Atmospheric Rivers

At the end of January 2012 the Department for Environment, Food and Rural Affairs (Defra) published the first Climate Change Risk Assessment (CCRA) outlining the potential impacts of climate change on the United Kingdom (UK). The largest risks identified in the report concern water, with summer shortages and winter flooding expected to become more commonplace in the UK. With an increased risk of winter flooding, it may be necessary to develop and improve flood defences to protect UK infrastructure and society. But before considering future flooding, what is the key atmospheric driver of UK winter flooding? This is where Atmospheric Rivers (ARs) come in.

An AR is a narrow filament along which moisture is transported from the subtropics across the mid-latitudes (see Figure). At any one time there are four or five ARs present across the mid-latitudes where approximately 90% of poleward water vapour transport occurs. ARs are located in the lower troposphere within the broader warm conveyor belt of extra-tropical cyclones and are regions of high water vapour content and strong winds. The large water vapour transport in ARs is essential for water supply, but also a hazard due to the heavy precipitation and flooding that can occur when an AR makes landfall. Most research investigating ARs and flood occurrence has been in western North America. Colloquially known as the “Pineapple Express”, these ARs can transport moisture from near the Hawaiian Islands to North America.

Recent research has shown that winter flooding in Britain is connected to ARs. The devastating Cumbrian floods in November 2009 were caused by a persistent AR that was located over Cumbria for about a day (Figure a). The moisture transported in the AR was forced to rise over the Cumbrian Mountains causing intense rainfall and flooding. Even more recently on 17th November 2010, Cornwall experienced heavy rainfall and floods; the AR behind this event is also shown in the Figure (b). Moreover, ARs have been linked with the 10 largest winter floods in a range of British river basins further indicating that ARs are crucial in explaining winter flooding in the UK. One complicating factor in linking ARs with floods is the river basin itself. Each basin responds to rainfall in a different way depending on properties such as the geology underlying the basin, the steepness of the terrain and land use. With impermeable bedrock, steeper mountains and higher rainfall receipt in western Britain, basins in this region have a rapid-response to rainfall and consequently the strongest AR-flood connection in the UK.

With a clear link between ARs and UK winter flooding, what will happen to ARs under current climate change projections? Two factors could affect ARs in a warming climate. Firstly, a change in AR frequency is likely to impact the number of winter flood events. So if the large-scale atmospheric circulation alters as to cause more persistent extra-tropical cyclones and their associated ARs to hit the UK, then there is a possibility for more winter flood events. Secondly a warmer climate is likely to give rise to an increase in saturation vapour pressure and higher atmospheric water vapour content. It is thought that this will change the hydrological cycle and intensify precipitation extremes leading to a risk of larger floods. However, as yet the effects of anthropogenic climate change on ARs over the North Atlantic are not certain. An assessment of the latest climate change projections will be a good tool to aid our understanding of future changes to ARs that strike the UK.

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