Jan. 5 () –
A study led by researchers at Brown University shows how the melting of large glaciers it may end up causing droughts and floods in East Africa and Indonesia.
With a new analysis of long-term climate data, understanding of how climate change may affect and cause seawater temperatures on one side of the Indian Ocean to rise has improved. are much warmer or colder than the temperatures on the other, a phenomenon that can lead to sometimes deadly weather-related events such as mega-droughts in East Africa and severe flooding in Indonesia.
The analysis, described in a new study published in the journal Science Advances by an international team of scientists led by Brown researchers, compares 10,000 years of past climate conditions reconstructed from different sets of geological records. with simulations of an advanced climate model.
The results show that between 18,000 and 15,000 years ago, as a result of the discharge into the North Atlantic of fresh melt water from the huge glacier that once covered much of North America, the ocean currents that kept the Atlantic Ocean warm weakened, triggering a chain of events in response. The weakening of the system eventually led to the strengthening of an atmospheric loop in the Indian Ocean that keeps warmer water on one side and cooler water on the other.
This extreme weather pattern, known as a dipole, sees one side (east or west) see above-average rainfall and the other side experience widespread drought. The researchers saw examples of this pattern both in the historical data they studied and in the model simulation.
They say their findings may help scientists not only better understand the mechanisms behind the east-west dipole in the Indian Ocean, but also one day make more effective forecasts of droughts and floods in the region.
“We know that currently Indian Ocean temperature gradients are important for precipitation and drought patterns, especially in eastern Africa, but it has been difficult to show that these gradients change in the long term and to relate them to precipitation and drought patterns over time.” long-term on both sides of the Indian Ocean,” says it’s a statement James Russell, study author and professor of Earth, Environmental and Planetary Sciences at Brown.
“We now have a mechanistic basis for understanding why some of the long-term changes in precipitation patterns in the two regions have changed over time.”
In the article, the researchers explain the mechanisms that explain how the Indian Ocean dipole they studied formed and the weather events it caused during the period they studied, which spanned from the end of the last Ice Age to the beginning of the current geological epoch.
The researchers characterize the dipole as an east-west dipole in which the water on the western side, which borders present-day East African countries like Kenya, Ethiopia and Somalia, is cooler than on the eastern side, in the direction of Indonesia. They saw that the warmer water conditions of the dipole brought higher rainfall to Indonesia, while cooler water brought much drier weather to East Africa.
That fits with what is often seen in recent Indian Ocean dipole events. In October, for example, heavy rains triggered flooding and landslides on the Indonesian islands of Java and Sulawesi, killing four and affecting more than 30,000 people. At the opposite extreme, Ethiopia, Kenya and Somalia suffered severe droughts starting in 2020 that threatened to lead to famine.
The changes observed by the authors 17,000 years ago were even more extreme, including the complete drying of Lake Victoria, one of the largest on Earth.
“Essentially, the dipole intensifies dry and wet conditions that could lead to extreme events, such as multi-year or decade-long droughts in eastern Africa and floods in southern Indonesia,” said Xiaojing Du, a postdoctoral researcher at the Institute for Environment and Society for Brown and Brown’s Department of Earth, Environmental, and Planetary Sciences, and lead author of the study.
“These are events that affect people’s lives and also agriculture in those regions,” he adds. Understanding the dipole can help us better predict and better prepare for future climate change.”
The dipole the researchers studied was formed from interactions between the Atlantic Ocean heat transport system and an atmospheric loop, called the Walker Circulation, in the tropical Indian Ocean. The lower part of the atmospheric loop flows east-west through much of the low-lying region near the ocean surface, and the upper part flows from west to east at a higher altitude. The upper air and the lower air connect in one big loop.
The disruption and weakening of the transport of heat by the Atlantic Ocean, which works like a conveyor belt formed by ocean and wind currents, was caused by the massive melting of the Laurentide ice sheet, which once covered most of Canada and the northern United States.
The melting ice cooled the Atlantic and the consequent anomalies in the winds caused the atmospheric loop over the tropical Indian Ocean to become more active and extreme. This caused an increase in rainfall in the eastern part of the Indian Ocean (where Indonesia is located). and a reduction in rainfall in the western part, where East Africa is located.
The researchers also show that, over the period studied, this effect was amplified by falling sea levels and the exposure of nearby continental shelves.
Scientists say further research is needed to find out exactly what effect the exposed continental shelf and lower sea level have on the Indian Ocean east-west dipole, but they are already planning to expand the work to investigate the issue.
Although this line of work on sea level decline will not serve to model future conditions, the work they have done investigating How the melting of ancient glaciers affects the Indian Ocean dipole and the Atlantic Ocean heat transport system may provide key insights into future changes as climate change causes more melting.
“Greenland is melting so quickly that it’s dumping a lot of fresh water into the North Atlantic, affecting ocean circulation,” Russell said. “The work done here has provided a new understanding of how ocean circulation changes Atlantic Ocean can affect the climate of the Indian Ocean and, through it, to rainfall in Africa and Indonesia”.
