SUITLAND, MD — The U.S. National Ice Center has determined that the 2016 Arctic Sea Ice Extent Minimum, shown in Figure 1, occurred on Sept. 9. Using a threshold of 40 percent ice cover at 4km resolution, the Interactive Multi-Sensor Snow and Ice Mapping System (IMS) indicated a minimum three-day average of 4.19 million square kilometers recorded Sept. 9th through 11th.
The use of a three-day running mean to calculate ice extent reduces subjective variability in the determination of ice extent and provides a clear trend line. The 2016 minimum establishes this year’s Arctic areal ice cover at its third lowest since the 4km analysis began in 2004, behind 2012 (3.397 million sq. km) and 2007 (4.019 million sq. km).
Ice analysts at the National Ice Center distinguish various ice types using a combination of visible and infrared satellite imagery and space-based synthetic aperture radar to analyze the daily ice edge, produce weekly hemispheric ice concentrations, stage of development analysis, and create tailored products for operating units.
For comparison, the National Snow and Ice Data Center in Boulder, Colorado bases their Sea Ice Index on passive microwave data, and reported the 2016 sea ice minimum occurred on Sept. 10 with an Arctic sea ice extent of 4.14 million sq. km.
The determination of the annual Arctic sea ice minimum marks the end of the summer melt. This year's minimum occurred eight days before the Ice Mapping System 10 year-average minimum date of Sept. 18. However, this 10-year period includes two of the minimum recorded ice extents within the satellite record (since 1979).
The first week of October, which this year began on Sept. 29, traditionally benchmarks “the old from the new” in Arctic sea ice. New ice, which develops during the winter months each year, is termed “first year” ice. First year ice that survives the summer melt season becomes “second year” ice. Sea ice that survives two summer melts is known as “multiyear” ice.
Having persisted through the melt period in the northern hemisphere, this ice typically becomes thicker, harder, more dense, and can pose an increased obstacle for navigation. Arctic multi-year ice is an important climate factor because it is typically more resistant to future summer melt-seasons.
The greater density of multi-year sea ice is a result of two processes: deformation of the ice from the movement of individual floes as winds, tides, and currents and a freshening of the ice as salt is slowly leached out from increased pressure on the ice. As a result, second year ice is characterized by a rougher surface. It is fresher, clearer, stronger, thicker, and more brittle than first year ice.
Multi-year ice provides an increased hazard to navigation than first year ice, even to ice-hardened vessels. On Oct. 1 (or the first week that falls within) "first year” ice is declared "second year” ice and becomes part of the “multi-year” ice that has persisted through the summer melt-season, as seen is Figures 2. and 3.
The U.S. National Ice Center is a tri-agency operational center represented by the United States Navy, the National Oceanic and Atmospheric Administration, and the United States Coast Guard. The U.S. National Ice Center mission is to provide global to tactical scale ice and snow products, ice forecasting and other environmental intelligence services for the United States government.