Each winter, wide swaths（细长的列） of the Arctic Ocean freeze to form sheets of sea ice that spread over millions of square miles. This ice acts as a massive sun visor（遮阳板） for Earth, reflecting solar radiation and shielding the planet from excessive warming. The Arctic ice cover reaches its peak each year in mid-March, before shrinking with warmer spring temperatures. But over the last three decades, this winter ice cap has shrunk: Its annual maximum reached record lows, according to satellite observations, in 2007 and again in 2011.

Understanding the processes that drive sea-ice formation and advancement can help scientists predict the future extent of Arctic ice coverage -- an essential factor in detecting climate fluctuations（波动） and change. But existing models vary in their predictions for how sea ice will evolve.

Now researchers at MIT have developed a new method for optimally combining models and observations to accurately simulate the seasonal extent of Arctic sea ice and the ocean circulation beneath. The team applied its synthesis method to produce a simulation of the Labrador Sea, off the southern coast of Greenland, that matched actual satellite and ship-based observations in the area.

Through their model, the researchers identified an interaction between sea ice and ocean currents that is important for determining what's called "sea ice extent" -- where, in winter, winds and ocean currents push newly formed ice into warmer waters, growing the ice sheet. Furthermore, springtime ice melt may form a "bath" of fresh seawater more conducive for ice to survive the following winter.

Accounting for this feedback phenomenon is an important piece in the puzzle to precisely predict sea-ice extent, says Patrick Heimbach, a principal research scientist in MIT's Department of Earth, Atmospheric and Planetary Sciences.

"Until a few years ago, people thought we might have a seasonal ice-free Arctic by 2050," Heimbach says. "But recent observations of sustained ice loss make scientists wonder whether this ice-free Arctic might occur much sooner than any models predict … and people want to understand what physical processes are implicated in sea-ice growth and decline."

Heimbach and former MIT graduate student Ian Fenty, now a postdoc at NASA's Jet Propulsion Laboratory, will publish a paper, "Hydrographic Preconditioning for Seasonal Sea Ice Anomalies in the Labrador Sea," in the Journal of Physical Oceanography.