When sudden warming can mean cold weather

  • When sudden warming can mean cold weather
    10.02.2011 14:43


    Yet another snow storm has just crossed the USA, this time bringing close to one metre of snow to parts of northeast Oklahoma and northwest Arkansas through Tuesday and Wednesday.

    Coming on top of the blizzard of February 1, this has already given the city of Tulsa, Oklahoma, its snowiest month on record, with 57cm of snow, and its snowiest winter, with 66cm so far.

    The UK has been relatively benign after December’s snow and intense cold but there is still enough winter remaining, and indeed early spring, for snow to return anywhere.

    Whether it will is another matter entirely. The cold December across much of Europe and the USA, and parts of January and February in the latter, can be attributed to anomalously high pressure in high latitudes of the northern hemisphere and relatively low pressure in lower latitudes. This is measured by the Arctic Oscillation (AO) or the North Atlantic Oscillation (NAO), indices which are negative during such an episode.

    There is some evidence to suggest that a negative AO or NAO early in the winter implies a return to negative index late in the winter or in early spring but it is not certain and in any case does not guarantee severe cold and widespread snow across the UK.

    There is another phenomenon emanating in the stratosphere, high above the weather in the troposphere but sometimes affecting that weather. In fact, it can help to induce a negative NAO / AO and therefore colder weather. Paradoxical as it may seem, the phenomenon in question is called Sudden Stratospheric Warming (SSW).

    It tends to occur a couple of times during any winter to a lesser or greater degree, provoked by disturbances closer to the Earth’s surface radiating upwards before sometimes, not always, translating back down to disturb our weather patterns.

    What happens is that the stratosphere at about 60 degrees north rapidly warms by perhaps 40 to 50 degrees Celsius within days, and the usual westerly flow of winds at that level around what is known as the polar vortex slows down. The polar vortex is a massive semi-permanent cyclone centred near the pole, occupying the middle and upper troposphere and the stratosphere.

    During a major SSW event the wind direction completely reverses, and this is associated with a split in the polar vortex with two centres developing, approximately over Baffin Island, Canada, and northeast Siberia, and jets of wind circulating both separately.

    This can shift Arctic air in the troposphere much farther south than usual, producing severely cold outbreaks in places by inducing, for example, east to north-easterly winds at the surface across Europe

    Such a dramatic warming of the stratosphere and split of the polar vortex occurred in January and the start of February 2009, and the UK had the sort of cold January and early February that might be associated with such an event.

    This year a sudden warming of about 40 degrees Celsius occurred in the stratosphere in early February but in the UK at least has not translated into severe cold. SSWs are hard to predict, and their effects even more so.

    On the other hand, much of the USA has, as we have seen, been cold and snowy so far this February. Was the SSW a cause? Perhaps.

    By: Stephen Davenport