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The Atlantic pressure system that controls the gateway that allows cold northern air to flow south into Europe has been stuck in the same “open” position for a record 14 months. While the Arctic pressure system is amplifying the effect by driving even more cold air south.
Similar situations existed around the northern hemisphere winter of 1968-69 resulting in severe cold weather in both North America and northern Europe and also in 1962-63 which was a particularly harsh winter in northern Europe. It was also responsible for the harsh European winter of 1942 which helped the Soviets defeat Nazi invaders in the second world war. This is the first time since 1969 that there has been such a long unbroken anomalous period. If the pattern of those previous years is repeated then the coming winter could also be severe.
The atmospheric pressure system known as the North Atlantic Oscillation (NAO) consists of a high pressure area over the Azores and a low pressure area over Iceland. If the difference in pressures between the high and the low is large, known as the positive phase, then warm air is dragged north confining Arctic air to higher latitudes and contributing to relatively mild winter conditions in northern Europe and North America. But in its current state, the pressure difference between the Azores high and the Icelandic low is relatively small and this means warmer tropical air is trapped further south allowing colder air to flow from the Arctic – in this situation the NAO is said to be in a negative phase.
US National Oceanic and Atmospheric Administration (NOAA) data shows that the NAO has been in its so called negative phase for 14 consecutive months up to and including November 2010. It is almost certain to be negative for December making an unprecedented 15 month run. Previously the longest continuous periods were for nine month across the northern hemisphere winter of 1968-69 and for six months across the winter of 1962-63.
Furthermore, these periods co-incided with a negative phase in the related Arctic Oscillation (AO) which links high pressure systems of the Arctic with low pressure systems at middle latitudes. The AO has been negative for four consecutive months since August, according to NOAA data, although this is not quite so unusual as the situation with the NAO. A negative AO compounds the situation by driving even more cold air south. The AO went strongly negative in November and contributed to the cold weather then. It is negative at the moment although not as strongly negative as it was in November.
In northern Europe the NAO determines whether cold arctic air or warm tropical air dominates the winter months. The NOAA data indicates that the NAO index in November was the second highest in the last 60 years which means that the colder arctic air is being drawn much further south than usual. The NAO index is a measure of pressure difference. The pressure difference controls where the boundary lies between the warm air pushing northwards and the cold air pushing southwards If the pressure difference is large then the warm air meets the colder air further north and if it is not then they meet further south and at the moment that boundary is further south.
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