When "El Niño" gathers its strength along the coasts of South America, at times it goes on to have global consequences. In many places in the southern hemisphere, but also in the north Pacific, the tropical climate phenomenon leaves traces in its wake in the form of floods and droughts. Until now it was not clear whether effects of El Niño can be felt in Europe, as attempts to provide proof of an effect at such a distance had been foiled. Working together with colleagues, Stefan Brönnimann, from the Institute for Atmospheric and Climate Science at ETH Zurich (1), has now succeeded in finding just such proof. As the team reports in their article, published in the latest edition of "Nature" (2), the harsh winters in Eastern Europe during the war years 1940–42 were a consequence of an El-Niño event.

Thousands of time series

Brönnimann started off by wanting to investigate something entirely different. He wanted to find out why higher than average ozone values were recorded in the stratosphere at the beginning of the 1940s at the meteorological station in Arosa (3). In order to find an explanation for this, the climatologist had to reconstruct the circulation of the air prevailing at the time, right up to the lower stratosphere. Because, via complex interdependencies, currents in the atmosphere influence the thickness of the ozone layer.

Traces of El Niño in the northern hemisphere 1940–42: In the stratosphere (top) the air over the Arctic Circle is warmer, the polar wind weaker and the ozone layer thicker than normal. The climate on the Earth's surface was also out of the ordinary: extreme cold in central and eastern Europe, mild weather in Alaska and bitterly cold in the north Pacific. large

Stefan Brönnimann had to analyse thousands of data series for his climate study. (Picture: Andrea Kaiser) large

Before he could get down to the actual work, however, Brönnimann had to collect, in onerous detail, the necessary data. The meteorological data he needed for the study were not systematically collected until after 1948 when, in the course of reconstruction, the World Meteorological Organization (WMO) (4) was established. In old archives Brönnimann finally tracked down thousands of so-called vertical profiles from radio probes and flight measurements dating from the years of World War II. For his work, in total, the researcher digitalised 33,000 of these records.

Traces right up to the stratosphere

The recorded data show that it not only were the ozone values in the stratosphere exceptional for the period in question, but also the climate on the Earth's surface. Central and eastern Europe experienced very harsh winters–in fact, the coldest of the 20th century. This is one of the reasons for the failure of the German campaign in Russia. Simultaneously, Alaska was passing through a mild winter, while it was bitterly cold in the north Pacific. What Brönnimann also discovered was that unusual currents were present under the stratosphere as well. At precisely the same time, climate data document an especially long-lasting El-Niño event in the tropical regions of the Pacific.

Using models to simulate the circumstances Brönnimann was then able to show that there is a causal connection between all these observations. Over the computed 650 model years there were a number of El-Niño events. Simultaneously, the computer showed similar climatic anomalies in other regions, such as those that had been recorded for the early 1940s. According to Brönnimann, this proves that El Niño had significant effects in Europe and in the stratosphere, which could have led to changes in the ozone layer. meteorologists have, in fact, observed similar climatic conditions on the heels of later El-Niño events, such as in the winter of 1986/87, when a number of lakes in Switzerland froze over.