Freeze Up

Summer has certainly ended here in the Arctic at 85N. The vernal equinox has passed and the sun stays very low in the sky all day, normally behind thick cloud.

Air temperatures have been between -2C and -7C depending on whether the wind is blowing onto or off the ice. Seawater temperature has been hovering around -1.5C, the approximate freezing point for water this saline (about 26 PSU, normal sea water is around 35 PSU).

We have been in mostly open water for the last couple of weeks but the conditions are causing the ocean surface to begin to freeze. This dramatically changes the appearance and behaviour of the ocean and is fascinating to see. It is also of scientific interest, as the interplay of air and water temperatures, winds and waves and the surface energy balance that affect the freeze up are complex and not well understood.

freeze up 1

calm sea with grease ice at the surface

The first visible stage in sea ice formation is the formation of ‘grease ice’, a thin layer of ice at the surface, giving the ocean the appearance of being covered in a grey rubbery mat (Photo 1). In the presence of waves, either from wind, swell or the wake of a ship, the ice layer suppresses the smaller wind waves, but longer wind waves and swell are still present. The suppression makes the wave construction and interference less chaotic, and creates beautiful patterns in the water that look more like computer simulations than natural phenomena (Photo 2).

freeze2

The ship’s walke supressed by grease ice

The next stage is the formation of pancake ice (Photo 3). Sometimes this is quite spread out, giving the ocean surface a mottled effect looking like the top of a whale shark. In the photo here, there is thicker multi-year ice in the background and the wind has blown the pancake ice onto it, pushing it together as if washing up on a beach.

freeze3

pancake ice pushed up against multi-year pack ice

 

Advertisements

Comments are closed.

%d bloggers like this: