December 2015 will go down in meteorological history as one of the wettest – and warmest – on record. It will also be remembered for the devastating floods in Cumbria, Lancashire, Yorkshire and Scotland. The extensive flooding of homes and businesses, loss of electrical power, major damage to roads and bridges, and disruption to the rail network have caused great misery and incurred huge losses.
In this blog our Chief Scientist, Professor Dame Julia Slingo, discusses what factors may have influenced the record breaking weather we have seen in recent weeks.
As with all high-impact weather, the meteorological set-up was critical in defining the severity of these events. Throughout the month, the winds have come from the south or southwest, bringing both extreme warmth but also very high levels of moisture.
There has been a lot of debate whether this has been associated with El Nino – an intermittent warming of the equatorial Pacific Ocean which has been very strong this year – or whether this is a sign of a changing climate. The links to El Nino are certainly very clear in the set up of large waves (troughs and ridges) in the atmospheric circulation, which we expect to see in these events.
However, it does seem that this year the unusual warmth of the North East Pacific Ocean may have altered the position of these waves across North America and into the Atlantic sector, setting up the conditions for the devastating tornadoes in the US and for the southerly feed of moisture-laden air into the UK.
Storm Desmond in early December was associated with a strong west-south-westerly flow around the ridge over the eastern seaboard of the US, reaching far back across the Atlantic, as far as the Caribbean. With ocean temperatures well above normal in the southern part of the North Atlantic (see above) – possibly due to the much weaker than normal hurricane season this year associated with the current El Nino – the air was primed with more moisture than normal. This river of atmospheric moisture fed the storms that formed on a stronger than normal jet stream, and as the air impinged on the mountains of Cumbria, large quantities of rainfall were released.
Later in the month the southerly flow intensified, with a high pressure system to the east of the UK over continental Europe providing a block to the normal passage of the westerly jet. With colder than usual ocean temperatures over the northern part of the North Atlantic (see above), a strong temperature gradient formed which acted to strengthen the jet and set up the conditions for the formation of rapidly deepening cyclones, such as Storm Frank. These cyclones drew in warm, moist air from far south leading again to heavy rainfall and further flooding on already saturated ground. And the southerly winds on the eastern flank of Storm Frank, and strengthened by the high pressure to the east, enabled extremely warm air to penetrate, temporarily, the deep Arctic leading to very high temperatures.
The potential for December to be stormy and wet was picked up in the three-month outlook and is consistent with what we expect in early winter when there is a strong El Nino in place. However, early analysis suggests that the specific nature of this December’s extreme weather might be linked to the detailed structure of this El Nino, to the warmth of the north-east Pacific Ocean and to their combined effects on the atmospheric circulation.
As for whether climate change has played a role, we know that the overall warming of the oceans increases the moisture content of the atmosphere by around 6% for every 1°C warming. This extra moisture provides additional energy to the developing weather system, enabling even more moisture to be drawn in to the system, so that the overall enhancement of rainfall when the moisture-laden air impinges on the mountains of Wales, northern England and Scotland may be even more significant. So from basic physical understanding of weather systems it is entirely plausible that climate change has exacerbated what has been a period of very wet and stormy weather arising from natural variability.