October 26, 2020

How does Antarctic sea ice differ from Arctic sea ice?

Besides residing on the opposite side of the planet, Antarctic sea ice differs from Arctic sea ice in important respects. Compared to Arctic sea ice, Antarctic ice melts back to a smaller extent in summer, and grows to a greater extent in winter. Whereas Arctic sea ice shows a strong declining trend, Antarctic sea ice shows little or no trend, and exhibits much greater year-to-year variability. During the 2010s, Antarctic sea ice set records for both the highest and lowest extents in the satellite record, which dates back to 1979.

Differences in Arctic and Antarctic sea ice behavior result largely from different land-ocean configurations.

The Arctic and Antarctic are polar opposites

Because the Northern and Southern Hemispheres experience opposite seasons, Antarctic sea ice usually reaches its annual maximum extent in late September or early October, and reaches its annual minimum in late February or early March—roughly the reverse of Arctic sea ice minimums and maximums. But the differences between the Arctic and Antarctic go beyond opposite timing.

“The two polar regions are essentially geographic opposites,” said Sharon Stammerjohn, a sea ice expert at the University of Colorado Institute for Arctic and Alpine Research (INSTAAR). “Sea ice in the Arctic Ocean is land-locked, while sea ice in the Southern Ocean is surrounded by open ocean.” These roughly opposite land-ocean configurations have significant implications for sea ice behavior.

Minimum and maximum Antarctic sea ice extents
Sea ice concentration in the Southern Ocean around Antarctica during February 2020 (summer minimum) and September 2019 (winter maximum). The gold line shows the median ice extent (the total area that is at least 15 percent ice covered) from 1981 to 2010. Maps by Climate.gov, based on data from NSIDC

Arctic sea ice is heavily influenced by conditions on surrounding landmasses. In recent decades, temperature has been the largest factor contributing to sea ice decline. As North American and Eurasia have experienced warming conditions and decreasing snow cover, the Arctic Ocean has warmed and sea ice has dwindled. When Arctic sea ice does grow, surrounding landmasses limit how far that ice can spread.

The Antarctic presents the opposite configuration. Unlike Arctic sea ice, which can extend all the way to the North Pole, Antarctic sea ice fringes Antarctica’s coast. The ice can get no closer to the South Pole than the coastline will allow. So, compared to Arctic ice, most Antarctic ice forms at a lower latitude, closer to the Equator. When that ice melts back in the spring and summer, it can shrink to a smaller extent than what is observed in the Arctic because it only has to melt back as far as the Antarctic coastline before it is gone. When it grows in the fall and winter, Antarctic sea ice can spread out across the Southern Ocean, pushed by winds and waves, its spread unimpeded by land.

These geographic differences make for greater year-to-year variability in Antarctic ice. Antarctic sea ice grows to about 18 million square kilometers (7 million square miles) in winter, versus 15 million square kilometers (5.9 million square miles) in the Arctic. Antarctic sea ice melts back to about 2.6 million square kilometers (1 million square miles) in summer, versus 6.5 million square kilometers (2.5 million square miles) in the Arctic.

Antarctica has younger, thinner ice

Compared to the Arctic, Antarctic sea ice is less likely to survive the summer melt season. On average, about 40 percent of the Arctic Ocean’s winter ice cover remains at the summer minimum. In the Southern Ocean, only about 15 percent remains at the end of summer. Most of Antarctica’s sea ice is only one winter old at most. So even though it may cover a larger area in winter, Antarctica’s sea ice is thin, often 1 meter (about 3 feet) or less.

Relationship status: complicated but crucial

“Antarctic sea ice is governed more by wind than by temperature,” said Earth Science Observation Center senior research scientist Ted Scambos. “The effects of climate change play out differently in the Southern Hemisphere than the Northern Hemisphere.”

Surrounding the Antarctic continent, Antarctic sea ice occupies the vast Southern Ocean. Natural cycles in this ocean can exert a strong effect on sea ice, in part through a phenomenon known as the Southern Annular Mode (SAM). SAM is a pattern of westerly winds circling the frozen continent. Besides sea ice, SAM influences weather patterns as far away as Australia.

El Niño-Southern Oscillation conditions affect SAM, meaning it is partially driven by natural oscillations, but human activities also play a part. Greenhouse gas emissions spur global warming, which nudges SAM into more frequent positive mode. The resulting wind behaviors generally increase Antarctic sea ice extent. SAM’s behavior also affects the Amundsen Sea Low (ASL), a low-pressure center related to climate variability in West Antarctic and nearby ocean waters. ASL has a complex influence on sea ice movement near the Antarctic Peninsula.

Scientists are still working to fully understand SAM, ASL, and their influence on Antarctic sea ice. What scientists do understand is that neither their relationship with each other nor with sea ice is straightforward. So while climate change has a discernible effect on Arctic sea ice, it has a scattershot, complicated relationship with Antarctic sea ice. Sea ice scientists continue to research how that tricky relationship works.

Sea ice affects land ice

Antarctic sea ice influences the fate of Antarctica’s land-based ice, especially where that ice meets ocean water.

Ice shelves are thick slabs of ice that extend from shorelines over adjacent ocean water. Glaciers feed most ice shelves along the Antarctic coast. Buoyed by ocean water, ice shelves act as emergency brakes on the glaciers feeding them. Warming conditions can weaken ice shelves, however, making them vulnerable to disintegration. When an ice shelf is already weakened by warming, the presence or absence of sea ice takes on a bigger role in the shelf's fate.

Wilkins Ice Shelf disintegration
A Nature study concluded that sea ice retreat contributed to the disintegration of the Wilkins Ice Shelf in 2008. The ice shelf disintegrated rapidly, with substantial changes apparent after a single day. Credit: NASA

Sea ice absorbs wave energy. When sea ice in front of an ice shelf retreats, offshore storms send ocean swells toward the shelf. If the ice shelf is already flooded and fracturing along its outer margin, the ocean swells can further weaken it, leading to breakup events. In the wake of an ice shelf disintegration, the glacier(s) feeding the shelf may accelerate, introducing new ice into the ocean that then melts and increases sea levels.

Researchers suspect that sea ice retreat in the Weddell Sea along the northern tip of the Antarctic Peninsula probably contributed to historic losses in the Larsen Ice Shelf in 2002, and a rapid collapse of the Wilkins Ice Shelf in 2008 and 2009. A 2018 study in Nature called for models of ice sheet behavior to consider the impacts of sea ice and ocean waves.

References

Massom, R. et al. 2018. Antarctic ice shelf disintegration triggered by sea ice loss and ocean swell. Nature 558: 383-389. doi:10.1038/s41586-018-0212-1.

NSIDC. 2020. Sea ice index. https://nsidc.org/data/seaice_index/

NSIDC. 2020. Charctic. https://nsidc.org/arcticseaicenews/charctic-interactive-sea-ice-graph/

New South Wales Government. 2011. Southern Annual Mode. https://www.youtube.com/watch?v=G-S-YmE-Lkc

UCAR. 2020. Amundsen Sea Low. https://climatedataguide.ucar.edu/climate-data/amundsen-sea-low-indices