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Heat, drought, flooding, record rainfall, and wildfires – the weather phenomenon El Niño affects various regions of the world in different ways. Climate models forecast a particularly strong El Niño this winter. While heat and drought are expected to afflict mainly the northern part of Australia and Indonesia, South America will face flooding and landslides. At the University of Potsdam, several working groups are researching the weather anomaly, which will likely increase in intensity and frequency in the coming years.
Pale and dark lines alternate, forming irregular patterns resembling the striped fur of an animal or the grain of wood, but in fact are the result of sedimentation. Over thousands of years, dead organic matter and chemical compounds trickle down through the water column to the bottom of a lake, where they form a thick layer. For climatologists and geoscientists, this sediment layer is a treasure trove.
Much like ice cores or tree rings, sediment at the bottom of a lake contains information about past climate events. Researchers can read these climate archives like books and learn about extreme droughts, rising temperatures, and exceptional precipitation events. The El Niño phenomenon regularly leaves its mark here as well. El Niño occurs every two to eight years – not more than the blink of an eye to climatologists and geoscientists. “To analyze the data, we need climate archives with a very high resolution,” says Christian Wolff, a geoscientist at the Institute of Earth and Environmental Sciences, who is intensively studying the phenomenon.Researchers in Kenya found a climate archive of this quality at the foot of Mount Kilimanjaro. Lake Chala is a crater lake with sediment layers displaying every single El Niño event, since its impact is enormous here, too. In southern Kenya, most of the around 500 millimeters of average annual rainfall occurs in November and December. In El Niño years, the amount of rainfall in these months sometimes triples, with extreme consequences for the population.In 2005, researchers obtained a drill core from the bottom of Lake Chala. Drilling at depths of up to 25 meters, they collected two 2-meter long sediment cores, sealed them, and transported them to Germany. They are now being kept shrink-wrapped at 4°C in the cold store of the German Research Center for Geosciences (GFZ) in Potsdam. The bottom layers of the core are about 25,000 years old. Many researchers use this archive to study various issues. Wolff uses it to study the history of El Niño.Under a microscope, the researcher measures the thickness of the dark and pale layers. Centimeter by centimeter, he examines the sediment layers of the past 25,000 years, a time-consuming task requiring great diligence and days of microscope work. “It takes about four hours to examine 100 years,” Wolff says. He evaluates the collected data statistically and looks for correlations between layer thickness and surface temperature of the Pacific Ocean, which the data confirm.One darker layer and a paler one make up a so-called seasonal year. “The dark stripes in the sediment result from the depositing of calcite during the months with higher precipitation, whereas the paler layers were formed during the dry months – from the remains of dead diatoms,” Wolff explains. The thickness of the layers indicates how wet or dry a year was. Broader stripes were formed in dry years, when strong winds raised nutrients from the bottom of the lake and the water was hardly diluted by rainfall. Due to the abundance of food, strong seasonal algal bloom occurred. Once dead, the algae sank to the bottom of the lake, where they formed thicker, pale sediment layers over time. In contrast, El Niño years starve the algae out, since a lot of rainfall dilutes the lake water, diminishing the algae bloom and leaving behind a much narrower, pale layer in the sediment. In a next step, the researchers examine the isotope distribution of oxygen in the calcite. They found that the composition of the calcite in Lake Chala correlates with the surface temperature of the Pacific Ocean – on the other side of the earth – and enables researchers to reconstruct historical temperatures and El Niño events.The analysis of these lake sediments sheds light not only on when El Niño events occurred, but also on their intensity. It has become clear that over the past 3,000 years – considered warm years in terms of climate history – El Niño has not only appeared more often, but has also been stronger than during the last glacial period, some 18,500-21,000 years ago. An overall relatively warm world climate seems to facilitate particularly severe effects of the weather anomaly, such as extreme flooding or sustained drought, whereas a comparatively cool world climate seems to curb them. The same holds for the opposite of El Niño, the La Niña phenomenon, which often immediately follows an El Niño, with the opposite effects. It can, therefore, be expected that global warming will contribute to a larger number of strong El Niño and La Niña occurrences, Wolff points out.
Other researchers are using the knowledge gained from looking back into the climatic past to make forecasts and concrete recommendations for action, which is known as climate modeling. To generate meaningful climate models for various scenarios, researchers rely on data produced by specialists like Wolff.
Doctoral student Franziska Hanf is researching precipitation in collaboration with researchers at the University of Potsdam and the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research. Her simulations explore the Indian subcontinent and the adjacent Indian Ocean. The meteorological data Hanf feeds into her model are provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). These global weather data – such as temperature, humidity, wind, and air pressure – result from analyses of ECMWF calculation models based on observation data. In this way, meteorological data not accounted for – because of the limitations of the global measuring network in the region – can be computed and made available. Hanf’s regional climate model runs on these data.
