Rossby waves and extreme weather

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  • Picture 1: Drawn and uploaded on de:WikiPedia by de:User:W.

    Picture 2: Source: Screen and Simmonds, Nature climate Change

  • Rossby waves and extreme weather
    16.08.2014 14:59

    Rossby waves and extreme weather

    Atmospheric Rossby waves are giant meanders in high-altitude winds with major influence on weather. Rossby waves are associated with pressure systems and the jet stream. Atmospheric Rossby waves emerge due to shear in rotating fluids, so that the Coriolis force changes along the sheared coordinate. In planetary atmospheres, they are due to the variation in the Coriolis effect with latitude. The waves were first identified in the Earth's atmosphere in 1939 by Carl-Gustaf Arvid Rossby who went on to explain their motion.

    Picture 1 shows Rossby waves.

    The terms "barotropic" and "baroclinic" Rossby waves are used to distinguish their vertical structure. Barotropic Rossby waves do not vary in the vertical, and have the fastest propagation speeds. The baroclinic wave modes are slower, with speeds of only a few centimetres per second or less.

    These Rossby waves are responsible for day-to-day weather patterns at mid-latitudes due to the fact that low and high pressure systems form within its boundaries. There is an ongoing debate about the possible influence of climate change in extreme weather mainly on hurricanes and tornados. However, it is less mentioned and there is a lot of research going on about flooding, cold snaps or heatwaves.

    What if the decrease in the Arctic ice cap is influencing somehow the Rossby waves? Is it interesting to find out if a change in these waves may produce more extreme events in our latitudes? The new paper by Screen and Simmonds recently published in Nature Climate Change (“Amplified mid-latitude planetary waves favour particular regional weather extremes”) studied this topic.

    The authors have tried to demonstrate statistically in this paper that there is a relation between the extreme years of precipitation and temperatures and slow wave propagation. By using, the 1979-2012 period they obtained the absolute values of precipitation and temperatures anomalies and then by using monthly-averaged data, fast-travelling waves are filtered out and thus only the quasi-stationary component remains, i.e. the persistent weather conditions. You can check on the next graphic that extremes in precipitation and temperatures will have a quasi-stationary wave associated. For example, 01/06, the most extreme temperature, is coincident with the first grey colour column on the left chart showing that there is not significant wave amplitude.

    Picture 2 shows:

    a) Temperature anomalies
    b) Precipitation anomalies
    c) and d) normalized wave amplitude anomalies
    Red signal amplified wave and grey signal quasi-stationary

    According to the abstract: ”The findings suggest that amplification of quasi-stationary waves preferentially increases the probabilities of heat waves in western North America and central Asia, cold outbreaks in eastern North America, droughts in central North America, Europe and central Asia, and wet spells in western Asia.”

    Interestingly they have found that in the Northern Hemisphere and on western boundaries of the continents a significant relationship exists between surface extremes and planetary wave activity: It is observed that there are more extreme temperatures with reduce wave amplitudes, although they do not explain the reason for that. In eastern parts of the continent this link is not so clear.

    This is an important step forward, but of course many questions remain. Has planetary wave activity changed in recent decades or is it likely to do so under projected future warming? And, if it is changing, is the rapid Arctic warming indeed responsible? Or perhaps, although wave activity had changed, is there a relation with climate change?

    Sources:

    J.A. Screen, and I. Simmonds, "Amplified mid-latitude planetary waves favour particular regional weather extremes", Nature Climate change, vol. 4, pp. 704-709, 2014. http://dx.doi.org/10.1038/NCLIMATE2271

    Realclimate

    By: Mario Cuellar