How does the Arctic Ocean freeze?

The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions.The International Hydrographic Organization (IHO) recognizes it as an ocean, although some oceanographers call it the Arctic Mediterranean Sea or simply the Arctic Sea, classifying it a mediterranean sea or an estuary of the Atlantic Ocean.Alternatively, the Arctic Ocean can be seen as the northernmost part of the all-encompassing World Ocean.

Arctic sea ice is that which forms on the Arctic Ocean during the winter months. Pure water, which contains no other molecules, substances or impurities, freezes at 0 degrees Celsius (32 degrees Fahrenheit). The world’s seawater, on the other hand, contains around 3.5 per cent dissolved minerals and salts. This additional material lowers the freezing point of the seawater to around -2 degrees Celsius (28.4 degrees Fahrenheit) because the freezing point depends on the number of molecules present in a solution, as well as the type of molecule(s). During the winter months, when the air temperature in the Arctic starts to fall dramatically, a deep layer of seawater begins to develop minuscule ice crystals; this slushy water is called frazil ice. A further drop in temperature causes the frazil ice to thicken. Pockets of salty slush accumulate until they become so heavy they start to sink. This leaves the top layer of icy crystals with significantly less salt content. The freezing point of this surface water therefore becomes higher and the falling temperatures enable the crystals to solidify into pack ice. This pack ice grows to become one huge floating sheet (made up of many smaller floes), the thickness and coverage of which varies over the year, but reaches its peak in March. During the warmer summer months, meanwhile, the ice begins to retreat and break up, reaching its lowest extent around September. 

Understanding how polar ice affects the world climate.

Sea ice at the poles is important because it influences the weather across the entire planet. The ice acts like a mirror, deflecting the Sun’s rays back into the atmosphere. As the ice melts, more of the ‘dark’ ocean beneath, capable of absorbing the Sun’s heat, is exposed. When the Arctic is frozen, warmer water entering from the Pacific or Atlantic begins to cool, becoming dense and sinking. This displacement of water drives the circulation of Earth’s oceans, affecting weather and conditions throughout the world. So, in many respects, the amount and extent of Arctic sea ice is absolutely critical to the global climate.

High reflection

The white sea ice cover acts like a mirror, reflecting the Sun’s rays back out to space, preventing the sea from heating excessively.

Sea exposed

As the ice melts, there is more dark seawater to absorb sunlight, which further melts the ice.

Low reflection

The more sunlight absorbed by the seawater, the more the ice melts until, eventually, significantly less light is reflected back into space.