by Len Shaffrey
The Arctic is changing rapidly due to human emissions of greenhouse gases. Arctic sea ice extent has been declining by 12% per decade since reliable satellite estimates began in 1979. By summer 2012, Arctic sea ice extent had been reduced by nearly half of its value in 1979. The reduction of sea ice has been accompanied by rapid warming, with Arctic surface temperatures increasing at nearly twice the pace of global temperature rise.
The dramatic changes in the Arctic have raised questions about whether the loss of sea ice is having an impact on mid-latitude atmospheric circulation and on the frequency of extreme weather events. One of the first papers to address this question was Francis and Vavrus (2012) (hereafter FV12). FV12 presented observational evidence that Arctic sea ice loss (and in particular the rapid loss of Arctic sea ice loss after Summer 2007) was associated with a weakening of the mid-latitude jet stream over North America and the North Atlantic. The apparent weakening of the jet stream from 1980 to 2010 can be seen in the time series of October-November-December (OND) 500 hPa zonal winds shown in Figure 1 (which is based on Figure 3b in FV12). FV12 went on to argue that the weaker jet stream was associated with large, persistent meanders that resulted in extreme weather events such droughts and severe cold spells.
Figure 1. Time series of OND 500 hPa zonal winds (averaged over 140°W to 0°E and 40°N to 60°N) from 1979 to 2010 (black) and with a 5-year running mean applied (blue). Note that for the running-mean time series the 2 years at the beginning and end of the time series are determined by averaging 3 or 4 years years rather than 5. This figure is based on Figure 3b of Francis and Vavrus (2012). Data from NCEP reanalyses.
FV12 received substantial attention from the scientific community. Subsequent a large number of papers were published that also argued for a link between Arctic sea ice loss and changes in mid-latitude circulation (e.g. Lui et al. 2012; Tang et al. 2013, etc.) or argued that the links weren’t statistically robust (Barnes, 2013; Screen and Simmons, 2013; Barnes et al. 2014, etc.). For more details of the debate, see the review papers of Cohen et al. (2014), Barnes and Screen (2015) and Shepherd (2016).
As five years have passed since the publication of FV12, I thought it might be worth revisiting the paper to see if a few extra years of data might provide some additional insight. Figure 2 extends the time series of 500 hPa seen in Figure 1 to OND 2016. It also extends the time series backwards in time to 1948 to provide some additional context. Figure 2 shows no evidence for a weakening of the OND 500 hPa zonal winds over North America and the North Atlantic in the extended time series between 1948 and 2016. It also suggests that the apparent weakening of the OND 500 hPa zonal winds seen in FV12 was due to the choice of the end date of OND 2010. OND 2010 was characterised by the extremely negative North Atlantic Oscillation (NAO) pattern over the North Atlantic, which resulted in a weakened jet stream. Within a few months, the NAO and the strength of the jet stream would return to normal. In the following year, OND 2011, the NAO was strongly positive and the jet stream was stronger than usual. It seems that the apparent link between Arctic sea ice loss and the mid-latitude jet stream is very sensitive to adding a few extra years of data. It also suggests that the arguments and conclusions of FV12 haven’t stood the test of time.
Figure 2. Time series of OND 500 hPa zonal winds (averaged over 140°W to 0°E and 40°N to 60°N) from 1948 to 2016 (black) and with a 5-year running mean applied (blue). Data from NCEP reanalyses.
References
Barnes, E. A., 2013. Revisiting the evidence linking Arctic amplification to extreme weather in midlatitudes. Geophys. Res. Lett. 40, 1–6.
Barnes, E. A., Dunn-Sigouin, E., Masato, G. & Woollings, T., 2014. Exploring recent trends in Northern Hemisphere blocking. Geophys. Res. Lett. 41, 638–644.
Barnes, Elizabeth A. and James Screen, 2015. The impact of Arctic warming on the midlatitude jetstream: Can it? Has it? Will it?. WIREs Climate Change, 6, doi: 10.1002/wcc.337.
Francis, J. A., and S. J. Vavrus, 2012. Evidence linking Arctic amplification to extreme weather in mid-latitudes, Geophys. Res. Lett., 39, L06801, doi:10.1029/2012GL051000.
Liu, J., Curry, J. A., Wang, H., Song, M. & Horton, R., 2012. Impact of declining Arctic sea ice on winter snow. Proc. Natl Acad. Sci. USA 109, 4074–4079.
Screen, J. A. & Simmonds, I., 2013. Exploring links between Arctic amplification and mid-latitude weather. Geophys. Res. Lett. 40, 959–964.
Shepherd, T.G., 2016. Effects of a warming Arctic. Science, 353 (6303). pp. 989-990. ISSN 1095-9203 doi: 10.1126/science.aag2349
Tang, Q., Zhang, X., Yang, X. & Francis, J. A., 2013. Cold winter extremes in northern continents linked to Arctic sea ice loss. Environ. Res. Lett. 8, 014036.