On the Processes Controlling the Ongoing Reduction of Multiyear Sea Ice in the Northern Hemisphere

Passive and active microwave satellite remote sensing observations are used to monitor the extent and concentration of sea ice in the northern hemisphere by a variety of operational agencies (e.g., Canadian Ice Service [CIS]), National Oceanic and Atmospheric Administration [NOAA]). In September 2009 aboard the Canadian Coast Guard icebreaker Amundsen, a much different sea ice cover than anticipated, based on remotely sensed products, was observed within the Southern Beaufort Sea, Canada. Remotely sensed products predicted 9 to 10 tenths multi-year (MY) or thick firstyear (FY) sea ice throughout most of the Southern Beaufort Sea in the deep water of the Canada Basin. In situ observations found a much different sea icescape consisting of a few heavily decayed, very small MY floes interspersed in a cover of mostly heavily decayed small first-year (FY) sea ice floes in combination with new ice between floes, in melt ponds, and thaw holes. This icescape contained approximately 25% open water, predominantly distributed in between floes or in thaw holes connected to the ocean below. In this paper we explain why two physically and radiometrically different sea ice regimes (a predominately first year sea ice area and an area dominated by multi-year ice) were not differentiable by microwave remote sensing under similar climatic conditions. Although these two regimes were quite different in terms of ice volume, their near-surface physical properties were found to be so alike that their backscatter to C-band SAR instrumentation was nearly identical. We conclude by examining how moisture and momentum fluxes from cyclones can dramatically alter the perennial icescape of the northern hemisphere.