Coldest Water - a misleading (sub)heading?
Tuesday, March 10, 2009 9:23:06 PM
It may well have been the the coldest surface water ever measured in the Arctic - but the paper does not really tell. It does however say that: ”The supercooling of 0.037±0.005°C observed directly in the Storfjorden polynya with frazil-ice growth is the strongest observed Arctic supercooling in recent years.“ An opportunity for me to explain what I understand by terms like supercooling, polynia, and frazil ice.
Super-cooled water is (liquid) water at temperatures below the normal freezing point of that water - a freezing-point temperature estimated from the measured salinity and pressure. With an assumed freezing point of -1.95°C and a super-cooling of 0.037°C the “real” temperature would have been -1.987°C - so this is the actual “coldest” temperature we are talking about.
Polynia (in the US spelled polynya) is a loan-word from Russian
which means a natural ice hole, and was adopted in the 19th century by polar explorers to describe navigable portions of the sea.
It is a semipermanent area of open water in sea ice. Polynias are generally believed to be of two types. Coastal polynias characteristically lie just beyond landfast ice (aka as fast ice), i.e. ice that is anchored to the coast and stays in place throughout the winter. They are thought to be caused chiefly by persistent local offshore winds, such as the foehn, or katabatic (downward-driving), winds typically found off the coasts of Greenland and Antarctica. The zone of open sea may be 50-100 km wide. Open-ocean polynias, the larger and longer-lasting of the two types, form within the ice cover and are believed to be caused by the upwelling of deep warmer water. This type is best exemplified by the vast Weddell Polynia in the antarctic Weddell Sea. The Weddell polynia was an enormous area of open ocean that reappeared in the Weddell Sea during 3 consecutive winters (1974-1976). At its largest it measured about 350 by 1000 km. The Weddell polynia reappeared in approximately the same position each year above a sea-bed topographic high, known as the Maud Rise.
Coastal polynias have been referred to as ‘sea-ice factories’. They ‘manufacture’ ice on an enormous scale, perhaps producing much of the ice in the adjacent ocean. It has been calculated that the heat flux to the atmosphere from a coastal polynia is more than 300 Wm-2, enough to supply a ten centimetre thick layer of ice to the adjacent sea each day. The Storfjorden polynia is a coastal polynia, and in fact the heat flux here was indeed estimated to be about 300 Wm-2 - and the wind during the super-cooling event to be about 8 m/sec.
Frazil ice is a collection of loose, randomly oriented needle-shaped ice crystals in water. It is a soupy suspension that resembles slush and has the appearance of being slightly oily when seen on the surface of water. It sporadically forms in open, turbulent, supercooled water. Under calm conditions the crystals freeze together to form a continuous sheet of new ice called nilas. It is up to 10 cm thick and looks dark gray. The ice crystals can be characterised as fine spicule, plate or discoid crystals about 1–20mm in diameter and 1–100μm in thickness.
Nilas is a thin elastic crust of gray-colored ice formed on a calm sea. It is characterized by a matte surface, and easily bent by waves and thrust into a pattern of interlocking fingers.
Grease ice is a slurry of ~25% pure frazil ice and ~75% sea water.
Katabatic wind. Down-slope winds flowing from high elevations of mountains, plateaus, and hills down their slopes to the valleys, planes or sea below are called katabatic winds. Katabatic is derived from the Greek, namely from katabaino - to go down. An upslope wind is called anabatic. Kata means down and ana means up. I mentioned another katabatic wind in my post on the (Adriatic) Bora.
Finally a cartoon showing the environment where coastal polynias are produced in the Antarctic by katabatic winds.
Did I remember to mention that Storfjorden is a fjord in Svalbard/Spitsbergen.
R. Skogseth, F. Nilsen and L. H. Smedsrud,
Supercooled Water in an Arctic Polynya : Observations and Modeling,
Journal of Glaciology, Vol. 55, pages 43-52, No. 189, 2009
PS: Colder temperatures are found in salty pockets of water trapped in polar sea ice (said to be) down to -20°C (I have no refrence for that).