How Ocean Warming is Triggering a Regime Shift in Antarctic Sea Ice

Recent scientific discussions have centered on the unexpected decline of Antarctic sea ice, which was previously considered resistant to climate change. Research indicates that robust winds have stirred warmer deep ocean water, disrupting the protective layer above the ice, leading to its accelerated melt.

While Arctic sea ice has seen a dramatic decrease of approximately 40% over four decades, Antarctic sea ice had shown slight expansion until recent trends reversed this. Since 2015, the extent of sea ice has shifted from record highs to remarkable lows, akin to the area of Greenland.

According to a study conducted by Antarctic researchers, rising temperatures are primary contributors to this melting. Further investigations reveal that ocean warming has played a pivotal role in this significant ‘regime shift’.

As stated by Simon Georgie from the National Marine Center in Southampton, UK, “A thorough analysis reveals a convincing sequence of events where oceans have significantly impacted ice melting, particularly starting in 2016.”

The circumpolar deep water, a warm, salty ocean body, flows southward from tropical regions, encircling Antarctica at depths under 200 meters. Two decades of temperature and salinity data suggest that this warm water is gradually surfacing, contributing to sea ice melt.

Antarctica is flanked by intense winds and storms in the “Roaring 40s,” “Roaring 50s,” and “Screaming 60s.” Climate change is shifting these storm paths southward, increasing precipitation in sea ice regions. Earl Wilson and colleagues from Stanford University highlight that additional precipitation formed a fresh water layer on the surface, temporarily insulating the sea ice from the warmer waters below.

However, these southward-moving storms bring strong winds that push surface water and ice forward. The Earth’s rotation causes this water to disperse at a right angle to the wind direction, facilitating a vortex comparable to the Weddell Sea circulation. As surface water shifts away, deep water replaces it, promoting further ice melt.

From 2014 to 2016, the upwelling of deep water began to outweigh the insulated layer of fresh water created by precipitation. This was evidenced in a simple computer model that mimicked real-world ice expansion and contraction based on observed temperature and salinity changes.

“Indications suggest a continued decline in sea ice,” Wilson remarks. “Although precipitation may reduce deep-sea heat temporarily, that heat remains a factor. A sudden change in conditions could unleash it back into the environment.”

A follow-up study indicates this reversal was instigated by a sequence of storms. Theo Spira and his team at the Alfred Wegener Institute in Germany found that the intrusion of warmer deep waters, coupled with winter water effects, is exacerbated by increasingly warmer global temperatures.

This warming causes deep water expansion, reducing winter water thickness, and has resulted in flooding of deeper waters over time. Since 2015 and 2016, strong winds have exacerbated these conditions, without allowing the lamellar structure to stabilize.

Importantly, while wind patterns may be a natural phenomenon, global warming has set the stage for these changes, as noted by Spira, emphasizing that the ocean’s reactions to these winds might mitigate the rapid ice decline.

Although melting sea ice will not directly contribute to rising sea levels, it poses risks to species such as krill and penguins that rely on this ice for habitat. Additionally, if sea ice retreats near significant ice shelves, it may disrupt global ocean currents, including the Atlantic meridional overturning circulation critical for maintaining Europe’s climate.

“The reduction of sea ice formation in these areas could lead to diminishing bottom water and decrease the meridional overturning circulation,” explains Wilson, while acknowledging that freshwater from glacier melt has a more pronounced impact on these dynamics.