The ability to deliver useful predictions of seasonal weather patterns from year to year is one of the most challenging endeavours for the Met Office and other meteorological institutions around the world.
It also has the potential to be of enormous benefit to society. With our growing understanding of the factors that influence weather over the coming months to years, we're already unravelling some of the key climatic factors at work and helping customers prepare for extreme conditions.
Blocked weather patterns, where large areas of high pressure remain in place for up to several weeks, occur every so often pushing the jet stream way off its normal course. The displaced jet stream can open the door to cold easterly winds in winter such as occurred in Europe in February 2012; while those directly influenced by the high pressure areas often endure prolonged spells of dramatically hot conditions in summer, such as occurred during the Russian heatwave in summer 2010.
The key question facing meteorologists and climatologists is why blocking occurs. Better understanding of the processes involved are necessary to progress the science of seasonal forecasting and could be of great benefit to contingency planners in government and the private sector alike. So Met Office scientists are looking at ways to improve our climate model's ability to capture blocking. For decades now, climate models have shown a deficit in Atlantic blocking when compared to observed frequency of blocking. However, recent work shows that blocking can now be simulated with close to the observed frequency in our latest climate model, through an improved simulation of the Atlantic Gulf Stream and its subsequent effect on the atmosphere. The aim is now to bring this latest model into operational use.
Another key piece in the near-term prediction puzzle is the regional response to climate change. Dr Adam Scaife, the Met Office's Head of Monthly to Decadal Prediction, has run our climate model with extended vertical resolution high into the stratosphere. Results from this work show a marked shift in winter storm tracks and subsequent changes in European rainfall compared to previous projections. This effect should also be included to give the best predictions of our future climate.
There are also other factors that can influence surface climate. For example, Met Office research has shown that if the variability in the Sun's ultra-violet (UV) levels is large enough, it can drive changes in the atmosphere that ultimately influence the winter climate in Europe and parts of the United States. In periods of low enough UV, changes in wind patterns rearrange air masses such that they could bring colder conditions to Europe and the USA. Importantly, this has no direct effect on global temperature but it does offer greater insight into regional climate over seasonal and decadal timescales.
An iconic impact of climate change is the melting of polar sea-ice. However, low Arctic sea ice cover is now becoming increasingly linked with significant changes in the North Atlantic jet stream in winter and hence the severity of European winters. A number of studies are indicating that Arctic sea ice depletion, in isolation, may increase sea level pressure over the Arctic in winter and drive more easterly winds across Europe in both observations and computer models. It is possible that continued low Arctic sea ice during the coming years might therefore drive additional changes in northern European climate due to changes in winds as well as the more direct warming effects of longer term climate change.
Dr Scaife concedes, "There is much still to be done, but these examples of leading edge science, coupled with our growing forecasting capability are already allowing us to take key steps towards improving climate prediction from months to years ahead."
With better understanding of the complex interactions within our climate comes the ability for us to accurately represent these in our climate models, which will ultimately give us a more realistic picture of the Northern Hemisphere's future climate.
Download the PDF Improving our near-term climate predictions (PDF, 351 kB)