The formation of sea ice requires specific conditions, but it can be observed in UK waters, says Karen Boswarva
OCEANSCIENCE
The science of sea ice
The formation of sea ice requires specific conditions, but it can be observed in UK waters, says Karen Boswarva
The winter months bring quite a chill to the air. Away from the sea in the fields and up mountains, snow and ice will form. If you’ re lucky, you’ ll see lakes and freshwater lochs freeze over, ducks comically waddling across slippery surfaces. If thick enough you may even get to do a spot of ice diving. But when we venture to the coast on a cold winter’ s day, the sea remains unchanged. Flakes of snow will fall onto salty waters of 6-10 ° C and immediately disappear. So, what does it take for the sea to freeze?
I first asked this question on Christmas Day 2010, a day which coincided with an unexpected flurry of snow! Typically, England was unprepared for the event and things came to a predictable standstill, even in our little northwest seaside town, which didn’ t normally see snow. The event sticks firmly in my mind – I was home for Christmas, and we had family visiting from Brazil. Understandably, they had NEVER seen snow before. We packed ourselves into the car like excited children and hurriedly headed to the coast.
If you’ ve ever been down the Southport coast road, you’ ll know that you’ re far from likely to see the sea! You certainly wouldn’ t want to try and shore dive it. Vast flat plains of sandy mud stretch as far as the eye can see, leading some to mistakenly think they can walk all the way to Blackpool. This day was somewhat different. There was something scattered across the beach, glinting in the sunlight. Huge slabs of broken ice had formed, filling the landscape, like giant stepping stones. Naturally we treated them as such, and so we became arctic explorers for the day, scaling the tundra and leaping over wide crevasses. A fortunate moment that we are yet to see it again.
To understand what caused this rare phenomenon we need to understand more about the properties of seawater. Firstly, salt does not freeze. For ice to form in the ocean, the sea temperature must reach-1.8 degrees C. But seawater doesn’ t cool down easily, it’ s usually a long and slow process, because cold salty water increases in density and sinks from the
Negotiating brash ice in Antarctica, in an ice-class research vessel
“ Frazil crystals bond together on sea surface, forming sheets of ice”
surface before it has time to freeze. Only once cold enough will needle-like crystals called frazil( 3-4 mm in diameter) start to form. During formation, frazil crystals expel briny droplets, with the resulting crystal consisting of nearly pure fresh water. When frazil crystals accumulate and bond together on the sea surface, sheets of ice are formed.
I recently watched frazil crystals in Stromness marina form a smooth thin ice called grease ice( named for its likeness to an oil slick). I observed the film appear as if out of nowhere and move gently with the tide, thickening into a thin white sheet called nilas. As this solid yet fluid matter moved past ropes and buoys, it deposited thicker layers that resembled treacle pouring into a bowl.
Karen’ s photograph of ice slabs at Southport
Once these thicker sheets settle, frazil production halts and slower, congelation ice crystals start to grow on the ice surface, adding thickness to the ice sheet. In choppy waters, frazil crystals form discs called pancake ice, which eventually bind to form thicker sheets.
New ice sheets are called first-year ice, while ice formed over subsequent years of melting and freezing are called multi-year ice. They have very different properties, which can be detected using satellite sensors.
Going back to my Southport memory, we returned the next day; all trace of the sea ice had disappeared with the following tide, but it certainly wasn’ t one to forget. The event was likely caused by the rapid cooling of a high-water spring tide coinciding with a sudden drop in air temperature. Exacerbated by the calm conditions, it resulted in the swift formation of frazil crystals into thick sheets of ice. �
PHOTO: SIMON ROGERSON
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