The Arctic's Shrinking Sea Ice Cover
The Arctic is losing is floating sea ice cover. Negative linear trends in ice extent over the satellite record (1979 onwards) encompass all months, with the largest trend at the end of the melt season in September (11.7% per decade through 2008; http://nsidc.com/arcticseaicenews/). The ice cover is also thinning [ Nghiem et al.
, 2006;
Maslanik et al ., 2007a].
Understanding the evolution of the Arctic sea ice cover is a major challenge which requires better information on the sea ice mass budget. The ice mass budget is determined by energy fluxes at the top and bottom of the ice that control thermodynamic growth and melt, ice deformation in response to winds and ocean currents, and the mass flux of ice out of the Arctic into the Atlantic Ocean, primarily through Fram Strait between
Greenland and Svalbard. There are complex interplays and feedback between budget terms. For example, changes in the ice mass flux out of the Arctic Ocean may alter ice thickness, in turn altering the heat flux through the ice to the atmosphere that affects ice growth.
While an externally forced component to the observed September extent trend is supported from simulations with coupled global climate models [ Zhang and Walsh , 2006], the observed trend is on the upper end of these hindcasts [ Stroeve et al ., 2007; Wang and
Overland , 2009]. Natural variability in the ice-ocean-atmosphere system is has clearly contributed to the loss of ice. There is evidence for roles of increased spring cloud cover, augmenting the longwave radiation flux to the surface. [ Francis and Hunter , 2006] and for altered ocean heat transport [ Polyakov et al., 2005; Shimada et al ., 2006]. The phase of the Northern Annular Mode and other atmospheric patterns has had impacts on ice age, the mass flux out of the Arctic, and patterns of atmospheric heat transport [ Rigor and
Wallace , 2004; Maslanik et al ., 2007b; Strove et al ., 2008]. For example, the record low ice extent of September 2007 can be linked to strong summer melt driven by anomalous horizontal atmospheric heat transpor and regional reductions in cloud cover [ Kay et al,
2008; Stroeve et al ., 2008].
A full accounting of the relative roles of different forcings on the ice mass budget has nevertheless remained elusive. The challenge can be appreciated by noting that a bias in the net surface flux of only 1 W m
-2
, maintained over a year, s the energetic equivalent of melting 0.1 m of ice. Surface fluxes from atmospheric reanalyses and satellite retrievals, while valuable for process understanding, likely have errors on the order of tens of W m
-2
[ Serreze et al.
, 2007] Accurate estimates of ice thickness across the Arctic Ocean could provide a valuable constraint on the mass budget. While retrievals from satellite altimeter data [ Kwok et al , 2007] and ice age tracking algorithms [ Maslanik et al ., 2007a] show promise, no accurate assessment of thickness are yet available. Upward looking sonar arrays provide estimate of the ice thickness comprising the Fram Strait outflow, [e.g.,
Kwok et al., , 2004] but spatial sampling is inadequate.
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