May is the month when melt ponds first start to form on the floes of the Arctic sea ice pack. Melt ponds are important because they soak up more sunlight than the ice would, speeding up the melting process and thus preconditioning the sea ice. When there is lots of melt ponding, something I call melting momentum is built up, sustaining a stable melting rate throughout the rest of the melting season, as we saw in 2011 and 2012. Of course, the reverse is also true, as we saw in rebound years 2013 and 2014.
Because this preconditioning through melt ponds seems to be correlated to the September minimum, scientists are trying to determine the so-called melt pond cover fraction (the percentage of total ice pack area covered by melt ponds) in different ways. Some look at satellite observations, like Anja Rösel from the University of Hamburg did back in 2011, using MODIS satellite data (see here). Others use models, like David Schröder from the University of Reading (see here, and more on that below). Yet others, like Christopher Cox from CIRES, look at radiation data around the Arctic to estimate how much sunlight is reaching the ice pack (I attended Dr. Cox' presentation on this subject at the EGU general assembly this year).
Even though it's very difficult to determine the exact amount and distribution of melt ponds, we can at least compare weather conditions from year to year, as melt ponds come into being due to above freezing temperatures and solar radiation (ie open skies due to high pressure), or a combination of the two. Chris Reynolds has a detailed blog post on atmospheric conditions during all of May on his Dosbat blog, but below I'll be looking at those conditions compared to those of other years during May.
As can be seen, May started out extremely cold in the Central Arctic Basin, with anomalously warm temps along the coasts of the Beaufort and Kara Seas. Except for 2011, no other year in this comparison was as cold in the heart of the ice pack. In that sense 2015 started out even worse (for melting) than the two rebound years 2013 and 2014, where a very low melt pond fraction cover during May (and June) made it virtually impossible for those melting seasons to reach the top 5 lowest September minimums.
These extremely low temperatures are also reflected on the DMI graph that shows (modelled) temperatures above 80° latitude:
The graph also shows that the extreme dip was followed by a steep climb, even traversing the green average temperature trend line. This suggests weather conditions flipped during the second half of May, so here's the comparison map for May 16th-31st:
The cold has disappeared from the Central Arctic, with the heat expanding in the Beaufort and Kara Seas, the effects of which were discussed in ASI 2015 update 2. Only 2011 seems to have been warmer, with 2010 and 2012 showing anomalously high temperatures spread all over the Arctic. If you didn't notice, go back and check how 2010 looked on the first map an second map as well: extremely warm (relatively speaking) and extremely high sea level pressure, meaning clear skies. It's no wonder PIOMAS shows a huge volume drop in this period in 2010. I guess it's the perfect preconditioning for a high melt year.
So, with 2015 starting out extremely cold during the first half of May, and then almost the reverse in the second half of May, what can be said about the melt pond cover fraction? Luckily I received some info on that from Dr. David Schröder, who I've met at the EGU General Assembly two months ago. In an e-mail he sent me three distribution maps, produced by his melt pond model, showing melt pond anomalies for the years 2012, 2014 and 2015 compared to the average for the last 10 years (click for a larger version):
Dr. Schröder added this caveat: "Be aware that we do not know how accurate the locations of our pond maps are and that the relative and absolute numbers for May pond fraction are very small."
2015 can't compare to 2012 really, but it does show a slightly less negative melt pond anomaly than 2014. Just like last year Dr. Schröder and his team use this result as a prediction for the SIPN 2015 Sea Ice Outlook (the first summary of which will be released soon), and Dr. Schröder has given me permission to reproduce the text accompanying the three maps above:
Based on our simulated May melt pond fraction we predict a September 2015 mean ice extent for the Arctic of 5.1 +/- 0.5 Mill. km2. Our value is slightly lower than in 2013 (5.4 Mill. km2) and 2014 (5.3 Mill. km2), but considerably larger than in 2012 (3.6 Mill. km2). The attached 3 figures show the anomaly of May melt pond fraction in May 2015, May 2014 and May 2012 with respect to the mean over the period May 2006 to May 2015. Locations which show no correlation with the September ice extent are masked.
In May 2012 there are positive anomalies of pond fraction with values between 0.5 and 2% above the last 10-year average, whereas in May 2014 negative anomalies between -0.2 and -1% occurred. In May 2015, the anomalies are mainly negative, but weaker than in 2014. The pond fraction does mainly depend on the atmospheric conditions in May and the pre-conditioning of sea ice (sea ice thickness, area fraction of thin ice).
While the atmospheric conditions were quite "normal" in May in average, the ice is thicker in April 2015 in our model simulation compared to previous years. The increase in ice thickness and volume is also confirmed by the PIOMAS simulation: maximum ice volume in 2015: 24388 km3, in 2014: 23104 km3 and in 2011: 22677 km3. The given uncertainty of our prediction of 0.5 Mill. km2 is mainly caused by ice drift during summer and the atmospheric conditions during June. Given the current situation we do not expect a new record minimum for Arctic summer sea ice in 2015.
CAPIE, or compactness, is calculated by dividing Cryosphere Today sea ice area data with IJIS sea ice extent data, which gives us a percentage. Short explanation: the Arctic is divided into grid cells. When 15% or more of a grid cell is covered with sea ice it will be counted as 100% covered with ice for extent (meaning the total km2 of the grid cell will be counted for total sea ice extent), whereas the exact amount of km2 that is covered with sea ice will be counted for area. This means that area will always be lower than extent, because of leads or open water within the grid cell.
Here's the thing: melt ponds fool the satellite sensors into thinking that there is open water where there is none. This will get counted for area, but not for extent (unless a melt pond is so big that the 15% threshold is passed). So, if there are a lot of melt ponds, area will go down faster than extent, and the percentage will drop.
In this case the reverse is true: extent has been very low for weeks now, but area remains average, and so the percentage is high. Because CT SIA and IJIS SIE numbers aren't perfectly compatible (grid cells for CT SIA are much bigger, for instance), I also show this compactness graph, made and updated by commenter Wipneus, that uses SIE and SIA data with higher resolution (smaller grid cells) from the same data providers, which basically shows the same:
There aren't many melt ponds right now, relatively speaking, just like in 2013 and 2014, which means that not much melting momentum is being built up, which means that sea ice decline will slow down as soon as the Arctic is cloudy, which means that chances that 2015 will make the top 3, let alone get close to record territory, look rather slim right now. Which is good for the sea ice and the Arctic in general, of course.
At the same time nothing is certain as of yet, as there are still two weeks left in June for melting momentum to be built up, and going by this thickness map based on SMOS data for February and March - that I found in this presentation - there is a lot of relatively thin ice around the Arctic, similar to 2012:
Warm and sunny weather can still do a lot of damage, especially in the Beaufort and Chukchi Seas where a lot of multi-year ice has been transported to over winter. I'll report on current conditions in the ASI updates (next one coming out at the end of this week) and cover the melt pond situation during June next month.