J A Screen 2013 Influence of Arctic sea ice on European summer precipitation Environ. Res. Lett. 8 044015
http://iopscience.iop.org/1748-9326/8/4/044015/article

Running time: 4:06 min

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. What this study set out to understand was whether the was any connection between this loss of Arctic sea ice and the wet summers in recent years.

Summer rainfall over Europe is strongly influenced by the location of the jet stream. The jet stream is a channel of strong winds high up in the atmosphere, around ten kilometres about the Earth's surface. These winds blow generally from west to east but with large meanders, or wiggles, to the north and south. These strong winds steer our weather systems and the rain that they bring. In this animation, the coloured arrows show these upper level winds. The orange and red arrows show the regions of strongest winds, which make up the jet stream. The jet stream is very mobile and moves around from day to day. Sometimes it is found to the north of the UK and other times it is found to the south. This wandering of the jet stream has a big influence on our weather.

The study used a climate model to investigate how the melting sea ice impacts weather in Europe. In the model we can change the sea ice in a controlled way, whilst keeping other factors that are known to influence the climate constant. In doing this we can isolate and measure what impact the sea ice has, or the melting sea ice has, on our weather. 

In the model when the Arctic sea ice was decreased, we saw a shift towards wetter summers over the UK and over northwest Europe. The pattern of rainfall anomalies in the model looked very similar to those which have been observed, in reality, in recent years. This led us to the conclusion that the loss of Arctic sea ice has been one factor that has contributed to wetter summers seen over the recent period.

Normally in summer the jet stream lies between Scotland and Iceland and this steers weather systems to the north of the UK. What we found is that the melting sea ice shifts the jet stream further to the south, and that means more weather systems hit the UK and that brings more rain.

Recent summers have highlighted some of the implications of this increased rainfall. We've seen frequent occurrence of flooding, which have caused damage to property and fatalities. We've seen problems in the farming industry and the tourism industry - farmers have lost their crops, more people have taken their holidays abroad. Also wildlife has been affected. So, this really has a big impact on us all. 

The impacts are not just over northwest Europe. Actually in the model, what we find is that whilst the sea ice loss increases rainfall over northwest Europe, we actually find drier conditions over Mediterranean Europe. Also the jet steam shifts over North America, which can have implications for the weather there too.

This study only examined the impacts of sea ice loss over the past thirty years. We haven't looked at the impacts of future sea ice loss, so we can't make predictions of future weather. It is also important to note that the changes in ice are only one factor that will influence our weather and future conditions will depend on many things. But, what we can say is that if sea ice continues to melt as it has done over recent decades, there is an increased risk of wetter summers in the UK and northwest Europe.

Previous work has suggested that European summer climate may be influenced by a slow natural cycle of warming and cooling in the North Atlantic Ocean. Since around about the late 1990s, the North Atlantic has been in the warm part of that cycle and that tends to favour wetter conditions over the UK and northwest Europe in summer. So we think it's probably a combination of both this natural cycle of warming in the North Atlantic and the melting of sea ice that have together contributed to the wetter summers.

What we'd like to do now is look at how future estimates of sea ice may impact our weather and make predictions of how that impacts our climate, and see whether we can make predictions of future summer rainfall over decades to come.