Between cold snaps and heat waves, droughts and heavy rain that is either unseasonable or of unusual duration, the feeling is growing almost everywhere in the world that the usual weather patterns have gone haywire. The winter of 2013/14 is a good example: whereas it was almost non-existent in central Europe, the eastern regions of North America and Japan experienced record low temperatures and snowfalls. In California there was too little winter precipitation to avert the drought, while in Great Britain it rained constantly. What is going on?
Meteorologists are cautious about offering quick explanations. But they believe that one striking fact over the last few years is that weather patterns have persisted for unusually long periods, often weeks at a time, and that this can have extreme consequences. The culprit is believed to be pronounced waves in the jet stream, the band of strong, high-altitude winds from the west that moves at high altitude around the polar regions in both hemispheres, and which plays a key role in determining the weather in mid-latitudes.
To understand how this works in the northern hemisphere, we need to take a closer look at the jet stream. It separates cold Arctic air masses from warm subtropical ones. Features that produce permanent disturbance, such as mountain ranges, are what give this band of strong winds its wave structure. Arctic air masses then push far to the south in what are known as troughs (bulges in a southerly direction). Conversely, what are known as ridges (bulges in a northerly direction) transport subtropical air masses in the opposite direction. Due to dynamic processes, the troughs in lower regions of the atmosphere correspond to low-pressure areas, located beneath the section of high-altitude loop current leading out of the trough towards the Pole. The ridge corresponds to areas of high pressure in the section of the jet stream that flows from the ridge towards the Equator.
The greater the deflection (amplitude) of the jet stream, the more likely it seems that weather extremes will develop within it. The physical explanation for the wave pattern with large amplitudes is resonant amplification of the wave oscillation. In most cases, the troughs and ridges move from west to east around the hemisphere. As a result, lows and highs alternate over a region over the course of several days. "However, various studies suggest that the jet stream has more frequently come to a standstill over past decades – particularly when its amplitude is especially great. In this situation, the repercussions can be extreme," says Professor Peter Höppe, Head of Geo Risks Research at the Munich Re Corporate Climate Centre. The waves in the band of strong winds can persist over a region for up to several weeks, thus creating quasi-stationary weather patterns. In areas influenced by the troughs, large quantities of rain accumulate, while in summer, heat waves and droughts affect the areas influenced by the ridges. "All this would be unremarkable," says Höppe, "if the meteorological parameters were averaged over the latitude." But from an insurer's perspective, stationary weather patterns are a costly peculiarity, since they produce a disproportionate level of damage.
There is currently intensive discussion among climate researchers about whether the changed weather patterns of the jet stream are a consequence of climate change, and in particular of the rapid melting of sea ice in Arctic latitudes. "Current research results indicate that the more frequent occurrence of stationary weather patterns is linked to the excessive warming of the Arctic region," states Dr. Eberhard Faust, Head of Climate Risks and Natural Hazards Research at Munich Re. "However, the possible causal connection is not yet fully understood at this early stage of the research," he admits.