October 29, 2013
Does the Arctic Sea Ice Influence Weather in the South West?
It's not often that we devote an article to a learned academic article, but then again it's not often that such an article is published concerning one of our hobby horses here at econnexus.org. It's also not often that an academic paper is published just as the sort of weather it discusses is obligingly demonstrating itself. It's unheard of that such a journal article is introduced by a video! As the St. Jude day storm was causing flood alerts on the River Exe and many other Devon rivers yesterday Dr. James Screen of the University of Exeter had a new paper published in The Institute of Physics journal "Environmental Research Letters" entitled "Influence of Arctic sea ice on European summer precipitation". Here's his introduction:
In a familiar tale to those of us in the Exeter area Dr. Screen points out that:
In recent years, the UK and Northwest Europe has experienced a run of unusually wet summers. The six summers from 2007 to 2012 were all wetter than average.
In the UK, summer 2012 was the wettest summer for a hundred years, with frequent occurrences of flooding, which caused profound damage to property and some fatalities, and also caused havoc for farming and the tourism industry. At the same time we've been having these wet summers, the Arctic sea ice has been exceptionally low. The last six summers in the Arctic has seen the six lowest Arctic sea ice covers and summer 2012 was the lowest on record, with about half the ice that was there about three decades ago.
Although the recent precipitation anomalies are not without precedent, the recent sequence of consecutive wet summers is extraordinary. An important open question for scientists and decision makers is whether there are climate forcings, either natural or anthropogenic, that are increasing the chances of such events.
What this study set out to understand was whether there was any connection between this loss of Arctic sea ice and the wet summers in recent years.
The "Data and methods" section of the article explains that computer modelling has been used to investigate this potential connection:
Precipitation observations are derived from two sources: the Global Precipitation Climatology Project (GPCP) data set version 2.2 and the UK Met Office Hadley Centre England and Wales precipitation (HadEWP) data set. 300 hPa zonal and meridional winds from the ERA-Interim reanalysis, available for the period 1979–2012, are used to diagnose the mean position of the jet stream. Since the reanalysis is constrained by observations, it is considered to provide a realistic depiction of jet stream variability and is used to validate the model output.
Model output is from the UK Met Office Unified Model, which is the atmospheric component of the HadGEM and ACCESS coupled models that participated in the fifth Coupled Model Intercomparison Project (CMIP5).
In answer to the question posed in our title, James says this in his conclusions:
This study has provided evidence of a causal link between observed Arctic sea ice changes, the large-scale atmospheric circulation and increased summer Northern European Precipitation. The simulated NEP response is relatively small compared to simulated year-to-year variability. This means that whilst low sea ice coverage increases the risk of wet summers, other factors can easily negate this influence and lead to dry summers during depleted ice conditions, or wet summers during extensive ice conditions. This is consistent with the broader view that mid-latitude responses to past Arctic sea ice loss are, in general, small compared to internal variability. However, the simulated summer NEP response is statistically significant (p = 0.05) in the large ensemble presented here.
We've mentioned Dr. Jennifer Francis a few times here whilst discussing this topic. The same issue of Environmental Research Letters offered Dr. Francis the opportunity to present her own "perspective", in which she says:
This new study by Dr Screen contributes additional evidence that Arctic sea-ice loss is partly responsible for shifting weather patterns, and provides new detail about the timing, location, and types of patterns that are expected to emerge as Arctic and global warming continue unabated.
Does that sound like a collective if tentative "Yes" to you?
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