Early in July, a divergence of 2018 surplus ice resembled a hockey stick temporarily. Though the blade later drooped downward, ice extent remained above average throughout July. The graph above shows 2018 300k km2 above the 11 year average (2007 to 2017 inclusive). Only 2015 and 2008 had a higher July monthly average extent. Note that SII (NOAA’s Sea Ice Index) was lower by 436k km2 in 2018, and SII 11 yr. average is lower by 264k km2.
The surprise: This is the first 2018 month above the average. Indeed March 2018 (annual maximum) was almost 500k km2 lower than March 11 yr. average. But reduced rates of melting in May, June and July have resulted in more ice extent than most other recent Julys. At end of July, 2018 ,2017 and 11 yr. average are close together, with SII 600k lower and 2007 with 824k km2 less ice.
The table below shows ice extents by regions comparing 2018 with 11-year average (2007 to 2017 inclusive) and 2017.
2018 is 86k km2 above average (1.2%). Laptev, Kara, Greenland Sea and Central Arctic are down. Offsetting surpluses are in Beaufort and East Siberian seas, as well as Hudson and Baffin Bays. Since the two bays will melt out soon, the eventual annual minimum remains to be seen.
Interesting commentary from Dr. Judah Cohen at his July 30, 2018 Arctic Oscillation and Polar Vortex Analysis and Forecasts Excerpts in italics with my bolds.
In the rest of the Impacts section I want to discuss what I wrote in the June 4th blog as it seems to be very relevant for this summer and especially the European summer version of western North America’s winter ridiculously resilient ridge. In the blog I described how a diminishing cryosphere (snow and ice) might be contributing to more persistent and amplified waves in the atmosphere a physical mechanism different from what I typically describe in winter. This mechanism may just be the best explanation of what occurred this summer over Europe. Though probably the precondition of the soil or its desiccation is probably another important contributor to the heat and dry conditions over Northern Europe this summer:
“Over the past several blog posts I have been discussing blocking and ridging over northern Europe with a split Jet Stream across Europe with the polar branch way to the north across northern Scandinavia and a second subtropical branch across the Mediterranean. In between the two Jet Streams has been a sort of no-man’s land with weak zonal winds in the mid-troposphere across much of Central and Northern Europe. This has resulted in a warm spring so far and for the months of April and May, Europe has seen possibly the largest positive temperature departures from normal for the entire Northern Hemisphere (NH).
A warm spring is different than a hot summer and had this atmospheric circulation occurred in July and August instead of April and May it would have created greater news headlines, straining resources and resulting a likely spike in mortality. Europe has been experiencing more frequent hot summers over the past couple of decades with possibly the most infamous being 2003 but even as recently as last summer, when extreme heat accompanied by forest fires were common across Southern Europe. The atmospheric circulation across Europe this spring with blocking and a split Jet Stream is consistent with an idea that Arctic change is resulting in more frequent occurrences of extreme summer weather including flooding, drought and heat waves. This is admittedly not my expertise but I thought it could be interesting to give a brief discussion given the weather pattern across Europe this past spring, which could be laying the groundwork for an overall hot upcoming summer.
In winter the loss of sea ice has contributed to accelerated warming across the Arctic Ocean referred to as Arctic amplification. This is hypothesized by some including me to influence mid-latitude weather either by weakening the zonal Jet Stream or by favoring large scale anomalous atmospheric waves that project onto the climatological waves forced by the geography of the NH. Amplification of the climatological waves leads to a breakdown of the polar vortex followed by increases in severe winter weather across the NH. This is a topic that write about often in my blog posts in the winter months.
Other scientists have postulated something somewhat analogous but also different for the warm season. During the warm boreal months it is much harder for the Arctic Ocean to warm rapidly relative to normal because even with increased ice melt, the ocean remains colder than the overlying atmosphere so energy is transferred from the atmosphere to the ocean rather than vice versa as in winter. Therefore we have not observed in the Arctic Ocean basin the extreme warm events in summer as we do in winter. Instead the rapid disappearance of snow cover in the spring and early summer has allowed the land masses that ring the Arctic Ocean to heat up much more quickly today than they used to two or three decades ago. So the accelerated Arctic warming in summer is not observed over the Ocean but rather across the adjacent land masses of Eurasia and North America.
The accelerated warming to the north can still cause a slackening of the zonal Jet Stream as the south to north temperature gradient weakens. Instead of one Jet Stream across the mid-latitudes, the Jet Stream splits into two pieces one to the north and a second to the south. The northerly Jet forms along and just north of the land regions that are experiencing the most accelerated warming that ring the Arctic approximately along the 70°N latitude, as a the strong warming along the north slope of the continents with a still relatively cold Arctic ocean maintains a strong temperature gradient and a Jet Stream. The southerly Jet Stream forms where the normal south to north temperature gradient resumes across the southern mid-latitudes or in a band between 30-45°N latitude. In between the two Jet Streams the winds are very weak. It turns out this atmospheric configuration with a Jet Stream to the north, a Jet Stream to the south and very weak winds in between is ideal for trapping waves that are persistent in one location and can even amplify. This is referred to as quasi-resonant amplification (QRA). When QRA occurs, atmospheric waves become trapped and persist for much longer periods than normal. This in turns leads to an increased probability of extreme weather whether it be floods, drought or heat waves. The split Jet Stream, the persistent atmospheric waves and extreme weather have all been observed to be increasing over the past two decades. Some early papers on the subject are Petoukhov et al. 2013 and Coumou et al. 2014. The weather models are predicting this latest example of QRA over Europe to dissipate over the coming two weeks but a recurrence of QRA over Europe or a different region this summer is not only of meteorological interest but of societal importance.”
via Science Matters
August 1, 2018 at 02:21PM