October characterized by a negative NAO, is this a new trend?

  • Sources: Figure drawn by Matt Dobson of MeteoGroup based on data collected by the US National Center for Atmospheric Research.

  • October characterized by a negative NAO, is this a new trend?
    23.10.2016 06:26

    The North Atlantic Oscillation (NAO)  is an important index concerning the weather over the UK and the Europe. October 2016 will soon end, confirming and probably strengthening a recent trend of having negative values of NAO during October. What are the implications in the European synoptic pattern? How does this affect the winter season?

    The North Atlantic Oscillation is an index that shows the difference between the atmospheric pressure in Lisbon, Portugal, and Reykjavik,  Iceland. When this index is positive, over the Europe we usually have a westerly flow with low pressure in the northern areas and high pressure to the south.  This causes wet, mild and windy conditions over the UK, which was the normal during the autumn, at least until a few years ago.  

    When the values of NAO are negative, the general circulation in western Europe becomes easterly, with high pressure to the north and low pressure in the south of Europe. This kind of circulation usually brings drier and calm condition in the UK, especially in the western areas, where a lot of rain would be expected at this time of the year.
    As  we go towards the winter season,  this easterly flow tends to be colder than mild Atlantic winds, increasing the chance of early frost in October. On the contrary, the Mediterranean areas experience wetter and stormier condition, which is what has happened so far during this month.

    Apparently, this October 2016, characterised by a strong blocking area of high pressure over Scandinavia, has been quite similar to a big part of the recent months of October. The graph on the left hand side shows the values of NAO index from October to April between 2003 and 2016. As we can observe, the first line of each group has been usually negative (blue line) over the past 13 years, indicating negative NAO values during October.

    Looking at the chart, it is also possible to try to speculate on this season, since most of the time, a negative NAO on October has been followed by positive values during the winter. This would mean wet and stormy conditions at times over the UK, but also mild with less chance to live a very cold spells. In fact, to experience long periods of cold weather over the UK, the flow must come from east, allowing the Polar continental air to invade the western areas of Europe, and as we said previously, this happens when the NAO has negative values. Just to choose an example, this is what happened during the extremely cold and snowy winters of 2009 and 2010.

    To summarise, in recent years we have experienced more frequently negative values of NAO during October, and these have been often followed by positive values during the winter season with mild and wet winters…. will it be so again this year?

    By: Alessio Martini