What could have caused this open water? Let's go through some of the background.
North Hemisphere snow cover has been low for some time. Snow cover in May 2013 was the lowest on record for Eurasia. There now is very little snow left, as shown on the image right, adapted from the National Ice Center.
Low snow cover is causing more sunlight to be absorbed, rather than reflected back into space. As can be expected, there now are high surface temperatures in many areas, as illustrated by the NOAA image below. Anomalies can be very high in specific cases. Zyryanka, Siberia, recently recorded a high of 37.4 C, against normal high temperatures of 20 C to 21 C for this time of year. Heat wave conditions were also recorded in Alaska recently (satellite image of Alaska below).
|NASA image June 17, 2013, credit: NASA/Jeff Schmaltz, LANCE MODIS Rapid Response Team, NASA GSFC - from caption by Adam Voiland: "Talkeetna, a town about 100 miles north of Anchorage, saw temperatures reach 96°F (36°C) on June 17. Other towns in southern Alaska set all-time record highs, including Cordova, Valez, and Seward. The high temperatures also helped fuel wildfires and hastened the breakup of sea ice in the Chukchi Sea."|
Sea surface temperatures have been anomalously high in many places around the edges of the sea ice, as also shown on the NOAA image below.
Nonetheless, as the above images also make clear, sea surface temperatures closer to the North Pole have until now remained at or below zero degrees Celsius, with sea ice cover appearing to remain in place. The webcam below from the North Pole Environmental Observatory shows that there still is a lot of ice, at least in some parts around the North Pole.
|Webcam #2 of the North Pole Environmental Observatory monitoring UPMC's Atmospheric Buoy, June 21, 2013|
Firstly, as discussed in earlier posts, there has been strong cyclonic activity over the Arctic Ocean (see also Arctic Sea Ice blog post). This has made the sea ice more prone and vulnerable to the rapid decline that is now taking place in many areas.
Furthermore, Arctic sea ice thickness is very low, as illustrated by the image below.
|Arctic sea ice volume/extent ratio, adapted by Sam Carana from an image by Neven (click to enlarge)|
|Monthly insolation for selected latitudes - adapted from Pidwirny, M. (2006), in "Earth-Sun Relationships and Insolation", Fundamentals of Physical Geography, 2nd Edition.|
"The ocean surface freezes if the temperature falls below -2.5C. The reason for the negative melting point is the presence of 4-5% of sea salt. Only in the polar regions does the sea surface cool sufficiently for sea ice to form during winters.Indeed, vertical mixing of the water column was enhanced due to cyclonic activity, and this occurred especially in the parts of the Arctic Ocean that also are the deepest, as illustrated by the animation below.
The sea ice cover is currently thinning near the North Pole between 80-90 degrees north. This part of the ocean is very deep. It receives heat of the Gulf Stream from the south: as the warm water vapourises, its salt content to water increases. This densifies the Gulf Stream which then falls onto the sea floor where it dissipates its heat to the overlying water column. The deep basin of the Arctic Ocean is now getting sufficiently warmed for the thin sea ice cover to thin on top of it. The transportation of heat to the icy surface is combined with the winds that push cold surface water down while rising heat to surface."
|Legend right: Ice thickness in m from Naval Research Laboratory|
Legend bottom: Sea depth (blue) and land height (brown/green)
in m from NIBCAO Arctic map at NOAA
|[ click to enlarge ]|
- Getting the Picture
- Supplementary evidence by Prof. Peter Wadhams