Arctic Amplification

The influence of Arctic amplification on mid-latitude summer circulation

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. We review the scientific evidence behind three leading hypotheses on the influence of Arctic changes on mid-latitude summer weather: Weakened storm tracks, shifted jet streams, and amplified quasi-stationary waves. We show that interactions between Arctic teleconnections and other remote and regional feedback processes could lead to more persistent hot-dry extremes in the mid-latitudes. The exact nature of these non-linear interactions is not well quantified but they provide potential high-impact risks for society.

The observed increases in the frequency and intensity of extreme heat and heavy rainfall events since the late 1980s, especially in mid-latitude regions, have been linked to anthropogenic global warming. Scientists are generally confident in the thermodynamic drivers of these changes but are less so in dynamic aspects. Another pronounced signal of anthropogenic global warming is the rapidly increasing near-surface temperatures in the Arctic at a pace two to four times faster than the rest of the globe, known as Arctic amplification (AA). The extent to which AA affects the mid-latitude circulation and possibly contribute to the observed increases in weather extremes has been a subject of active debate.

Future impacts from extreme weather are likely to be most pronounced in summer, as most ecological activity and agricultural production takes place in this season. Though the uncertainties are large, changes in atmosphere dynamics have the potential to cause rapid transitions at a regional scale leading to surprises for society. In summer synergistic effects between thermodynamic and dynamic drivers of extreme weather could act in the same direction to cause very-extreme extremes. Recent summers have seen such anomalous weather and these events are not well understood. This presents risks for society and in particular for global food production, given that the major breadbasket regions are located in the mid-latitudes with many crop types vulnerable to heat extremes.

The current literature provides robust evidence that AA influences mid-latitude summer circulation substantially by weakening the storm tracks. The uncertainties to do with other dynamical aspects and with how dynamical changes ultimately affect regional weather conditions are admittedly large. Nevertheless, we identified several possible feedback mechanisms for how storm track weakening can lead to persistent and therefore extreme weather in the mid-latitudes. Several studies suggest that Northern Hemisphere summer weather is indeed already becoming more persistent.

In summary, this review shows that AA is likely to have substantial impacts on mid-latitude summer circulation. The societal impacts can be severe due to tail risks arising from radiatively forced mean summer warming combined with local and remote processes that favor more persistent summer weather. A coordinated research agenda focusing on summer circulation, its drivers and extremes is needed to resolve the key knowledge gaps.

This is an extract from the article published on 20th August 2018.  The full article can be found here:  https://www.nature.com/articles/s41467-018-05256-8

 

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