its effects on the local weather conditions, including air temperature, snowfall, wind speed, and cloud cover. 

The study, published in Geophysical Research Letters, could help to improve future projections of East Antarctica’s contribution to global sea level rise.  

PhD candidate Dominic Saunderson, the SAEF researcher based at Monash University who led the study, says that most of East Antarctica’s surface melt occurs on ice shelves, which are floating extensions of the ice sheet surrounding ~75% of the continent. This melt potentially has global implications. 

“Ice shelves are an important control on global sea level rise because they can slow the flow of ice into the ocean. When melt occurs on ice shelves, they can become weaker, and in some cases, they can even disintegrate entirely. Because surface melt is expected to increase in a warming climate, understanding these processes is crucial for improving future projections.” 

In Antarctica, the amount of ice that melts each summer is influenced by the Southern Annular Mode (SAM), a weather pattern that describes the north-south shift of westerly winds in the Southern Hemisphere. It has three phases – positive, neutral and negative – and affects rainfall, snowfall, and temperature across the Southern Hemisphere, including in Australia and Antarctica. 

When the SAM is in a positive phase, the westerly winds are further south than normal,  meaning the winds around Antarctica are stronger, and the continent becomes more atmospherically isolated. When the SAM is in a negative phase, the westerly winds are further north, and the winds around Antarctica are weaker. 

“During a negative phase of the SAM, there is more surface melt in East Antarctica,” said  Saunderson. “People have reported this relationship before, but in this paper, we look at the reasons why this relationship exists and show that the reasons are different for different parts of East Antarctica.”

This figure shows the amount of surface melt and the physical processes responsible for it in December and January (1980 – 2018)

Saunderson and the team used a climate model adapted for the Antarctic region to study the influence of the SAM on 27 ice shelves across six regions. The team investigated the amount of surface melt and the physical processes (including air temperature, snowfall, wind speed and cloud cover) responsible for it each summer over the past 40 years. 

“In the Wilkes Land sector, a “negative” SAM corresponds to warmer air temperatures, heating the surface and leading to more melt. However, in the Dronning Maud Land sector, a “negative” SAM is associated with less snowfall and therefore darker surfaces, which absorb more sunlight and cause more surface melt,” Saunderson said.  

Median summer melt flux (1980-2018). Dashed lines separate the 27 ice shelves.

On average, 95% of an ice shelf’s total melt occurs in December and January each summer. However, the study also found the strength of the relationship between the SAM and surface melt differed throughout the summer, with the relationship being stronger in December than in January. This difference highlights the importance of the snowmelt-albedo feedback for driving melt – when snow begins to melt, it becomes darker than fresh snow, causing it to absorb more sunlight and thus melt even further. Therefore, the SAM in December is particularly important for how much melt occurs in a summer, because it can control how early this process is initiated.

“Future work needs to build on these results to better understand how other climate drivers, such as the El Niño-Southern Oscillation, and weather events, such as atmospheric rivers, affect surface melt, as well as how they interact with the SAM. These answers could be critical for helping us to predict how likely, and how large, extreme melt events could be in the future,” Saunderson said. 

Read more

Saunderson, D., Mackintosh, A. N., McCormack, F. S., Jones, R. S. & van Dalum, C. T. (2024) How Does the Southern Annular Mode Control Surface Melt in East Antarctica? Geophysical Research Letters, 51(6), e2023GL105475. https://doi.org/10.1029/2023GL105475