Seasonality and long-term trend of Arctic Ocean surface stress in a model

• Main Authors: Torge Martin, Michael Steele, Jinlun Zhang
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Arctic; Arctic Ocean; Sea ice
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.649.352; http://psc.apl.washington.edu/zhang/Pubs/Martin_etal2014_jgrc20607.pdf

Abstract A numerical ocean sea-ice model is used to demonstrate that Arctic sea ice retreat affects momen-tum transfer into the ocean. A thinner and thus weaker ice cover is more easily forced by the wind, which increases the momentum flux. In contrast, increasing open water reduces momentum transfer because the ice surface provides greater drag than the open water surface. We introduce the concept of optimal ice concentra-tion: momentum transfer increases with increasing ice concentration up to a point, beyond which frictional losses by floe interaction damp the transfer. For a common ice internal stress formulation, a concentration of 80–90% yields optimal amplification of momentum flux into the ocean. We study the seasonality and long-term evolution of Arctic Ocean surface stress over the years 1979–2012. Spring and fall feature optimal ice conditions for momen-tum transfer, but only in fall is the wind forcing at its maximum, yielding a peak basin-mean ocean surface stress of0.08 N/m2. Since 1979, the basin-wide annual mean ocean surface stress has been increasing by 0.004 N/m2/ decade, and since 2000 by 0.006 N/m2/decade. In contrast, summertime ocean surface stress has been decreasing at20.002 N/m2/decade. These trends are linked to the weakening of the ice cover in fall, winter and spring, and to an increase in open water fraction in summer, i.e., changes in momentum transfer rather than changes in wind forcing. In most areas, the number of days per year with optimal ice concentration is decreasing. 1.