The 2020 Arctic sea ice melting season is officially over – and not a moment too soon.
Sep 15th, the NSIDC extent measure bottomed out 2nd lowest by a comfortable margin.
Meanwhile, PIOMAS sea ice volume estimates also reached 2nd lowest on record, narrowly below 2019.
This has come after an early March maximum over ½ million square km above those of 2015-2018, facilitated by a cold, stormy winter. It wasn’t enough to outweigh a strong 2020 melting season.
In terms of regional volume estimates, 2020 was on par with 2012 in all but a few seas. This pattern correlates well with that of temperature anomalies not just in the melting season but the preceding winter too. You can check that out in this early March blog post, in which it was used to successfully predict one of the strongest melting seasons on record.
Coverage Matters More Immediately
Our focus here, however, is how much and how fast the sea ice extent reduced, leading to exposed Arctic Ocean. You see, open water takes up sunlight far more than ice (see: albedo). Much of the Arctic sees 24-hour daylight in Jun-Jul and sunny weather is not uncommon. So, open water causes more heat energy to be stored in the Arctic.
This year, open water developed at an unprecedented pace in the ‘Asia side’ of the Arctic Ocean. Below is a visualisation of the sea ice coverage on June 20th, 2020. This was at the summer solstice when the received sunlight is at its most intense. This is usually too soon in the melting season for there to be much open water taking up that strong sunlight – but not this year!
The huge heat uptake initiated further record-fast melt on that side, opening yet more water to sunlight during Jul-Aug. The consequence will not surprise you…
An Overheated Ocean
There’s been much discussion over the sea temperatures of the Arctic Ocean this month – and rightly so. The situation there will likely have serious ramifications for the 2020-2021 refreezing season (Oct-Feb).
Comparing mid-September sea surface temperatures (SSTs) with the rest of the top four seasons for minimum sea ice extent, none can rival 2020 for the warmth on the Asia side of the Arctic. Especially that of the Siberian seas (Laptev and East Siberian).
When we compare with the 1981-2010 long-term average (right-most image), the picture is even more dramatic. I can honestly say I’ve never seen anomalies of +5°C and above cover such a large area.
All this oceanic heat carries much more ‘weight’ compared to equivalent °C anomalies of air temperature. Water holds almost four (3.84) times as much additional energy per °C of warming.
All this energy is bound to take a long time to release into the atmosphere. This will likely cause substantial delays to the development of ice in the Asia-side seas.
So, what does this mean for Northern Hemisphere weather patterns in Nov-Dec 2020?
A Conundrum in Asia
This is not an easy question to answer. For starters, there are few historical years in which Arctic SSTs have been similar enough to make for useful comparison.
Even so, a number of recent scientific studies (summarised by Koenigk et al. 2015) have identified significant weather pattern responses to regional losses of autumn sea ice cover (i.e. anomalously open, warmed sea surface).
Of those, one stands out by far: An enhancement of a large clockwise circulation feature known as the ‘Siberian High’. This is a clear feature in the years with September Arctic SSTs similar to in 2020 (below). The strengthened Siberian High moves cold air from the Arctic to the central belt of Asia. Meanwhile, warm, often relatively moist air is moved across northern Russia from eastern Europe.
This is very striking, but it would be irresponsible to predict Nov-Dec 2020 based on this one factor.
This winter, a weak to moderate strength La Niña event is expected. This is a major driving force behind winter weather patterns (teleconnection). The same goes for SST patterns in the North Pacific and North Atlantic.
Below is a composite made up of only the years since 1990 in which those three driving forces behaved similarly to what’s expected in Nov-Dec 2020.
Here, we see the anomalous warmth in northern Russia, but the central belt of Asia doesn’t look so cold. Studying the associated sea-level pressure composite (not shown), the Siberian High is a little weaker than normal.
On balance, it’s hard to see the Arctic forcing an enhanced Siberian High in Nov-Dec 2020. Having said that, the sea ice losses are so extreme this year that it would be foolish to rule out the possibility. So, that circulation pattern will be worth looking out for as we go through Nov 2020.
Steering our eyes away to the west of Asia, there are a couple of other weather tendencies worth noting.
Scarcely Cold in the USA
Both the weather tendency plots above feature widespread anomalous warmth across the USA.
This means it’s unlikely that the Arctic situation will interfere with the predominantly mild pattern encouraged by the other teleconnections.
Chilly Western Fringes of Europe?
In Europe, when considering the two tendency plots together, it generally looks colder the further west you are. The main question is how strong that pattern is.
Arctic aside, the teleconnections favour an above-normal frequency of cold northerlies affecting western Europe. However, the Arctic sea ice edge will be unusually far away to the north of Europe this autumn into winter. That will cause northerlies to be less cold than was typical as recently as a decade ago.
It could be that these two factors cancel each other out, resulting in near-average temperatures overall. Any longer-lasting northerlies will still be able to bring notably cold conditions, however – and believe it or not, I’d not be surprised to see that happen at least once.
James Peacock MSc
Head Meteorologist at MetSwift
Cover Photo by NASA Goddard Space Flight Center (operated from Greenbelt, MD, USA) is licensed under CC BY-SA.