Scientists have observed for some time that the level of sea ice concentration tends to increase shortly after an Arctic cyclone passes over. But in August of 2012, the powerful Great Arctic Cyclone traversed the entire Arctic. Shortly after its passage, scientists recorded the lowest sea ice levels ever, so they thought that the cyclone may have contributed to the sea ice loss. This conundrum sparked the interest of Erika A. P. Schreiber, a graduate student with the National Snow and Ice Data Center (NSIDC) at the University of Colorado Boulder, and her supervisor, Mark Serreze.
They began to study how cyclones affect the Arctic sea ice in different seasons by tracking cyclones from 2012 to 2016 and measuring how the sea ice concentration changed over 2 days after the event. Temperature changes seem to be the main reason why sea ice concentration fluctuates, Schreiber described in a poster presented in December at AGU’s Fall Meeting 2018 in Washington, D. C.
Fierce Winds and Changing Temperature
Sea ice plays a key role in the energy balance of the polar regions, which influences the global climate and ocean circulation. “Tracking is especially important now that the ice extent is decreasing all the time,” said Schreiber.
As the total sea ice decreases in the Arctic Ocean, traffic from tourism and other sources is becoming more common, as is drilling for oil and natural gas. This increase in activity has brought attention to the need to understand sea ice behavior year-round, but scientists have not yet been able to accurately predict the waxing and waning through the seasons.
Changes to sea ice concentration are mostly caused by the interplay of dynamic and thermodynamic processes, both of which are affected by the passage of cyclones. Dynamically, cyclonic wind promotes ice movement and wave action that can break up the ice. Thermodynamically, cyclone clouds trap bounced longwave radiation, which warms up the ice surface, while also blocking shortwave radiation from the Sun, which cools the ice surface.
First, Schreiber and Serreze used a cyclone tracking algorithm developed by the NSIDC. They found that on average, there were as many as 40 days with cyclone activity in a 3-month season from 1990 to 2017 in the Arctic. However, cyclones were more common around the North Pole during the summer, whereas in the winter they appeared more frequently at the Arctic’s edges.
Using passive microwave satellite data from the Advanced Microwave Scanning Radiometer 2 (AMSR-2), a remote sensing tool that records weak microwave emission from the surface of Earth, Schreiber and Serreze found that the sea ice concentrations are greater after a cyclonic event, especially in the fall and winter, and at the edges of the Arctic, where the change was up to 10% per day.
The team wanted to know whether this concentration buildup was mainly due to dynamic or thermodynamic processes. For this, they measured how much the sea ice moved after the passage of a cyclone using passive microwave data. They found that the sea ice movement was minimal, so they presume that thermodynamic factors are resulting in the greater sea ice concentration.
But these conclusions might not be accurate, Schreiber admits. “I don’t fully trust this,” she said. Measuring sea ice movement, or divergence, and assuming in its absence only thermodynamic processes could be affecting the sea ice change may not be the best approach, according to David A. Bailey, an oceanographer at the National Center for Atmospheric Research who was not involved in the study. “It’s a little more complicated than that.”
More Ice on Warmer Water
Next, the team looked at the temperature changes in the sea ice surface after a cyclone passed. Using ERA-Interim data, the researchers found that although closer to the pole temperatures dropped, nearer the Arctic’s edges, temperatures actually rose up to 10°C after a cyclone passed, even in winter. Therefore, ice concentration in the southernmost regions rises despite the higher temperatures.
“I’m really surprised by the temperature anomalies,” said Schreiber. “Ice freezes more when there’s a cyclone—but it’s warmer.” Those warmer temperatures are still below zero, but the results add to the complicated behavior of sea ice after a cyclonic event.
“There’s still some work to be done,” said Bailey, who studies the sea ice component of the Community Earth System Model (CESM). “But it’s a good step in the right direction.”
The team will next look at the sea surface temperature at the Arctic’s edges to see whether warm water coming up from the Atlantic during a cyclone explains why the temperature in that area rises. Another interesting factor to look at, Schreiber said, would be the cyclonic wind’s direction and velocities. “It’s complex.”
“Under a climate change scenario, you can imagine more cyclonic activity in the Arctic,” Bailey said. Cyclones may not increase in number but definitely increase in strength, he added. “As they penetrate into the central Arctic, they’re going to definitely have an impact on the sea ice.”