The Arctic Ocean is an integral part of the global climate system and an area that is observing record breaking seasonal fluctuations. This study investigates the spring snowmelt onset conditions in the Arctic sea ice cover from 1979 to 2010. Snowmelt onset over Arctic sea ice is defined as the point in time when liquid water appears in the snowpack. Physically, the timing of snowmelt onset is important because surface energy absorption increases rapidly at snowmelt onset, owing to changes in surface albedo values. Monitoring the timing of snowmelt onset over Arctic sea ice is facilitated by using satellite passive microwave data, because surface microwave emission changes rapidly when liquid water appears in the snowpack, and data acquisitions are relatively unaffected by cloud cover or solar illumination. The Advanced Horizontal Range Algorithm (AHRA) exploits the changes in passive microwave brightness temperatures between 18GHz (19GHz on SSM/I) and 37GHz brightness temperatures to derive snow melt onset dates over Arctic sea ice from 1979-2010. Discussion also focuses on how there is consistency between the different platforms (SMMR and SSM/I) and sensors (SSM/I F8, F11, F13 and F17). Comparison between AHRA-derived melt onset dates and temperatures from International Arctic Buoy Program/Polar Exchange at the Sea Surface (IABP/POLES) and NCEP/NCAR Reanalysis-2 illustrates melt onset typically occurs when air temperatures near 0°C.

In general, melt onset usually begins in the lower latitudes in the first week of March, and progresses northward towards the central Arctic by the middle of July. The latest melt onset dates are usually observed in the Lincoln Sea, north of Greenland. In comparison with the roughly radial northward melt progression of the annually averaged melt onset, specific years show a high degree of spatial variability. Most years typically have some regions of earlier than average melt, and other regions with later than average melt. The marginal ice zones also show a great variability in the melt onset date, with greater than 30 days difference in melt onset between years. The central Arctic Ocean has a smaller variability with 15-20 day differences. The results for the Arctic Ocean region as well as most sub regions continue to show a trend to earlier melt onset dates. However, the melt onset dates for the more recent years do not show extremely early dates, even though the ice cover at the end of the melt season continues to show reduced perennial ice cover in October; this was especially true in 2007, still the smallest ice area for the end of the summer season. These results would indicate a shorter melt season and thinner ice cover which is not melting earlier, but is being removed quicker in the summer melt season.

In addition to the annual melt onsets for the Arctic Ocean and sub-regions, daily melt onset conditions for each year are presented and discussed to further understand the melt characteristics during individual years. Daily maps of melt onset will further show the annual variations between years and the atmospheric conditions which delay or accelerate melt within a region for a given year. The spatial maps also show regions of melt in the Arctic and the extent of the melt during warming periods and how these periods differ within a year and between years. This is accomplished by accumulating the daily area coverage of melt.