Through great efforts by researchers to raise awareness and understanding outside of scientific circles, it has become widely accepted that losing the Arctic Ocean’s sea ice cover would have serious negative ramifications.
The 2019 sea ice melting season (mid-March to mid-September) has been a dramatic one, to say the least. Prolonged episodes of exceptional warmth resulted in the warmest May-July on record for the Arctic region. There was also an abundance of sunshine beating down on the ice.
Yet, as I write this, the September minimum of sea ice coverage looks unlikely to rival the record low set in 2012.
That year’s weather patterns were near-optimal for inflicting unprecedented loss of sea ice coverage. This hasn’t been the case in 2019, but its own extraordinary weather patterns have reduced the sea ice volume very close to that of 2012.
‘2019s extraordinary weather patterns have reduced the sea ice volume very close to that of 2012. – The record low year for sea ice coverage.’
Read on to delve deeper into the how this has come about and why 2019’s exceptional warmth could deal an even harder blow in 2020.
2012 & 2019 During May to July: Similar but Different
The majority of Arctic sea ice melt occurs in the months of May, June and July. During these months, the Arctic region experiences a full 24 hours of sunlight, and air temperatures climb toward a late-July peak.
2012 and 2019 each had an abundance of both sunny and unusually warm weather. Their weather patterns were also similar overall, but with some key differences. These effectively caused the melt-inducing weather to be focused on different regions of the Arctic.
Figures 1 and 2 display analysis data relating to weather patterns and air temperatures, for 2012 and 2019 respectively.
Figure 1: Maps showing, for May-Jul 2012, the mean 500 mb geopotential height* (GPH; left) and the mean surface mb air temperature (right) compared to the 1981-2010 average. I’ve illustrated on the GPH map key associated movements of warm airmasses into the Arctic (red arrows) and sea ice out of it (blue arrow).
Figure 2: Same as Figure 2, but for 2019. Note the misalignment of the arrows with the North Pole this time.
Note how 2012 focused the movement of unusually warm air toward the region around the North Pole (known as the Central Arctic Basin; CAB), but 2019 toward a region on the Pacific side of the ocean (Beaufort, Bering & East Siberian Seas).
During the biggest melt seasons, the CAB becomes the last stronghold of the Arctic sea ice. It contributes to the sea ice extent and area minimum more than any other region of the Arctic.
This means that a year like 2019, which focuses sea ice melt on a different region, will struggle to rival 2012 for the extent and area minimum. Even with record-breaking warmth and abundant sunshine.
That record warmth and abundant sunshine have made themselves felt in another, more serious manner. For an unusually long time, more or less the entirety of the Arctic sea ice was experiencing at least some surface melting. To see the effect of this, we need to know about the sea ice volume**.
Fragile Floes – Arctic Sea Ice Volume Close to Record-Low
You can see a plot of sea ice volume figures (from PIOMAS**, updated every half-month) here.
The data for May-Jul 2019 tells a troubling tale of precipitous volume losses. It went from being typical of the past decade (which is already below the 1978-2018 average) to the lowest values on record.
‘The data for May-Jul 2019 tells a troubling tale of precipitous volume losses.’
During August, 2019’s sea ice volume remained close to 2012 despite that year’s sea ice coverage falling much lower. On 31st August, 2019 had 239 cubic kilometres more volume than 2012, but 891,000 square kilometres more sea ice extent.
This tells us that 2019’s additional sea ice coverage consists of some of the thinnest, most fragile ice on record.
Easy Pickings in 2020
Potentially, this could set the stage for unprecedented loss of sea ice in 2020.
This is far from set in stone, however. As is very apparent when studying the years that followed some of the other lowest few years on record for September volume.
Both 2011 and 2012 were followed by unimpressive volume increases Oct-Apr. Yet while 2012 fully exploited the thin state of the sea ice, 2013 delivered an unusually ice-friendly melting season, leading to significant increases in sea ice coverage and volume.
2016-2017 was a similar story to 2012-13, but with volume substantially lower that time around. This was a consequence of an extraordinarily ‘warm’ Arctic winter.
This past decade, the refreeze season has ranged from poor to very poor. So, it seems unlikely that 2019-2020 is going to do the Arctic sea ice a favour.
On this basis, I believe that we’re likely going to need another melting season like 2013 or 2017 to avoid chasing the 2012 records again in 2020. It’s possible, but sadly, not something I’d bet the farm on.
James Peacock MSc
Head Meteorologist at MetSwift
* Broadly speaking, above-normal 500 mb geopotential height (GPH) corresponds to more area of high pressure – hence sunny weather – than usual at the surface. Similarly, below-normal 500 mb GPH is associated with more surface low pressure systems and cloud cover than usual. In the Northern Hemisphere, surface highs have a clockwise circulation, and surface lows anticlockwise. Interplay between the two determines the large-scale motion of relatively warm and cold airmasses.
** Sea ice volume is not readily observable via direct measures, but it’s possible to use related observations to provide a good quality estimate to work with. This is what the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS, Zhang and Rothrock, 2003) does.