An Arctic Expert's View of the Great Ice Melt of 2012

1. A Highly Specula/ve Discussion of Bathymetry-‐Ocean Controls on Ice Extent Jamie Morison 8/29/2012 Acknowledgments: Florence FeMerer for the MASIE image in kmz format, Son Ngheim for ﬁrst poin/ng out the idea.

2. Claim: At minimum ice extent, the ice edge mainly corresponds the deep-‐ water / shallow-‐water boundary (e.g., 500 m isobath. The sense of bathymetry-‐ocean controls on ice edge posi/on come in two ﬂavors. MASIE Ice Extent for Aug 28, 2012 Overlaid in Semi-‐transparent White on Google Bathymetry

3. Ice Survives in the Shallower Region: The prime example is the East Greenland shelf, which almost always has a sea ice cover and where the the ice edge almost always lies along the 500 m isobath. The reason for this is that Arc/c Surface Water near the freezing point flows out of the Arc/c Ocean over the East Greenland con/nental slope and thermodynamically buffers the oulowing sea ice against the effects of the atmosphere. The front associated with the East Greenland Current lies roughly over the shelf break, and any ice that dri]s east across this front encounters much warmer water of the Greenland Sea cyclonic gyre circula/on and rapidly melts. We saw this up close in the MIZEX cruises of the early 1980s and it looks like it s/ll happens today. Greenland Sea Gyre MASIE Ice Extent for Aug 28, 2012 Overlaid in Semi-‐transparent White on Google Bathymetry

4. Ice Survives in the Deeper Region: This is shown along most of the Eurasian con/nental shelf break. Here the inflow of warm Atlan/c Water via the Barents Sea Branch and Eurasian runoff input heat to the ocean over the shelf. This and the fact that radia/ve hea/ng is trapped in the shallow water provides a thermodynamic environment that is rela/vely hos/le to the overlying sea ice compared to deeper water off the shelf, which has the chance to develop a freezing point mixed layer and even a cold halocline that provide an ice bath buffer for the sea ice. Consequently the ice disappears first off the con/nental shelves and only disappears from the deep water areas if there is a major disturbance in the upper ocean. The examples of this are where we are seeing important ice losses. War m Ba rent s Sea Bra nch Greenland Sea Gyre MASIE Ice Extent for Aug 28, 2012 Overlaid in Semi-‐transparent White on Google Bathymetry

5. Example 1 oﬀ the Laptev Sea: The ice edge has retreated away from the Laptev Sea shelf break and we even see reduced ice out over the eastern Amundsen Basin. This is specula/ve, but over the past couple of decades cyclonic circula/on aligned along the Russian shelf break has tended to shi] Eurasian runoﬀ eastward to Canada Basin, which tends to weaken the cold halocline layer that insulates sea ice from Atlan/c Water heat in the Eurasian Basin. Hence ice disappears more easily over deep water. War m Ba rent s Sea Bra nch Greenland Sea Gyre MASIE Ice Extent for Aug 28, 2012 Overlaid in Semi-‐transparent White on Google Bathymetry

6. Example 2, Beaufort Sea: As recently as a couple of years ago, major chunks of the ice edge would align with the Beaufort shelf break. This year’s storm event separated the ice into two divergent chunks, one ice edge moving north and one south. The southern por/ons melted most quickly over the shallowest regions of the Chukchi shelf, likely for the reasons men/oned earlier with respect to the typical “ice-‐remains-‐over-‐deep-‐ water” case. It is noteworthy that some of this ice remains over the deeper parts of the shelf associated with the bathymetry around Herald Canyon (unfortunately the google bathymetry doesn’t resolve this bathymetry). War m Ba rent s Sea Bra nch Greenland Sea Gyre MASIE Ice Extent for Aug 28, 2012 Overlaid in Semi-‐transparent White on Google Bathymetry

7. Implica/ons for Future Ice Extent: I think that in the past even under warm atmospheric condi/ons, the ocean has provided refuge areas for summer sea ice. Major reduc/ons of ice extent into these refuge areas s/ll require special circumstances. For the sake of argument, I’ll speculate that we won’t have an ice free Arc/c Ocean un/l winter freezing is radically decreased or the Arc/c Ocean circula/on changes significantly. As long as freezing occurs in the winter, the ocean will tend to build up a buffer that can see major areas of sea ice through the summer. Substan/al increases in ocean heat fluxes through Fram and Bering straits could change this. More subtle, if changes in atmospheric circula/on cause changes in upper ocean circula/on, structure, and mixing that allow more ocean heat to get to the ice (e.g., weakening of the cold halocline) the ocean might lose this buffering capacity and summer sea ice will mostly disappear. MASIE Ice Extent for Aug 28, 2012 Overlaid in Semi-‐transparent White on Google Bathymetry

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An Arctic Expert's View of the Great Ice Melt of 2012

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