Drift ice, also calledbrash ice, issea ice that is not attached to the shoreline or any other fixed object (shoals, grounded icebergs, etc.).[1][2][3] Unlikefast ice, which is "fastened" to a fixed object, drift ice is carried along by winds andsea currents, hence its name. When drift ice is driven together into a large single mass (>70% coverage), it is calledpack ice.[1] Wind and currents can pile up that ice to formridges up to dozens of metres in thickness. These represent a challenge foricebreakers and offshore structures operating in cold oceans and seas.
Drift ice consists ofice floes, individual pieces of sea ice 20 metres (66 ft) or more across. Floes are classified according to size:small – 20 metres (66 ft) to 100 metres (330 ft);medium – 100 metres (330 ft) to 500 metres (1,600 ft);big – 500 metres (1,600 ft) to 2,000 metres (6,600 ft);vast – 2 kilometres (1.2 mi) to 10 kilometres (6.2 mi); andgiant – more than 10 kilometres (6.2 mi).[4][5]
Drift ice affects:
Drift ice can exert tremendous forces when rammed against structures, and can shear off rudders and propellers from ships and strong structures anchored to the shore, such as piers. These structures must be retractable or removable to avoid damage. Similarly, ships can get stuck between drift ice floes.
The two major ice packs are theArctic ice pack and theAntarctic ice pack. The most important areas of pack ice are the polar ice packs formed fromseawater in the Earth'spolar regions: the Arctic ice pack of theArctic Ocean and the Antarctic ice pack of theSouthern Ocean. Polar packs significantly change their size during seasonal changes of the year. Because of vast amounts of water added to or removed from the oceans andatmosphere, the behavior of polar ice packs has a significant impact on global changes inclimate.
Seasonal ice drift in theSea of Okhotsk by the northern coast ofHokkaidō,Japan, has become a tourist attraction,[6] and is one of the100 Soundscapes of Japan. The Sea of Okhotsk is the southernmost area in theNorthern Hemisphere where drift ice may be observed.[7]
