Why is extent so low?

This post looks at why Northern Hemisphere (NH) sea ice extent is so low at present. A commenter Dreesen asked what I thought about it, and due to work pressures I hadn't looked at sea ice for weeks. The low sea ice is likely due to a combination of long term reduction due to global warming and weather in the North Atlantic. But temperatures in January were extreme, and I lack a convincing answer as to why.

(This post was largely written on Sunday but I've been too busy to do a final check and post until now.)

NSIDC Extent for January was the lowest on record, at 13.53M km^2 this beats the previous record of 13.62M km^2 in January 2010.

2016 has been tracking the lower end of the envelope of extent (NH) throughout this year so far.

By breaking extent into regions using Wipneus's calculations of regional extent it is possible to see where the deficits that lead to the low extent are. Here are the sectors used.

Anomalies are used with a baseline of 1981 to 2010 in the following two plots.

First a reminder of the state last year.

Then, as discussed last year, the greatest deficit compared to the long term climatology was in the Pacific Sector. This year the greatest deficit is in the Atlantic Sector.

So what regions in the Atlantic Sector are contributing to this deficit in sea ice? Here I turn to my usual bars and point plot. All data is transformed to a series of anomalies from a 1981 to 2010 baseline. The bars show the range of anomalies for 1979 to 2016 for 5 February, the red triangles show the anomaly on 5 February 2016.

Total extent is very low and further calculations show that Barents Sea is 44.9% of the Total negative anomaly, Greenland Sea is 12.1% of the total, so 57% of the Total (i.e. all regions) anomaly is due to the Atlantic sector on 5 February 2016.

Before I go on to look at the reasons for the low extent in the Atlantic Sector I need to digress. NCEP/NCAR temperatures north of 70degN are at record high levels for the winter so far (October to January).

Most of this is due to January, which for the surface is -18.3degC, that compares with an average of -23.6 for the preceding 10 years (Jan 2006 to Jan 2015).

It is worth starting by looking at the temperature plot for north of 70degN expressed as the same anomaly as above. Note that all of the scale is positive and the temperature scale runs from 0 to 15 degC, with dark red being in excess of 15degC and dark purple being below zero.

There is a massive warm anomaly over the Pacific half of the Arctic Ocean (bottom right quadrant), but the anomaly over the Atlantic sea ice edge is over 15degC (upper left quadrant). The former feature is unusual, the latter is common to recent years but is far warmer than normal.

The Atlantic anomaly is common to recent years and is caused by ice being there in the 1981 to 2010 average, but absent in recent years. Ice over the ocean keeps the air above cooler, remove the ice and temperatures soar. Examining vector wind anomaly for January shows that the Atlantic Sector warmth is caused by winds blowing towards the ice pack from the North Atlantic.

This has the effect of compressing the ice against the ice pack and will draw in warmer ocean water under the pack resulting in a combination of melt and compression. The reduced ice edge and warmer air intrusion results in warmer temperatures. Further analysis (not shown) shows that the state of ice in Barents Sea is not likely to be a good predictor of September extent.

Turning back now to the extreme warmth of January 2016, I have been unable to come up with a convincing explanation. However here's some more information and comment. Note that a possibility is a problem in the NCEP/NCAR reanalysis model, that's noted but I don't consider it a likely explanation in that I have never seen anything like it. I have just rechecked and the extreme warmth is still in the data.

Having calculated the trend of January surface temperatures for the four quadrants and the whole region north of 70degN I have then calculated the difference from trend for each year, these are known as the residuals from the trend.

Residuals for all four quadrants and for the whole region north of 70degN exceed 4degC for January 2016, a situation that has not happened since 1948, the start of the data. Residuals for the entire region north of 70degN are 3 standard deviations, using the standard deviation calculated for 1948 to 2016. This is extreme and suggests that a process not related to that driving the trend is at work.

The percentage contribution for all four quadrants is shown below, the warming in the lower right quadrant of the above plots (180 to 270 degrees) is the greatest single contribution, the other regions are similar in contribution. However there is no domination from a single quadrant (unlike the case of the sea ice sector extent discussed above).

So I step back and look at the wider pattern over the Northern Hemisphere.

And in the above map plot we see a familiar pattern, the Warm Arctic Cold Continents pattern, the warmth over the Arctic is surrounded by a band of below average temperatures. This is associated with the low phase of the Arctic Oscillation (AO), and the AO has been low in January.

However there have been low AO indices in winter months without such extreme warmth. An additional factor might be the PDO, this has generally been negative in recent years but was positive last year and remains positive.

One thing that might explain the warmth are the wind patterns into and out of the Arctic, but I lack the experience to say whether this is unusual enough to account for such unusual temperatures. So I show this graphic for information, in no way claiming it's the cause of the warmth. Red arrows show large flows, the black line shows the region of temperatures below -10degC in the January average.

One thing that is notable to me is the cold outbreak towards China. That's because I have recently read Zuo et al 2016 "Predictability of winter temperature in China from previous autumn Arctic sea ice". The time may have come to revisit the impacts of sea ice loss on weather.