The effect of lead formation by ridging on the Arctic climate in coupled model simulations

Leads are stretches of open water within the pack ice of the Arctic Ocean formed by divergent and shear motion of a fractured sea ice cover. Ridging compacts thin, weak ice into thick ice when the drift ice converges. By piling up blocks of thin ice space for leads is created within the ice pack once the floes part. During winter when the ice cover is most compact and ridging occurs frequently the formation of leads stimulates enhanced loss of heat from the ocean to the atmosphere and hence sea ice formation.

We conduct two experiments with an atmosphere-ocean-sea ice coupled model: one without and one with an advanced ridging scheme applied. The effect of ridging most important to the climate system is the enhanced formation of open water. The sea ice concentration in the run with riding is about 5% lower in the Arctic basin on annual average than in the run without. This has two implications: first, a reduced surface albedo results in the increased absorption of solar insolation by the ocean over summer, and second, an enhanced turbulent heat exchange between ocean and atmosphere leads to increased heat loss in winter. The first causes sea ice loss while the second stimulates ice growth. The latter dominates over the enhanced loss and we find an increase in total ice volume of 2.4*10^3km^3 in the run with ridging compared to the one without. Moreover, the reduction in surface albedo causes the annual mean surface air temperature to warm by up 2˚C. The warming spreads out over the continents surrounding the Arctic Ocean with a potential to affect land surface processes.

Our simulations show that sea ice dynamics and lead formation in particular have a non-negligible effect on the mean state of the Arctic climate and thus need to be appropriately represented in global climate models. However, the parameterization of ridging used in large-scale climate models is not sufficiently well constrained by observations. Moreover, observations of lead frequency and size distributions are needed to further the understanding of how sea ice deformation moderates heat exchange between ocean and atmosphere and how this is affected by the current shrinking of the Arctic sea ice cover.