Dr Flora Whitmarsh, Science Communications Research Analyst in Climate Change at the Grantham Institute, Imperial College London

The widespread flooding in the south-west of the UK this winter was associated with unusually persistent stormy weather and unusually high rainfall totals in December 2013 and January 2014. The causes of this weather and any possible link to climate change are discussed below.

Extreme rainfall

Warmer temperatures are increasing the frequency and intensity of heavy rain. This may have contributed to high rainfall totals on individual days, but does not explain the remarkable persistence of the heavy rain. Read more.

Sea level rise

Sea level rise will have contributed to coastal flooding. We know that sea level along the English Channel has already risen by 12 cm over the last century. Increases in mean sea level translate into increases in the most extreme wave heights. IPCC Working Group I found that there had likely been an increase in the incidence or magnitude of extreme high sea level globally since 1970, and that this was due to the increase in mean sea level. The reasons for sea level rise are of course well understood: the total amount of ice in glaciers and ice caps is reducing globally due to warmer temperatures; and warmer sea water takes up more space than cooler water due to thermal expansion [1].

Unusual weather patterns

The exceptional persistence of the stormy weather this winter was the result of the unusual path taken by the jet stream. In the Pacific, the jet stream propagated further north than usual over Western Canada, bringing cold weather to the United States when its path came southward. This cold air increased the temperature contrast between the pole and the mid-latitudes in the North West Atlantic. This enhanced temperature contrast strengthened the Atlantic jet stream; it was observed to be up to 30% stronger than normal. Instead of its usual tilted orientation, passing off North West Scotland, the jet went straight across the Atlantic towards The Bay of Biscay. Weather systems grew on this strong southward displaced jet stream and moved up into the United Kingdom throughout December and January, bringing wind and rain.  

The unusual behaviour of the jet stream was linked to global weather systems. The warm ridge over the west coast of North America, as observed this December and January, is usually associated with La Niña conditions. However, the surface temperatures in the Pacific were not consistent with La Niña: they were characteristic of the ongoing negative phase of the Pacific Decadal Oscillation. Despite this, tropical precipitation patterns in December and January were consistent with La Niña conditions: there was more than the usual precipitation over Indonesia. This enhanced rainfall over Indonesia helped disturb the North Pacific jet stream, leading to the warm ridge. The unusually strong high altitude polar vortex [2] this year may also have played a role in strengthening the jet stream. This may have been influenced by the current westerly phase of the Quasi-Biannual Oscillation [3].

Is the jet stream changing?

The jet stream is driven by the temperature contrast between the Arctic and the mid-latitudes (temperate zone); a contrast that is now weakening in the lower atmosphere due to enhanced polar warming [4]. Francis (2012) argued that this is making the jet stream slower on average, and its path is becoming more meandering as a result, leading to more frequent blocking episodes [5]. Although the impacts of climate change on the jet stream are still the subject of active research and much debate, this theory has been put forward in the mainstream press as an explanation for the extreme winter weather this year in both the UK and US.

The theory cannot however explain the weather we have observed this winter because the jet stream was in fact stronger than usual, not weaker. The strengthening of the Atlantic jet was due to the enhanced temperature contrast between the cold polar air and warmer mid-latitude air in the western Atlantic. This enhanced temperature contrast was caused partly by the cold air that was deflected south by the Pacific jet stream. It was further enhanced by unusually warm conditions in the North Atlantic at 30 degrees north.  In addition, the longer term weakening in the temperature contrast between the poles and the mid-latitudes is found only at very low levels in the atmosphere.

Wind speed
Figure 1: Average January wind speed (colours) and direction (arrows) (left). Wind speed and direction in January 2014 (right). The faster than usual winds over the Atlantic are clearly seen. Image: Met Office. The wind speeds are shown at an altitude of 850 millibars (measured in pressure units), or around 1.5 km.

figure 2
Figure 2: The difference (anomaly) in the wind speed and direction in the period December 16th 2013 to January 15th 2014 compared to the average for that period. The wind speed anomaly is in metres per second (given by the colours) and wind direction is given by the arrows. The wind speeds are shown at an altitude of 250 millibars (measured in pressure units) or around 10 km. This corresponds roughly to the top of the troposphere, the lowest atmospheric layer.

Notes

[1] At warmer temperatures, the water molecules have more kinetic energy on average, so the spaces between molecules get larger, meaning that the water takes up more space than it would at lower temperatures.

[2] The polar vortices are persistent cyclonic (i.e. anticlockwise in the Northern Hemisphere) circulation patterns located above each of the poles. In this case the North Polar Vortex is referred to.

[3] The Quasi-Biannual Oscillation (QBO) refers to the changing winds in the tropical stratosphere (roughly 18-50 km above the Earth’s surface). Winds go either from west to east or from east to west, changing roughly every couple of years.

[4] The poles warm more quickly than the mid-latitudes. A significant cause of this is the high albedo (reflectivity) of ice. When ice melts, the remaining water or land is less reflecti ve, meaning less solar radiation is reflected back to space, enhancing the warming.

References

The Recent storms and Floods in the UK. Met Office briefing report. February 2014.http://www.metoffice.gov.uk/media/pdf/1/2/Recent_Storms_Briefing_Final_SLR_20140211.pdf Retrieved Feb 27th 2014.

Met Office web page: http://www.metoffice.gov.uk/climate/uk/interesting/2014-janwind.pdf  Retrieved 26thFeb 2014.

Francis, J.A. and Vavrus, S.J.,  2012. Evidence linking Arctic amplification to extreme weather in mid-latitudes. Geophysical Research Letters. Vol 39. Issue 6.