April sets the course for drought in summer

Forest

Forest in drought stress (Image: André Künzelmann / UFZ)

In the last 20 years, Germany and Central Europe have experienced more summer heat and drought than seldom before. Now climate researchers have found out that a month plays a special role in this: whenever April is particularly dry and sunny, there is also a risk of heat and drought in summer. And the weather patterns that lead to the unusually constant and warm April weather have been occurring more frequently since 2007, as the analyzes showed. The reason for this is the weakening jet stream, as a result of which high pressure areas remain longer over Central Europe in spring.

2003, 2010, 2013, 2015, 2018 and 2019 – Central Europe has experienced hot summers with severe drought six times in the last 20 years. “Since the turn of the millennium, Central Europe has been repeatedly hit by summer heat waves and periods of drought that have caused billions in damage,” explains first author Monica Ionita from the Alfred Wegener Institute (AWI) in Bremerhaven. “For the drought in 2018 alone, the financial damage is estimated at around 3.3 billion euros – making it the most expensive single annual weather event in Europe.” In view of these accumulations of extreme weather conditions and their effects, the question arises as to what causes them and what whether this trend might continue like this in the future. In their search for answers, however, researchers have so far mainly looked at weather patterns and atmospheric conditions in summer or winter – with only limited success.

Cumulative weather anomalies in April

Ionita and her colleagues have therefore taken a closer look at another season of the year: spring. “To predict these extreme events correctly, however, has so far failed because the influence of spring was underestimated,” explains the researcher. “For this reason, we decided to investigate the relationships between the weather conditions in spring and the following summer more precisely – for the entire period in which sufficient observation data was available.” For the study, the team evaluated weather data on temperatures, precipitation, the Evaporation and soil moisture in Europe for the past 140 years. Using statistical methods, they then determined whether there have been any noticeable deviations from the long-term mean in recent years.

The evaluations revealed that there had indeed been anomalies in the spring of the past two decades. “While there were hardly any changes in the months of March and May, April was on average three degrees Celsius warmer in the period from 2007 to 2020 than in the same period from 1961 to 1999,” reports Ionita. “In extreme years like 2018, April was so warm that the snow that fell in winter evaporated almost directly in spring and had no chance of seeping into the ground in the form of meltwater.” the precipitation in April decreased significantly. According to the researchers, these deficits were particularly pronounced in Germany, Poland and parts of the Czech Republic, Hungary and the Ukraine. “In Germany alone, April rainfall decreased by an average of 30 millimeters in the period from 2007 to 2020,” report the scientists. “In some regions that represents a statistically significant reduction to just 50 to 60 percent of the normal April rainfall.”

This meant that the soils lacked water supplies in two respects: On the one hand, there was too little rain, and on the other, the water evaporated faster than normal at this time because of the higher temperatures. “That means: the summer drought situation of the soil was preprogrammed in April,” says co-author Rohini Kumar from the Helmholtz Center for Environmental Research (UFZ) in Leipzig. Because if the soils in Central Europe, but above all in Germany, show a significant moisture deficit in spring, they can usually no longer compensate for this deficit by summer. The temperature and precipitation patterns in April are decisive in determining whether or not the soil is above average in the summer that follows.

Blockade-Hoch and Jetstream as originators

Ionita and her colleagues have also investigated why these weather anomalies have been increasing recently in April. To do this, they primarily looked at the large-scale atmospheric currents that shape the movements of the high and low pressure areas over Europe. “Our analysis shows that a blocking high pressure system forms over the North Sea and parts of northern Germany during this period, which deflects the jet stream towards the north,” explains Ionita. As a result, the low pressure areas coming from the west no longer cross Central Europe, but are diverted to the north and south. As a result, the typically changeable April weather does not occur and instead it remains sunny and warm for an unusually long time for this month.

Studies have been suggesting for a long time that such blockade weather situations have increased in recent times. One reason for this is a weakening of the jet stream – the westerly wind band that determines the weather in our latitudes. This current is driven by the difference in temperature and air pressure between the Arctic and temperate latitudes. However, this gradient has weakened in recent decades – among other things due to the disproportionately strong warming of the Arctic. “The jet stream, which determines the weather in Central Europe, runs on a wave course under these conditions and allows the high-pressure system to settle over the North Sea,” explains Ionita. That could mean that in the course of climate change there could be an abnormally warm and dry April more frequently in the future. “The month of April will then continue to be much warmer and drier in Central Europe than it was 20 years ago, thus paving the way for extensive water shortages and parched soils all summer long,” said the researcher.

However, such a prognosis cannot yet be made with certainty, because the climate is also subject to natural fluctuations that can temporarily mask or offset the general trend. “Understanding the conditions under which such dry periods arise is crucial in order to be able to take precautions or protective measures in good time,” Kumar concludes.

Source: Monica Ionita (Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, Bremerhaven) et al., Npj Climate and Atmospheric Science, doi: 10.1038 / s41612-020-00153-8

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