Heavy rain in the north – heat in the south: how these weather conditions develop
On July 24, 2023, the thermometer in south-eastern Sardinia rose to 48.2 degrees, a new European heat record for that year, just 0.6 degrees less than on August 11, 2021 in Syracuse, Sicily.
On the same day, between 30 and 35 millimeters of rain fell in southern Norway and south-west Sweden. It was around 20 degrees warm and therefore two to three degrees colder than the long-term average.
A cool, rainy and blustery July
Across Scandinavia, but also in England and Ireland, it was a cool, rainy and stormy July, so not exactly what you want for a summer holiday either. In Sweden, twice as much rain fell in July as in the same months of the reference period 1991 to 2020. Nowhere did the thermometer rise above 30 degrees and it was unusually windy, especially in the south-west. The Swedish weather service SMHI has only counted such lousy July months seven times in the past 32 years.
SMHI meteorologist Sverker Hellström explained the enormous weather difference between northern and southern Europe to the Swedish holidaymakers with the location of the jet stream, the stream of jets that meanders around the Arctic. It showed a bulge far to the south, a so-called “trough” that reached as far as the Alps.
The jet stream is a strong wind blowing at 200 to 500 kilometers per hour, which meanders in waves from west to east around the poles at the upper edge of the troposphere at an altitude of eight to twelve kilometers. When it retreats far to the north in the northern hemisphere, forming a “ridge”, it directs warm air masses from the south to the north. However, when it curves far to the south, as it did last July, the cooler arctic air and lows north of this storm band remain locked in, making for volatile, mostly inclement weather in the regions below. Sometimes such a bulge just gets stuck for a while, so that the general weather situation does not change as usual over a longer period of time.
The jet stream is the result of a transient current that carries warm, rising air north from the equator. The greater the temperature difference between the low and high latitudes of the earth, the more air flows from the equator to the poles in the upper troposphere.
On its way north, the Coriolis force, the rotation of the earth, pushes this air current eastwards. With the decreasing radii of latitude, the speed increases and finally becomes the north polar wind band that determines the general weather conditions in Europe.
On three continents: heat vs. rain and coolness
Blocking of the troughs and ridges is by no means unusual in climate history, but three continents were hit this year: in addition to southern Europe, southern North America and eastern China also suffered from extreme heat and drought. In contrast, rain with flooding and coolness in the northern United States, China and Japan.
‘The fickle nature of weather patterns determines where and when extreme weather events occur, but the giant waves (of jet streams) spreading across the hemispheres can often connect areas of extreme heat, drought and flooding,’ explains Professor Richard Allan in Climate Science from the University of Reading.
All of this is no coincidence. Climate experts agree: the simultaneous occurrence of heat waves in different regions of the world and their predicted intensity fit very well with the expected impact of climate change on global temperatures. Because the natural weather phenomena, such as the jet stream, are superimposed on the global climate warming of 1.3 degrees, which intensifies their effects.
“As far as I know, much of what we are currently observing is very consistent with our model projections – above all the extraordinary heat waves over land and in the sea, but also the occasional extreme precipitation,” confirms Jakob Zscheischler from the Helmholtz Center for Environmental Research in Leipzig. Daniela Domeisen from the University of Lausanne and the Swiss Federal Institute of Technology in Zurich is also certain: “The current global extremes are not short-term exceptional situations, on the contrary. Research shows that the extremes will continue to increase in the coming years and decades. “
Warmest three-week period on record
The bottom line is that due to this not yet everyday large-scale weather constellation, the first three weeks of July were the warmest three-week period worldwide since the start of systematic weather recording, as reported by the European Climate Change Service Copernicus and the World Weather Organization WMO. With 98 percent certainty, however, one of the next five years will surpass the record of 2023.
“This means that we have experienced the warmest month on global average for many millennia. Possibly since the Eemian warm period, which ended around 115,000 years ago,” said Karsten Haustein from the Institute for Meteorology at the University of Leipzig -Media center one.
Other factors certainly contributed to the fact that such an extreme summer was possible, such as the abnormally warm oceans and neighboring seas. The North Atlantic was already one degree warmer in mid-June than in the comparative period from 1982 to 2011, as calculated by the US climate agency NOAA from satellite measurements. The Mediterranean Sea, Baltic Sea, Black Sea and southern Arctic are warming even three times faster than the open oceans.
In contrast, the El Niño that has just occurred, the natural warming in the Pacific that occurs at irregular intervals, is unlikely to have had any effects in Europe this summer, according to many climate scientists – but maybe in 2024 and 2025.
Physically based climate models divide the atmosphere and oceans into a grid of cells. They calculate quantities such as temperature, air pressure, humidity and flow velocity for each cell and each time step. They take advantage of the conservation of energy and mass. This means that everything that flows into a cell must also flow out again.
In contrast to weather forecasts, it is not the initial values that are as precise as possible that are important for climate projections, but rather the marginal values – i.e. the development of greenhouse gas and aerosol emissions. This data comes from scenarios ranging from no reduction in emissions at all to compliance with the Paris Agreement.
To project future rainfall, the global models are scaled down to regional conditions. These regionalized models work with a cell size of just a few kilometers. But that’s not enough to model clouds – the grid width is still too coarse here, too.
In a regionalized model, precipitation occurs when the humidity exceeds certain values, explains Nora Leps from the German Weather Service (DWD). Clouds and precipitation are described by parameters. “The amount of precipitation is calculated from empirical formulas that incorporate a large number of observations. That can get very complicated,” says Leps.
In order to rule out that the models are running in the wrong direction, climate researchers have them recalculate historical periods and then compare the results with the measured values. They also have an ensemble of different models calculated, the results of which are then statistically evaluated. While the global temperature development can now be projected quite well, the precipitation projection therefore has a wide range.
An additional problem is that several weeks of drought have a different effect than a constant change between drought and rain – even if the number of dry days is the same. Climate researchers must therefore also analyze the duration of dry, rainy and hot phases.
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