Icebergs approach New Zealand’s sub-Antarctic islands every few years.

Life cycle of an iceberg

Icebergs are biggest when they calve from the ice shelf. After that, the sun and the ocean start to melt the iceberg. Waves erode its sides and small pieces of ice constantly break off. But perhaps the most dramatic way that icebergs get smaller is by splitting, from waves flexing the iceberg, or losing their rams. Rams are portions of an iceberg that jut out horizontally from an iceberg underwater. These can break away and surge to the surface under their own buoyancy. An iceberg’s route is determined by the action of wind, waves, and ocean currents. When icebergs are large the ocean currents are the most important in determining their direction. As the icebergs get smaller, the wind and the waves become more important.

Icebergs around New Zealand

Icebergs approach New Zealand’s sub-Antarctic islands every few years. The closest they have come to New Zealand was in November 2006 when they were off the coast of Otago and Canterbury coast for over a month. Prior to 2006 the last sighting from the mainland was off Dunedin in 1931. We know icebergs have been visiting New Zealand for a long time. NIWA has identified scour marks in the sea floor on the Chatham Rise, which were probably left by icebergs during the last Ice Age 20 000 years ago.

The 2006 icebergs originally came from one of six large icebergs that calved from the Ross Ice Shelf near New Zealand’s Scott Base, between 2000 and 2002. The largest of these, B15, was initially 295 km long by 38 km wide. On their journey to the New Zealand the  icebergs will have drifted along the western coast of the Ross Sea, before spending years grounded in one of Antarctica’s  “iceberg graveyards”. After escaping from the graveyards by breaking up and melting they resumed their northward journey crossing the Southern Ocean in around 7 months. Near New Zealand, the icebergs came between the Auckland Islands and Stewart Island, instead of taking the more usual path to the southeast and out in to the Pacific Ocean. By the time they were east of Stewart Island the original large iceberg had broken into an armada of smaller icebergs, the largest of which was over 500 m long and 300 m when first seen. Although most of the iceberg is under water it’s likely to have been up to 350 m thick, with about 300 m under water. From here most of the icebergs headed away from New Zealand, but some made it into the Southland Current. This current runs up the east coast of the South Island to about Banks Peninsula, before heading towards the Chatham Islands.

It is likely the icebergs currently near Macquarie Island have a similar origin to those that made it to New Zealand in 2006, but have been trapped in the “iceberg graveyards” for longer. So far they have made it most of the way across the Southern Ocean, but there remains the possibility that like most icebergs near New Zealand they will be pushed east by the main Southern Ocean current, the Antarctic Circumpolar Current, out into the Pacific Ocean.

Antarctic iceberg - Antarctic peninsula. Credit: Katja Riedel