November 25, 2020

I, Science

The science magazine of Imperial College

Although this Christmas was unusually mild the severe cold and snow of the past two winters still sticks in my memory, my long-johns ready for imminent use. I remember friends having train journeys that had 4-hour delays and roads being brought to a standstill whilst we made structurally poor igloos and trans-gender punk snowpeople. These were scenes I’d never witnessed before, with snow being something that lasted for half a day each year at most through out my childhood. When looking at historical records my memory appeared surprisingly accurate, with both winters being two of the coldest three in my 23-year lifetime and December 2010 was the coldest for over 100 years. But what controls our winter climate and caused the winters of 2009/2010 and 2010/2011 to be so severe and will we see more of these cold winters in the future under a changing climate?

During the snow last year I had returned home for Christmas (going to old sledging spots and realising they appeared much steeper that year than to my seven-year–old self) and I was frequently told by family and friends that this weather was produced by a drastic change in the path of the Gulf Stream up towards Greenland. This idea had not come from nowhere but was being widely disseminated by the TV weather programmes with appropriate graphics of schematic Gulf Streams providing all the proof anyone needs. It is true that the Gulf Stream transports large quantities of warm water northwards, however strictly the Gulf Stream only refers to the current up to the latitude of Canada’s east coast when it stretches out into the Atlantic. This is the Gulf Stream extension and it is only up to here where the large quantities of heat are transported. A current continues north of here up towards the UK but it is much more diffuse, has an inconsistent path and carries far less heat.

Kevin Trenberth and Julie Caron from the National Centre for Atmospheric Research in Colorado used a combination of atmospheric models heavily constrained by observations and top of the atmosphere radiation budgets from satellites to estimate the heat transported in the Atlantic Ocean at different latitudes. They found that ocean heat transport was insignificant as you approach the latitude of the UK in comparison to the heat transport of the atmosphere, which agreed with other teams’ estimates from ocean observations. Furthermore, Richard Seager and colleagues investigated the heat release from the ocean surrounding northern Europe and found that the majority of winter heat release was from local storage and not due to heat transported by ocean currents. They went even further and ran models where the effects of ocean currents were not included. These model experiments saw little change in our winter climate.

Although the Gulf Stream appears a simple explanation of our changing winter climate, the experiments and calculations discussed above show that the transportation of heat by the ocean has little direct effect on our winters. The recent finding of deep atmospheric warming by the Gulf Stream along the east coast of the USA means that soon an indirect effect of ocean currents on our winter climate could be discovered; however the idea of the Gulf Stream lapping, or not lapping in the case of the past two winters, its warm waters onto our shores is a myth.

If the source of our recent cold winters cannot be found in the ocean then it is only sensible to look up in to the atmosphere for an explanation. The jet stream is a lane of fast moving air, centred at the height air liners fly, which flows from America to Europe across the Atlantic. However the jet is not straight but has waves that undulate north and south and which travel against the jet stream, and so can be almost stationary depending on its speed resulting in persistent weather systems. Like waves in the ocean, these atmospheric waves can also break when the wave overturns, often resulting in an isolated air mass of warm southern air breaking off into the northern air called blocking highs (not unlike an oxbow lake breaking off from a meandering river!). Due to conservation of angular momentum this mass of air, from more equatorward air with less spin, spins clockwise. It is these clockwise spinning blocking highs that caused our last two cold winters as they brought cold arctic air in from the north-east instead of the warm oceanic air we normally receive from the Atlantic. These blocking highs deflect the relatively warm westerly jets southward of the UK and it is this north or southward movement of the jet that forms the North Atlantic Oscillation. We experience warm winters when the North Atlantic Oscillation is positive and the warm westerly jets can travel north to the UK and we experience cold winters when the North Atlantic Oscillation is negative and a blocking high deflects the jet stream to the south and draws cold arctic winds from the north-east.

However, by studying winters of a similar negative North Atlantic Oscillation, and therefore atmospheric dynamics, to the last two winters Julien Cattiaux and French colleagues concluded that the last two winters should have been much colder. This implies that climate change is possibly reducing the chance of further cold winters and reducing the severity of ones we have. So next time someone remarks, as you walk through thick snow, that global warming is disproved tell them without it the winter would be even colder. Although by increasing the background temperature we reduce the severity of winters caused by blocking and negative North Atlantic Oscillations without understanding what causes changes in the North Atlantic Oscillation and blocking then we cannot know whether more cold winters could be heading our way. Essentially climate change could cause dynamical changes in our atmosphere which increase the likelihood of cold winters.

There are two main ideas for what could have caused the extreme North Atlantic Oscillations and blocking that produced the last two cold winters. The first is natural solar variation and the other is the diminishing sea ice. Until recently solar variation was thought to have minimal effect on our climate however recent measurements of the UV spectrum of the solar cycles is much greater than previously observed. When a team at the Met Office put these new measurements into climate models they discovered that the increased middle atmosphere temperature change produced dynamical changes that cascaded down to the surface and caused variations in the North Atlantic Oscillation. From measurements they were able to link the 11-year solar cycle to causing the cold winters of 2009-2011. Studies have also shown that the increase in heat released from the ocean into the atmosphere as the insulating sea ice cap melts can cause clockwise atmospheric rotations that bring in cold air into Europe.

These two processes are not mutually exclusive, they could both be occurring, but as one is part of a natural cycle of variability and the other a product of our warming climate it is important to know their relative roles for understanding the trajectory our future winters are likely to take. Although, as discussed above, a warming climate will ameliorate the winter extremes we could still experience more frequent cold winters depending on the cause of the blocking highs and negative North Atlantic Oscillations. If these are caused mainly by the decreasing sea-ice then cold snowy winters could be a more common feature of the future.

More > Feeling chilly? Take a look at the amazing science Scott’s team brought in from the cold: Captain Scott’s 100-Year Legacy.