Golden leaves fluttering from our wooded oaks, bramble bushes in full bloom, tomatoes flourishing under cover – yet not the typical weather we might expect in Britain during late November. The outdoors play with our perception of flowing time, our shopping behaviour, the very psychology of Christmas decorations – with the weather almost forgetting to prompt us with those crisp frosts and the odd snow flakes that spark our seasonal imaginations. So as we approach the advent month, and as Weather Logistics UK approaches an end to its operational weather forecasting, we investigate the deeper goings on in the atmosphere that are responsible for these little quirks of nature. After two winters of extreme cold and snow, we address the question of what is causing our unseasonable weather …
Intense blocking patterns located over Poland and the northwest Pacific Ocean have played havoc with our seasonal weather, driving southerly rivers of warm air into Britain and the southeast of the US. This wacky weather phenomenon has seen temperatures in England soar into the high teens during November. In a sense the east coast of the US and the UK are often teleconnected in their seasonal weather, as broadly speaking the planetary waves that are spun-out by our Earth’s rotation operate in their most stable state with a 60 to 70 degree wavelength. So it’s rather unsurprising that our northern hemisphere jet stream has deflected wildly northward at both longitudes, rather like two rather aggressive slinky shaker partners.
Yet as the jet stream flings northward, there have been equal arguments in the northwest America where the vigorous north Pacific upper air flow has plunged far south. This has resulted in unseasoned cold and winter storms that have sent soggy shivers across the northwest, with snow reported in parts of California much earlier than the seasonal norm. Meanwhile, short and sharp Arctic bites are expected across the East Coast of America this season. In part, this is due to the prevalence of relatively cooler surface waters over the tropical Pacific that is associated with a La Niña. A deep cold snap over the East coast of the US points toward an abrupt and intense Arctic-type weather scenario in the UK in 14 to 21 days time. This Arctic deep freeze will see temperatures plummet, with significant widespread snowfall and perhaps blizzard conditions across Britain during mid-December 2011.
Good news for cold weather haters, is that our resident high pressure blocking patterns are weakening somewhat and slipping southward. As the meandering wobbles in our upper air flow give way to a more laminar flow. On the other hand, the bad news is that unsettled weather with high winds and heavy rainfall are anticipated to in parts of northern of England and Scotland. Yet again the cause is our jet, situated around 11km above the surface it is fiercely blowing along the western shores of Greenland, it is expected to introduce cool surface Arctic airflow to us. As this air mass comes into contact with the mild and most south-westerlies in the north-east Atlantic it will activate instability and rapid cyclogenesis, the process involved in the development of our frontal weather systems. As a result, we expect a series of deep and compact low pressures to storm toward Scotland that may lead to flash floods and structural wind damage.
So whilst the exact cause of all this wacky and topsy-turvy weather patterns perhaps remains unknown, many answers may lie in character of our polar jet. As temperatures soar over the Arctic, the gradient between the surface temperatures from the subtropics to poles has begun to intensify, thus increasing the likelihood of future “blocking patterns”.
It is foreseeable that unusual patterns may arise in the behaviour of our jet as our climate warms, with long-term and predictable blocking features dominating the regional weather conditions over parts of the globe. One noticeable trend is that the European jet stream has been taking a two-way path of deflection, leading to interesting northerly bands in both weather patterns and temperature anomalies that deviate substantially from our typical climate. One upper air-stream has dived into North Africa and has traveled through the Mediterranean. Toward the North Pole, another jet stream has been observed crossing Scotland and Scandinavia, ending its extensive fetch in central Siberia. The key players in these “blocking weather patterns” are: emerging sea surface temperature anomalies through weakening of the over-turning meridional circulation(1), downward trends in the northern hemisphere ice coverage(2), expansion and poleward migration of the subtropical anticyclones(3), and rapid increases in the land-surface temperatures in contrast to the oceans(4). Low solar activity also plays a role in the meandering of the polar jet stream, with particular implications for extremes of winter cold over Europe(5).
Our growing concern is that science can be lost in the midst of secrecy, that’s why our energy demand calculator is now publicly available. The calculator provides vital information on the current temperature thresholds for several heating increments and is reported for 19 UK towns and cities. It can easily be adapted to predict your household heating bills. The heating degree day (HDD) data is based on the 1961 – 1990 averages that are reported as daily maximum and minimum temperatures. Furthermore, we have also released our jet stream blocking diversion model, which has been produced in the IDL computer programming language. Please stay in tune for our final winter weather prediction.
Further contact details …
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(1) Mignot, J., A. Ganopolski, A. Levermann, 2007: Atlantic Subsurface Temperatures: Response to a Shutdown of the Overturning Circulation and Consequences for Its Recovery. J. Climate, 20, 4884–4898
(2) Climate Change 2007: Working Group I: The Physical Science Basis, FAQ 4.1 Is the Amount of Snow and Ice on the Earth Decreasing?
http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-4-1.html [Accessed November, 2011]
(3) Lu, J., G.A. Vecchi and T. Reichler, Expansion of the Hadley cell under global warming, Geophysical Research Letters , 34(6), L06805, 2007
see also: Correction to “Expansion of the Hadley cell under global warming”, Geophysical Research Letters, 34(L14808), 2007, doi:10.1029/2007GL030931
(4) Sutton, R. T. Dong, B. Gregory, J. M., Geophysical Research Letters, Land/sea warming ratio in response to climate change: IPCC AR4 model results and comparison with observations, 2007, 34 (2), pages L02701
(5) Lockwood, M., Environmental Research Letter, 5(2), Are cold winters in Europe associated with low solar activity?, Environmental Research Letters, 2010