Abstract
Abstract. We present a method for analysing changes in the modelled volume budget of the Arctic sea ice as the ice declines during the 21st century. We apply the method to the CMIP5 global coupled model HadGEM2-ES to evaluate how the budget components evolve under a range of different forcing scenarios. As the climate warms and the ice cover declines, the sea ice processes that change the most in HadGEM2-ES are summer melting at the top surface of the ice due to increased net downward radiation and basal melting due to extra heat from the warming ocean. There is also extra basal ice formation due to the thinning ice. However, the impact of these changes on the volume budget is affected by the declining ice cover. For example, as the autumn ice cover declines the volume of ice formed by basal growth declines as there is a reduced area over which this ice growth can occur. As a result, the biggest contribution to Arctic ice decline in HadGEM2-ES is the reduction in the total amount of basal ice growth during the autumn and early winter. Changes in the volume budget during the 21st century have a distinctive seasonal cycle, with processes contributing to ice decline occurring in May–June and September to November. During July and August the total amount of sea ice melt decreases, again due to the reducing ice cover. The choice of forcing scenario affects the rate of ice decline and the timing and magnitude of changes in the volume budget components. For the HadGEM2-ES model and for the range of scenarios considered for CMIP5, the mean changes in the volume budget depend strongly on the evolving ice area and are independent of the speed at which the ice cover declines.
Highlights
Arctic September sea ice cover has declined at a rate of over 13 % per decade since satellite observations began (Serreze and Stroeve, 2015), and the ice that remains is becoming thinner (Kwok and Rothrock, 2009), younger (Maslanik et al, 2011), and faster moving (Rampal et al, 2009; Spreen et al, 2011)
As HadGEM2-ES projections are available for a range of different 21st century forcing scenarios, we evaluate the impact of the forcing scenario on the evolving volume budget
We have presented a method for investigating changes in the volume budget of the Arctic sea ice as the ice declines due to increasing greenhouse gas forcing
Summary
Arctic September sea ice cover has declined at a rate of over 13 % per decade since satellite observations began (Serreze and Stroeve, 2015), and the ice that remains is becoming thinner (Kwok and Rothrock, 2009), younger (Maslanik et al, 2011), and faster moving (Rampal et al, 2009; Spreen et al, 2011). Holland et al (2010) evaluated the annual mean changes in ice growth, melt and divergence during the 21st century for a range of models submitted to the CMIP3 model archive, finding considerable variation in the magnitude and relative importance of changes in the budget components. We use a CMIP5 model for which the budget components are already available as model output and consider the processes contributing to 21st century changes in the volume of the Arctic sea ice and overlying snow in the Met Office Hadley Centre model HadGEM2-ES (Martin et al, 2011; Collins et al, 2011).
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