One fundamental issue in Earth's climate system is the impact of large-scale atmospheric temperature waves on our extreme weather. An international team of physicists and meteorologists led by Miklós Vincze of the HUN-REN Institute of Earth Physics and Space Science (HUN-REN EPSS) examined this phenomenon using laboratory fluid dynamics models. In their paper published in Scientific Reports, the team was looking to answer how the frequency of certain 'extreme' phenomena in Earth's mid-latitude weather changes in relation to the temperature difference between the Arctic and the Equator, known as the meridional temperature contrast.
Thermal camera images of various states of Rossby waves in one of the experimental setups used in the study (above). The atmospheric Rossby wave traced by the wind field – which has a similar shape to those seen in the second and third laboratory recordings – from two perspectives. (below – source: NOAA).
The importance of the problem is highlighted by the fact that the observed increase in the Earth's global average temperature over the past decades is accompanied by a decrease in temperature contrast. This is because the rate of warming in the Arctic is multiple times greater than the global average. Temperature contrast plays a fundamental role in determining the characteristics of temperature waves (Rossby waves) with wavelengths of thousands of kilometres, which greatly influence the variability of mid-latitude weather. The extent of meandering, their north-south extent, and their west-to-east progression speed, known as the "waviness" of these Rossby waves, are highly sensitive to the magnitude of the temperature contrast. In the literature of the past decade, contradictory results have emerged regarding how the rearrangement of Rossby waves due to changes in temperature contrast affects temperature variability at mid-latitudes. Clarifying this question is significantly complicated by the fact that wave dynamics are just one of many factors influencing local weather in our complex climate system, making them difficult to isolate.
The authors of the paper, therefore, examined experimental setups where Rossby waves could be studied in isolation, applying the principle of fluid dynamics similarity. In this context, it involves recognising that the nature of fluid flow primarily depends on the relationship between two time scales: the rotation period of the axis and the circulation time due to temperature contrast. By adjusting the ratio of these two timescales, very similar waves can develop in a scaled-down and "accelerated" laboratory-sized system as in the atmosphere. Experiments were conducted in cylindrical water-filled containers at the geophysical fluid dynamics institutes of Florida State University in the United States and Brandenburg University of Technology in Germany, as well as at the von Karman Laboratory of Environmental Flows at the Institute of Physics and Astronomy at Eötvös Loránd University. During the measurements, controlled heating and cooling of the walls ensured the presence and modulation of the temperature contrast driving the flow.
The results obtained in the arrangement considered as a model of mid-latitude circulation by the researchers suggest that the Rossby waves, on their own, do not exhibit behaviour that would lead to greater temperature extremes – extreme heat and extreme cold weather situations – as a result of a decrease in temperature contrast. However, they have demonstrated that the "predictability" of weather, meaning the extent to which one day's weather is likely to be the same as the previous day's (persistence), significantly decreases with a decrease in the temperature difference between the Arctic and the Equator.
The authors hope that these experimental results can help in better understanding the complex processes of our climate system, which constantly reorganises itself over various time scales. They aim to separate simultaneously occurring climate phenomena in reality, thereby uncovering the complex causal connections between them.