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    Editor
    Keymaster

    What drives changes in weather?
    There appear to be two main modulators for the weather we can experience in the Northern Hemisphere, sea ice albedo loss (related: Warming from Arctic Sea Ice Melting More Dramatic than Thought) and depending on the state of ENSO.

    As we better learn to understand the interconnected climate system we can now connect another dot (ENSO), as the Met Office explains:

    During December and January 2013/14 the pattern of winds over the North East Pacific and North America was very disturbed. The North Pacific jet was deflected a long way north, with a secondary branch extending southwards into the tropical Pacific accentuating the separation of the Pacific and Atlantic jet streams. The effects of this over North America and into the North Atlantic were profound.

    The deflection of the jet to the north led to colder air being carried south over Canada and the northern US to enter the North Atlantic jet and establish a stronger than normal temperature gradient at the entrance of the North Atlantic Jet. This acted to strengthen the jet and provide the conditions for active cyclogenesis, which in turn led to a sequence of strong storms across the UK throughout December and January. As Figure 13 indicates, the North Atlantic jet was, on average, as much as 30% stronger than normal. Similar, but weaker, conditions can be seen in the southern hemisphere, mirroring those to the north and supporting the view that the tropics were driving at least some of what has been experienced this winter.

    The polar jet stream is then deflected a long way south over the US bringing cold air with it It is well understood that El Nino and its cold counterpart La Nina have major effects on weather patterns around the globe. Indeed the changes in the jet stream over the North Pacific, described above, are typical of what is observed during La Nina events, with the jet being deflected to the north by anomalously high pressure off the western seaboard of the US, and with a variable jet to the south along which disturbed weather forms.

    The polar jet stream is then deflected a long way south over the US bringing cold air with it In the North Atlantic, ocean temperatures continue to be above normal near 300N which would also contribute to a strengthened north-south temperature gradient across the storm track, aiding the development of storms. As Figure 16 shows, the sub-tropical Atlantic is currently warmer than the average for the last 30 years (1981-2010), but substantially warmer than it was 30 years prior to that (1951-1980).

    This in itself will potentially increase the moisture being held in the atmosphere, above the ocean, and entering the storm systems as they moved towards the UK. In terms of the global influences of El Nino/La Nina, it is the changes in tropical rainfall patterns that ultimately drive the perturbations to the atmospheric circulation described by Figure 15. So whilst the sea surface temperatures suggest neutral conditions in the tropical East Pacific, it seems that tropical rainfall patterns in December and January are consistent with a La Nina signal, with higher than normal rainfall over the West Pacific, Indonesia and the eastern Indian Ocean throughout December and January (Figure 17). Bearing in mind that the average rainfall in this region is between 8 and 12 mm/day, these anomalies in rainfall are substantial. http://www.metoffice.gov.uk/media/pdf/n/i/Recent_Storms_Briefing_Final_07023.pdf

    For more science on the jet stream see for instance: The Science of the Polar Vortex and Jet Stream.

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