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Data shows that the atmospheric pressure system responsible for the current cold weather in northern Europe has been in the same anomalous state for a record 14-month period and could indicate that a harsh winter is in store.
Similar situations existed around the northern hemisphere winter of 1968-69 which resulted 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. This is the first time since then 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. 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.
“The situation is unusual,” commented Nick Grahame of the UK Meteorological Office. “The implication for Europe is that it is more open to extremes of cold.”
“Really bad winters are related to negative NAOs,” agreed climate scientist Mark Maslin of University College London in the UK.
However, Met Office researcher Chris Folland said: “There is a huge amount of internal variability in the climate over timescales of less than a year and we can not draw any real conclusions (about the winter forecast)”. But he confirmed that if the NAO phase is negative then the weather will be cold and if this is compounded by a negative AO then it may be even colder.
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.
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,“ explained UCL's Maslin.
There is no significant link between the variations in the NAO and the AO and climate change, according to the Met Office's Folland. Equally, the cold weather associated with these cycles does not mean that global warming is not happening. “It is important that people understand that the climate is more than what they can see out of their window,” he said.
The World Meteorological Organisation recently announced that 2010 would be one of the three warmest years on record based on average global temperatures.
See NOAA data here.
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