A key input variable is the surface temperature of the Pacific and Indian Ocean. Here is where El Niño comes into play. “The temperature of the Pacific Ocean influences the Indian Ocean and vice versa,” the researcher says. This affects the amount of precipitation in the Indian subcontinent. Below-average summer monsoon rainfall tends to lead an El Niño event. Analyses show further, that extended droughts over India tend to occur more frequently during El Niño years. With the help of her model, Hanf can calculate the volume of rainfall for various regions of India – based on the surface temperatures of the Oceans and other weather data. Although her simulations do not allow for making projections, since they use current and past meteorological data, they are an important milestone in better understanding the complex interrelationship of ocean and atmosphere in the South Asian monsoon system and its variability over time.
At the Institute of Biology and Biochemistry, Istem Fer is studying El Niño from a third perspective. As a member of its working group on vegetation ecology and nature preservation, the doctoral student is researching how a changing climate and El Niño affect the East African vegetation. She also uses a computer model, which shows how the vegetation develops depending on the climatic conditions. “East Africa is quite arid, since the mountain chains in the east prevent the humid air from the Atlantic Ocean from coming in,” Fer explains. Forests cannot grow under such conditions. The region is instead characterized by savannas, i.e. grassland scattered with shrubs and isolated trees. Based on pollen data and up-to-date vegetation maps, Fer can verify whether the results produced by her model match reality, both in the past and present. Once she has calibrated her model to carry out correct simulations, she will be able to test various future scenarios. For instance, what will happen if more frequent El Niño occurrences cool and humidify the East African climate? Which plant species will benefit and which ones will be pushed aside? The answers Fer hopes to find with her model are vital for the people living there. Farming in the region has primarily consisted of animal husbandry, since the savanna provides ample grazing for cattle. Should the grassland turn into shrub land, farmers will also have to adapt.“The aim is to build up an early warning system,” Wolff says of the overall objective of the research projects. “Once we understand the entire system and can put the pieces together, such a warning system will be within reach.”
The pattern repeats every two to eight years: The southeast trade winds in the Pacific Ocean that normally push warm surface water towards Southeast Asia and Australia, thus facilitating the influx of cold, nutrient-rich deep water off the west coast of South America, die down or decrease. The pool of warm water that has built up near Indonesia swashes back to the South American coast, and the cold Humboldt Current weakens or ceases. This phenomenon is referred to as El Niño – the Child Jesus. It is a phase of the El Niño Southern Oscillation (ENSO), a coupled circulation system between the ocean and the atmosphere. El Niño is generally followed by La Niña, when particularly strong trade winds push the warm surface water back to the west coast of South America. The consequences of the change of sea currents are also felt onshore: Instead of abundant monsoon rain, El Niño brings aridity to Southeast Asia, while South America may experience extreme precipitation with flooding and landslides. La Niña, on the contrary, causes flooding in Southeast Asia and droughts in South America. Weather patterns in other parts of the world are also shifting. The causes of the El Niño weather anomaly are not yet completely understood.
Dr. Christian Wolff studied geosciences in Trier and earned his doctorate in Potsdam. At the Institute of Earth and Environmental Sciences, he researches climate change over the past 25,000 years using geoscientific methods
Institut für Erd- und Umweltwissenschaften
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Franziska Hanf studied meteorology at Berlin’s Free University and is currently doing her doctorate at the Alfred Wegner Institute Helmholtz Centre for Polar and Marine Research and at the University of Potsdam. Her focus is on the variability of the South Asian summer monsoon and humid-dynamic processes of monsoon interruption phases as well as the effects of atmospheric aerosols on the climate in Southeast Asia.
Istem Fer studied molecular biology, genetics, and computer science in Istanbul. In her doctoral thesis she is researching changes in the vegetation of East Africa over the past millennia in relation to climatic factors.
This research is linked to the research initiative NEXUS: Earth Surface Dynamics, which clusters approaches from various scientific disciplines in the Berlin-Brandenburg area within the overarching theme of Earth surface dynamics. The University of Potsdam, along with its partnering institutions the Helmholtz-Centre Potsdam - German Research Centre for Geosciences (GFZ), the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) and partners from the Potsdam Institute for Climate Impact Research (PIK), the Museum für Naturkunde - Leibniz Institute for Evolution and Biodiversity Science (MfN) and the Technische Universität Berlin (TUB) therefore combines the outstanding expertise from geo-, bio-, climate and data sciences.
Text: Heike Kampe, Translation: Monika Wilke
Online-Editing: Agnes Bressa
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