The impact of variable sea ice roughness on changes in Arctic Ocean surface stress: A model study

Abstract

The Arctic sea ice cover is thinning and retreating, causing changes in surface roughness that in turn modify the momentum flux from the atmosphere through the ice into the ocean. New model simulations comprising variable sea ice drag coefficients for both the air and water interface demonstrate that the heterogeneity in sea ice surface roughness significantly impacts the spatial distribution and trends of ocean surface stress during the last decades. Simulations with constant sea ice drag coefficients as used in most climate models show an increase in annual mean ocean surface stress (0.003 N/m2 per decade, 4.6%) due to the reduction of ice thickness leading to a weakening of the ice and accelerated ice drift. In contrast, with variable drag coefficients our simulations show annual mean ocean surface stress is declining at a rate of −0.002 N/m2 per decade (3.1%) over the period 1980–2013 because of a significant reduction in surface roughness associated with an increasingly thinner and younger sea ice cover. The effectiveness of sea ice in transferring momentum does not only depend on its resistive strength against the wind forcing but is also set by its top and bottom surface roughness varying with ice types and ice conditions. This reveals the need to account for sea ice surface roughness variations in climate simulations in order to correctly represent the implications of sea ice loss under global warming.

Arctic Ocean surface geostrophic circulation 2003–2014

“…Martin et al (2014) found that, in the 2000s, the summertime ice pack was experiencing longer periods of "optimal ice concentration" (80-90 % ice concentration) for momentum transfer to the ocean. However, in this study, and a follow-up study including variable form drag (Martin et al, 2016), they reported a negative trend in summertime ocean surface stress; despite increased ice-ocean stress, the loss of summer sea ice coverage led to an overall decrease in the ocean surface stress because the atmosphere-ocean stress is smaller than ice-ocean stress. Wintertime circulation changes are likely a result of reduced ice strength and reduced ice interaction forces.…”

“…However, Martin et al (2016) found reduced wintertime ocean surface stress due to ice smoothing and decreased form drag associated with loss of deformed thick ice. So, whilst observations make it clear that ocean surface stress increased in the Arctic Ocean, particularly in the late 2000s, contradictory model results show that more work is needed to refine the representation of atmosphere-ice-ocean coupling in models before we can fully attribute causality to these increases (Martin et al, 2016). An aspect of ice-ocean coupling that has been lacking is long-term observations of time-variable upper-ocean circulation, which this study has helped to provide.…”

“…Reductions in Beaufort Sea ice concentration in most seasons (apart from January-March) and thinning of the ice pack has likely reduced the ice interaction force (Petty et al, 2016) and, with less opposition to drift, the ice has drifted faster (Spreen et al, 2011) leading to an increased ocean surface stress (Martin et al, 2014). However, Martin et al (2016) found reduced wintertime ocean surface stress due to ice smoothing and decreased form drag associated with loss of deformed thick ice. So, whilst observations make it clear that ocean surface stress increased in the Arctic Ocean, particularly in the late 2000s, contradictory model results show that more work is needed to refine the representation of atmosphere-ice-ocean coupling in models before we can fully attribute causality to these increases (Martin et al, 2016).…”

“…However, we have not utilised these data for evaluating our satellite-derived currents because the moorings are typically instrumented at a minimum depth of 50 m and so do not directly measure surface currents. In order to extrapolate surface velocities from the mooring data we would have to introduce various assumptions about the velocity profile in the upper 50 m, regarding Ekman velocities and associated air-ocean/ice-ocean drag, turning angles and stratification, all of which are subject to substantial uncertainty (Tsamados et al, 2014;Martin et al, 2016). Seasonal and geographic biases in hydrographic measurement density would adversely affect any attempt to insert geostrophic shear from climatological data assemblies (e.g.…”

“…As such, changes in sea ice circulation are tightly coupled to upper ocean circulation. Arctic sea ice drift accelerated in the 2000s, and suggested causes include changing wind forcing, the reduction of sea ice concentration and thickness, and changing ice pack morphology which alters the coupling between the atmosphere and the sea ice (Ogi et al, 2008;Rampal et al, 2009;Spreen et al, 2011;Kwok et al, 2013;Olason and Notz, 2014;Tsamados et al, 2014;Martin et al, 2014Martin et al, , 2016Petty et al, 2016). Meanwhile, observations suggest that ocean surface stress increased during the 2000s, particularly in the Beaufort Sea where there was an accumulation of liquid freshwater due to increased Ekman pumping (Proshutinsky et al, 2009;Giles et al, 2012;Krishfield et al, 2014) and increased geostrophic currents due to doming of the regional DOT McPhee, 2013).…”