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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Atmosphere-Sea Ice interactions in the new Arctic

Alternative title: Interaksjoner mellom atmosfæren og havisen i det nye Arktis

Awarded: NOK 9.3 mill.

The Arctic is being radically transformed by ongoing climate change. This has important effects on peoples lives, not just in the Arctic where people are now experiencing a climate very different to the one they have experienced throughout the 20th century, but also further afield as the changes in the Arctic may also affect the weather across Europe. This transformation is particularly visible in the shrinking sea ice cover. The ice is getting thinner and this makes it easier for it to be broken up by the action of wind and waves. While in the atmosphere the weather is also strongly affected by the presence of sea ice. If the sea ice is thin, then a passing storm can break up the sea ice, possibly causing it to melt faster. But the storm may also be affected by the breaking of the ice cover as it is then 'fed' with more heat from the ocean. In ARIA we make use of a cutting-edge sea ice model driven by a regional atmospheric model to investigate the response of sea ice to the passage of cyclones in the Arctic. We have looked at rapid break-up events in the ice where large chunks of the sea ice break-off from the rest of the ice pack in quick secession, following the passage of a storm. And we have used an atmospheric model customised to simulate the dynamics of the harsh polar environment to drive the sea ice model. We took a case study of a particularly large break-up event that occurred in the Beaufort Sea during February and March of 2013, and we have shown that this sea ice model is capable of realistically capturing the fracturing of sea ice. This is the first time that this kind of rapid break-up of sea ice has been captured in a model. We found that during these events there is a temporary increase in the sea ice volume in the Arctic as new sea ice forms in the gaps where the sea ice has been broken up by the storm. However, the sea ice also gets exported out of the Arctic more rapidly, leading to a reduction in the thick multi-year ice cover. So the net effect of these storms is to create a thinner and weaker sea ice cover, which may precondition earlier breakup in Spring and accelerate sea ice loss. Another important result from our work is that it is necessary to have a high quality and high-resolution atmospheric model to accurately simulate such breakup events. We have investigated how big an effect Arctic cyclones have on sea ice from a long-term perspective by using coupled climate reconstructions. We looked not just at how the total amount of sea ice changes but also how it is redistributed by the strong winds during the passage of cyclones. We demonstrated that changes to sea ice concentration are amplified by extreme weather events such as cyclones (both increases and decreases in concentration) with cyclones leading to a net reduction in sea ice concentration. Since these events are relatively rare they may not directly have a large effect on the climatology of sea ice concentration, but the non-local effects can be really important. This is exemplified by the case study we analysed in the Beaufort Sea where the passage of a cyclone didn't have a large direct effect on the sea ice state but it caused the increased transport and melt of multi-year ice. Additionally we have used a high-resolution atmospheric model to investigate how the effect of Arctic cyclones on sea ice can be influenced by the shape and distribution of sea ice in idealized setups. The sea ice cover in the Beaufort Sea has been changing - it is getting thinner and weaker. This makes the ice more likely to break apart from strong winds. Using a computer model, we study how these changes may have affected the frequency of large sea-ice breakup events from 2000 to 2018. We find that the amount of open areas in the sea ice, called leads, is increasing during winter. This allows new, thin ice to form, but also causes more ice to move out of the region under the action of winds and currents. This movement of ice cancels the growth of new ice, resulting in less ice overall at the end of winter in this region. Interestingly, these events became more common after 2007 and the results suggests that bigger breakup events might happen more often as the sea ice continues to thin. This study highlights how important it is to include these changes in large climate models to better predict what might happen in the Arctic in the future. Our next steps will be to investigate how well these processes are captured in current climate models. A lot of the uncertainty around what the Arctic sea ice cover will look like in the near future is due to uncertainty in these dynamics between the atmosphere and sea ice, so by improving our understanding of these processes, we can reduce uncertainty in our climate projections.

Arctic cyclones can break up and reshape the Arctic sea-ice cover and can be expected to do so more readily as the ice grows thinner due to anthropogenic climate change. Processes driven by Arctic cyclones can enhance the rate of melting of the ice and increase its export out of the Arctic. The record minima in sea ice extent in 2012, which was partially attributed to the presence of an Arctic cyclone. However, despite their importance, Arctic cyclones have remained relatively un-examined. We hypothesise that surface coupling (interactions between the ocean, sea ice and atmosphere) play a crucial role in determining the life cycle of Arctic cyclones, and the effect they have on the sea ice. In ARIA we will take an important step towards understanding the role of sea ice-atmosphere interactions during the passage of cyclones, and how they might be expected to change in the future. We will quantify the dynamical feedback between the sea ice and atmosphere using a cutting-edge sea-ice model and a state-of-the-art atmospheric model. We will then evaluate the latest generation of climate models (CMIP6) to determine how well they reproduce the underlying conditions, and what is lost in the climatology by failing to resolve these processes. In particular we will focus on the impact cyclones have on the sea-ice volume, both in the short-term response and how it affects the inter-annual variability and overall decline observed in recent decades. The project results will therefore directly contribute to addressing the World Climate Research Programme’s grand challenge on the melting cryosphere.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek