The expedition was intense and felt more like going to the Moon than going on a typical research cruise. What took us by surprise were the many winter storms that battered the ice (and our ship and ice camp).
It has taken us years to collate these data but now we know the winter storms play a key role in the fate of Arctic sea ice, particularly in the Atlantic sector of the Arctic.
How winter storms amplify climate change
On average, about 10 extreme storms will reach all the way to the North Pole each winter. While these winter storms are short (they last on average 6-48 hours), they can be incredibly intense.
During a storm in winter 2015 we saw the air temperature rise from -40℃ (-40℉) to 0℃ (32℉) in just a day, and then fall back to -30℃ (-22℉) the next day, when cold Arctic air returned after the storm.
These storms bring heat, moisture and strong winds into the Arctic, and next we look at how they impact sea ice and its surroundings.
Warming and weakening the ice
The heat from the storms warms up the air, snow and ice, slowing down the growth of the ice. Moisture from the storms falls as snow on the ice. After the storm, the blanket of snow insulates the ice from the cold air, further slowing the growth of the ice for the remainder of winter.
The strong winds during the storms push the ice around and break it into pieces, making it more fragile and deforming it, more like a boulder field.
The strong winds also stir the ocean below the ice, mixing up warmer water from deeper waters to the surface where it melts the ice from below. This melting of the ice in the middle of winter can happen for several days after the storms when the air is already back to well below freezing.
Thinner ice, shelter for life and accelerated melting
The breakup of the ice opens big passages of open water between ice floes, called leads. In winter these passages end up refreezing rapidly, generating new super-thin ice.
These thinner refrozen patches of ice let more light through in the following spring, allowing ocean plants (phytoplankton) to bloom earlier.
The rougher sea ice landscape becomes a shelter for many ice-associated Arctic organisms, including ice algae, becoming biological hot spots in the following spring.
The broken up and deformed ice drifts faster, reaching warmer waters where it melts sooner and faster.
So really, winter storms precondition the ice to a faster melt in the following spring with an impact that continues well into the following season.
Why is Arctic sea ice declining?
The Arctic is particularly sensitive to human driven climate change. We know the decrease in sea ice is due to both the warming of the Arctic (air and ocean) and changing wind patterns that break up the ice cover.
But there are also amplifying mechanisms or “feedback” mechanisms, in which one natural process reinforces another. Their role in the decrease of sea ice is hard to predict. We now know winter storms in the Arctic contribute to these feedback mechanisms.
More storms ahead
Arctic winter storms are increasing in frequency and this is likely due to climate change.
With the thinner Arctic sea ice cover and shallower warmer water in the Arctic Ocean, the mechanisms we observed during the winter storms will likely strengthen and the overall impact of winter storms on Arctic ice is likely to increase in the future.
Two weeks ago, the Arctic sea ice reached its minimum extent for 2019, after another winter of intense winter storms. The minimum ice extent was effectively tied for second lowest since modern record-keeping began in the late 1970s, along with 2007 and 2016, reinforcing the long-term downward trend in Arctic ice extent. Arctic sea ice has been declining for at least 40 years, and amplifying mechanisms such as the winter storms are accelerating this retreat.
As highlighted in the recent IPCC Ocean and Cryopshere report, these changes in September sea ice are likely unprecedented for at least 1,000 years.
As we start taking into account feedback mechanisms like the winter storms, our predictions for the first Arctic sea ice free summer are indicating it will likely happen before 2050.