The Arctic sea ice maximum

NSIDC scientists announced today that the Arctic sea ice cover has likely reached its maximum extent, marking the beginning of the melt season. (For details, see Arctic Sea Ice News & Analysis). What is the sea ice maximum and why does it matter?

The maximum ice extent marks the beginning of the melt season for Arctic sea ice. Leads, long cracks in the ice, begin to open up and the ice cover starts to melt as sunlight brings warmth to the Arctic. Credit: Angelika Renner

What is the maximum and when does it happen?
Arctic sea ice melts and regrows in an annual cycle, freezing throughout the winter months and melting in the spring and summer. The ice cover generally reaches its maximum extent sometime in late February or March. After that, ice melts through the summer, hitting a low point in early or mid-September. NSIDC scientist Walt Meier said, “The maximum marks the point when the Arctic shifts from a freezing period into the summer melting period.”

While scientists pay attention to milestones like the daily maximum and minimum ice extents, they place more importance on monthly averages for studying climate. Ice extent can move up and down quickly during late winter, since thin and dispersed ice at the edges of the ice pack can move around easily, getting pushed together or spread apart by winds. Meier said, “The amount of ice at the maximum is a function of not only the state of the climate but also ephemeral and often local weather conditions. The monthly value smoothes out these weather effects and so is a better reflection of climate effects.”

What does the maximum mean for summer sea ice?
Ice extent is declining year-round, but the downward trends for winter months are less steep than for summer months. That is partly because most of the Arctic Ocean still gets covered in ice every winter, as the sun disappears north of the Arctic Circle.

At the beginning of the melt season, scientists look at data on sea ice extent, ice thickness and other factors that influence how much the ice cover will retreat during the coming summer. For example, the amount of thick ice that has survived multiple melt seasons plays a big role in how much ice melts away during the summer. Researchers also look at the date on which the maximum occurs, and how quickly the ice declines after the maximum extent.

NSIDC scientists will provide the monthly average data for March, along with ice age data and other indicators, on the Arctic Sea Ice News & Analysis Web site in early April.

For more information on sea ice in late winter, see last year’s post about winter sea ice and reader questions. http://nsidc.org/icelights/2011/04/04/what-about-sea-ice-in-winter/

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