Paper: The influence of Arctic amplification on mid-latitude summer circulation

[donsutherland1]

Since June 1, the Arctic Oscillation (AO) has been negative on 47/55 (85%) days and the North Atlantic Oscillation (NAO) has been negative on 52/55 (95%) days. In other words, summer 2019 has featured persistent and often strong blocking.

That persistent blocking may well have contributed to the two historic heat waves that have shattered all-time record high temperatures in Europe. Further, according to some literature, the tendency for summer blocking may be linked to Arctic Amplification (AA) and or the weakening Atlantic Meridional Overturning Circulation (AMOC), both of which are being driven by climate change.

A paper published in Nature on August 20, 2018 addressed AA's potential summer impact. Excerpts are below:

Accelerated warming in the Arctic, as compared to the rest of the globe, might have profound impacts on mid-latitude weather. Most studies analyzing Arctic links to mid-latitude weather focused on winter, yet recent summers have seen strong reductions in sea-ice extent and snow cover, a weakened equator-to-pole thermal gradient and associated weakening of the mid-latitude circulation...

Links between AA and summer circulation have received far less scientific attention, despite the potential for synergistic effects that might favor high-impact extremes. In summer, thermodynamic and dynamic drivers of extreme weather could act in the same direction, leading to tail risks17. For instance, any increased frequency in circulation regimes conducive to persistent heat extremes would act on top of the thermodynamically driven increase in heat, creating possibilities for very-extreme heatwaves. Many recent high-impact summer heatwaves indeed occurred in that far-tail of the distribution and cannot be explained by the direct thermodynamic effect of greenhouse gas forcing alone (Box 1)18–21. Such extreme heatwaves have been found to increase and intensify across most regions but more so in the mid-latitudes than over the rest of the globe20. Consistent with the increase in heatwaves, the hot tail of summer temperature distribution has been warming faster than the median and the cold tail. Figure 1 shows the warming trends in the 95th percentile (hot tail), 50th percentile (median), and 5th percentile (cold tail) of daily summer temperatures. Clearly, over most mid-latitude regions, in particular over Eurasia but less so in the US, the hot tail has been warming faster than the cold tail and thus temperature variability in summer has increased22. This increased variability indicates that more complex processes beyond simple radiative greenhouse gas forcing are important in driving heat extremes (Box 1). This is supported by recent studies that indicate that summer weather has become more persistent in several regions in the mid-latitudes23,24,25. In summer, the hot tail of the distribution is associated with persistent, blocking weather systems, and an increase in their persistence leads to more extreme temperatures.

Recent studies indicate that the Atlantic Meridional Overturning Circulation (AMOC, i.e., the large-scale north-south transport in the Atlantic ocean) has seen an unprecedented slowdown in recent decades103,104, something which is projected for future warmer climates as well. This slowdown results in anomalously cold SSTs over the northern Atlantic which can trigger a quasi-stationary Rossby wave response favoring blocking high-pressure systems over western Europe105. So, just like AA, a slowdown of the AMOC leads to weakening westerlies in summer over the Atlantic sector, favoring persistent hot-dry extremes over Europe106. Recent observational studies indeed indicate that weather persistence in Europe and some other mid-latitude regions has increased in boreal summer23,24,25,102.

https://rdcu.be/bLNgn

[chubbs]

Thanks Don, does appear relevant this year.

[Typhoon Tip]

I find the ocean conveyor stuff intriguing... I recall papers and forum discussion - not local to the history of BB, Wright and Eastern public social media/Internet outlets...or those since, but well prior.   Way back in the mid 1990s during the early days of web news-feeds, there were mindful exchanges regarding the fluxing of fresh water from glacial reduction, into the oceanic basins, and plausible climate ramifications.  The then more primitive early models of the era suggested density discontinuity --> breakdown of what was called the "Thermohaline Circulation"

There's plenty of information on the web within a mouse click of a simple phrase typed into any in the myriad of competing search engine, so not gonna bother with primer on what that is..

But the short version is that when the ocean surface density changes due to fluxes of fresh water, this water does not cold "chimney" as readily... In a more discrete conceptual level, it has to do with the specific gravity of fresh water versus an acquiesce solution of saline water at the same temperature. Salt water weighs more... tending to freshen water above.  Arriving warm saline waters from the south mixes with the fresher water in stable mass exchange, but with increased glacial flushing ...the fresh water is in positive anomaly from the added melt sourcing... disturbing that balance.

There is thus reducing mass-conservation, physical forcing that pulls the warm surface water from the Equatorial latitudes toward the N.  The end result is a slowing of the AMOC in total -

What we are observing is a veritable confirmation on those earlier studies... 

As far as how that effects the atmosphere and the propensity for blocking - I have mixed intellect on that.  

The temporal super-position of the 11, 22 and 300 year solar cycling are coincident over the next 15 years...staring some 5 years ago... Which means through almost 2030...we should be seeing unusually low frequency of solar events... These eras are correlated to cooling Global temperatures ... admittedly, prior to the onset of Industrialization of Humanity and the latter becoming more of an actual geological force on the environment. 

Therefore ... it is hard to deconstruct how much AMOC is directly causal in the blocky characteristic to the general circulation medium ... and how much would tend to take place anyway, because -AOs have historically shown more frequency during solar minima.

But, this discussion is heavily textured.  It's a loaded subject matter.  This latter atmospheric response stuff is one thing... But the AMOC possibly being causally linked to Anthropogenic activity is astounding..

[Vice-Regent]

What is remarkable is how warm the East Coast has been especially south of NYC despite the favorable indicators for below average temperatures. (-AO/-NAO)

The global warming signal is very strong and as a result the Hadley Cells are very expansive.

[Bhs1975]

What is remarkable is how warm the East Coast has been especially south of NYC despite the favorable indicators for below average temperatures. (-AO/-NAO)
The global warming signal is very strong and as a result the Hadley Cells are very expansive.

Yeah its seems like we need a deep trough just to get normal temps.


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[Bhs1975]

I also think the expanding Hadley cells are make landfalling hurricanes more likely.


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