A Model View of Arctic Sea Ice During Summer 2007 and Beyond SASS 3/2008, New York A Model View of Arctic Sea Ice During Summer 2007 and Beyond Jinlun Zhang Ron Lindsay, Mike Steele, & Axel Schweiger PSC/APL/UW Polar Science Center Jinlun Zhang

Motivation Satellite observed September sea ice extent from NSIDC Significant decline of arctic sea ice detected in recent years. Decline was particularly dramatic during summer 2007. What drove the dramatic retreat of arctic sea ice in summer 2007 seen by a model? How would the summer 2007 ice-ocean conditions affect 2008 sea ice? 1996 1979 2005 2007 Satellite observed September sea ice extent from NSIDC

PIOMAS (Pan-arctic Ice-Ocean Modeling and Assimilation System) 12-category thickness & enthalpy distribution (TED) sea ice model (Zhang/Rothrock 2003). LSR sea ice dynamics model to solve ice momentum equation (Zhang/Hibler 1997). Ice mechanics follow a teardrop viscous-plastic rheology (Zhang/Rothrock 2005). POP ocean model. NCEP/NCAR reanalysis forcing. Nested to a global ice-ocean model.

PIOMAS Simulated Ice Thickness and Satellite Observed Ice Edge

Simulated arctic sea ice extent and volume Anomaly 2000−2006 mean (climatology) 2007 Anomaly Years of shrinking/thinning set a stage for a huge decline in summer 2007

Simulated arctic ice outflow and production anomaly Aug-Sept increase: 0.23×1012 m3 Aug-Sept decrease: 0.57×1012 m3 Aug-Sept total loss: 0.8×1012 m3 (mass), 1.1×1012 m2 (extent)

NCEP/NCAR SLP and surface wind anomaly 2007 minus 2000−2006 mean

Simulated sea ice motion & advection anomaly Ice advection = ice mass convergence: One of every 36 ice velocity vectors plotted.

Simulated sea ice production anomaly

Simulated net shortwave radiation anomaly

Simulated surface albedo anomaly

Ice thickness Ice thickness difference

Conclusions Years of shrinking and thinning in arctic sea ice before 2007 set a stage for unprecedented ice retreat in summer 2007 − preconditioning. Anomalous winds cause anomalous sea ice motion and advection with a stronger transpolar drift, leading to a large reduction in ice thickness in the Pacific sector of the Arctic and a sizable increase in ice outflow at Fram Strait − a trigger. The reduction in ice thickness and concentration in the Pacific sector lowers the surface albedo, leading to increased surface shortwave radiation − ice-albedo feedback. Increased surface solar heating accelerates summer melting − “amplification.” Ice loss due to ice advection: 0.23×1012 m3; ice loss due to melting 0.57×1012 m3 − amplification factor: 0.57/0.23 = 2.5. Arctic sea ice is in a state vulnerable to anomalous forcing. Winter 2008 ice is low in Pacific sector, in favor of a summer low.