As expected Arctic sea-ice has grown during the month of October(albeit at an exceptionally fast rate) after reaching it’s seasonal minimum on September 14, 2008. This, in turn has contributed to unusually warm air temperatures near the ocean surface. At first glance, this might seem contradictory, but as the National Snow and Ice Data Centerr explains the higher temperatures in combination with the rapid ice growth are exactly what is to be expected.
Higher-than-average air temperatures
Over much of the Arctic, especially over the Arctic Ocean, air temperatures were unusually high. Near-surface air temperatures in the Beaufort Sea north of Alaska were more than 7 degrees Celsius (13 degrees Fahrenheit) above normal and the warming extended well into higher levels of the atmosphere. These warm conditions are consistent with rapid ice growth.
The freezing temperature of saline water is slightly lower than it is for fresh water, about –2 degrees Celsius (28 degrees Fahrenheit). While surface air temperatures in the Beaufort Sea region are well below freezing by late September, before sea ice can start to grow, the ocean must lose the heat it gained during the summer. One way the ocean does this is by transferring its heat to the atmosphere. This heat transfer is largely responsible for the anomalously high (but still below freezing) air temperatures over the Arctic Ocean seen in Figure 3. Only after the ocean loses its heat and cools to the freezing point, can ice begin to form. The process of ice formation also releases heat to the atmosphere. Part of the anomalous temperature pattern seen in Figure 3 is an expression of this process, which is generally called the latent heat of fusion.
In the past five years, the Arctic has shown a pattern of strong low-level atmospheric warming over the Arctic Ocean in autumn because of heat loss from the ocean back to the atmosphere. Climate models project that this atmospheric warming, known as Arctic amplification, will become more prominent in coming decades and extend into the winter season. As larger expanses of open water are left at the end of each melt season, the ocean will continue to hand off heat to the atmosphere.
Arctic air temperature composite anomaly (Degrees C), September 14 to October 31, 2008.
In this image, near-surface air temperatures show strong warming near the surface in the Beaufort sea region, an area with substantial open water at the end of the melt season. The anomalously high temperatures extend well up into the atmosphere, showing that the ocean is transferring heat to the atmosphere as ice forms. The vertical axis represents altitude measured by pressure, extending from the planet’s surface to 700 millibars (about 3,000 meters or 10,000 feet above sea level). The horizontal axis represents latitude from 50 degrees North (left) to 90 degrees North (right; North Pole).
—Credit: From National Snow and Ice Data Center courtesy NOAA/ESRL Physical Sciences Laboratory
It has been a very interesting year so far for sea ice. The sea-ice minimum extent fell just short of last year’s record, but recent research from the European Space Agency showed that a record was set for minimum sea-ice volume. Now the NSIDC reports that another record was broken: the total ice loss between the March maximum and the September minimum set a record for total ice loss in a single melt season, 10.56 million square kilometers slightly more than 2007’s 10.51 million square kilometers.
The NSIDC report underlines the extreme and heretofore unseen nature of the data now being recorded
Arctic sea ice and climate are behaving in ways not seen before in the satellite record—both in the rate and extent of ice loss during the spring and summer, and in the record ice growth rates and increased Arctic air heating during the fall and winter.
You can watch an interesting video from NASA’s Goddard Space Flight Center (requires Quicktime) of the progression of this year’s sea-ice melt from July 1 through the minimum on September 14, by clicking here and choosing the 640×360 MPEG-4 video near the top of the page.
Crossposted at Daily Kos