El Niño reduces rainfall in the Federal District and impacts soybean productivity

Much is said about the influence of the El Niño Southern Oscillation (ENSO) phenomenon on the climate in Brazil, but, for the first time, researchers from Embrapa have identified and quantified how the warming of the sea surface in the Pacific Ocean changes the rainfall regime and, consequently, the productivity of soybeans in a region of the Brazilian Cerrado.

In a study recently published in the journal Agrometeoros, Alfredo Luiz, from Embrapa Meio Ambiente (SP), and Fernando Macena, from Embrapa Cerrados (DF), found a relationship between El Niño years and a sharp reduction in accumulated precipitation in the months of October, November and December, in the administrative region of Planaltina, in the Federal District. The researchers analyzed records from the historical series from 1974 to 2022, in addition to daily observations at the meteorological station located in Embrapa's experimental area in the DF.

“It was impressive to see that the change in the surface temperature of an ocean thousands of kilometers away from the Brazilian Central Plateau changes rainfall in this region in such an impactful way,” says Luiz, commenting that the results show an average in El Niño years (372 mm) 40% lower than the average in La Niña years (623 mm).

How the study was done

Scientists used monthly values ​​from the Oceanic Niño Indicator (ONI), an index provided by the US Space Agency (NASA) since 1950 that provides an average of the sea surface temperature in the Pacific Ocean. They considered the 12 ONI values ​​from each year, from 1974 to 2022 (the period in which data from Embrapa's meteorological station were available - photo of the station on the side), to classify the years into five groups.

The group in which monthly ION values ​​were predominantly greater than +0,5 °C was considered as El Niño years. Similarly, the group in which variations smaller than -0,5 °C predominated was called La Niña years. Groups without a predominance of values ​​greater than +0,5 or smaller than -0,5 °C or with a mixture of them were called: End of El Niño; Neutral; and Beginning of La Niña.

After separating the years into groups, the average amount of accumulated rainfall in the months of October, November and December was calculated, the main period related to the soybean vegetative cycle in the region.

To complement the research, the scientists used a plant growth and yield simulator called STICS, which was fed with climate, soil and plant data, considering the planting of two soybean varieties in the soil of Embrapa Cerrados' experimental fields. The soil and plant data are always the same, however, for the climate data, the values ​​of rainfall, temperature and solar radiation actually observed daily for each year, from 1974 to 2022, were used.

Using this computational model, it was possible to simulate soybean productivity for planting on different dates, from September to December, in all years. Finally, the average productivity was calculated by planting date for each year and by groups of years derived from ENSO.

Soybean productivity was much lower in the El Niño group of years compared to the other groups, a difference that was even more evident when the simulated planting took place in the months of September and October. In the worst case, when sown in early September, the BRS 8383IPRO soybean cultivar produced, on average, 2.418 kg per hectare in La Niña years and only 493 kg per hectare in El Niño years. It is important to highlight that this difference decreased significantly when the simulated soybean sowing took place in the months of November and December. 

“The study proved that the effects of the Enos phases varied according to the sowing date in the Planaltina region, when the El Niño phases had a negative impact on soybean productivity on the first sowing dates and opposite results were found for La Niña years”, highlights Macena.

Next step: anticipation and predictability systems

“We know that early warning systems and early action can help reduce the negative impacts of extreme weather conditions exacerbated by ENSO or even take advantage of favorable conditions,” adds Luiz. “The next step will be to identify key periods, prior to soybean planting, in which the variability of ION or another ENSO indicator is strongly related to crop productivity. This is a way to recognize in advance the greater probability of increased or reduced rainfall during the critical phase of the crop cycle, so that decision makers can, in time, plan management alternatives, planting date, crop change or cultivar cycle to reduce losses with possible adverse effects or take advantage when the predicted weather conditions are more favorable.”

Understanding El Niño

ENSO is a large-scale natural climate phenomenon involving fluctuations in the temperatures of the equatorial Pacific Ocean, both at the sea surface and in the atmosphere above it. It is considered one of the most important sources of interannual global climate variability. El Niño is characterized by continued warming of the sea surface. In contrast, La Niña gives its name to cooling relative to the historical average in the same region and an intensification of the prevailing east-to-west surface winds.

The effects of each El Niño event, or its La Niña counterpart, vary depending on its intensity, duration, time of year of occurrence, and interaction with other sources of climate variability. Both are associated with characteristic patterns of precipitation and temperature, which can include extreme events such as floods and droughts. Not all regions of the world are affected, and even within a region, the impacts can be different.

In addition to rising global temperatures, regional climate influences commonly attributed to El Niño in Brazil include drier conditions in the North and Northeast and heavy rainfall in the South. The impacts of La Niña tend to be opposite to those of El Niño, with cooler global temperatures and wetter than normal conditions in the North and Northeast of Brazil.

“Despite the indication that in Brazil the most important effects would be on the North, Northeast and South regions, experience showed us that there were also changes in the central region, with an impact on agriculture. That is why we decided to investigate,” explains Macena.

By altering the quantity and temporal distribution of rainfall in a region, ENSO can result in food security being compromised by reducing agricultural production and productivity. These effects are caused by excess or lack of rainfall and extreme temperatures, which affect the development of plants and animals, including altering the distribution pattern and severity of pests and diseases that harm agricultural production.