Forecasting Climate, with Help from the Baobab Tree
In a three-part series for the International Research Institute for Climate and Society, graduate student Catherine Pomposi relates her experience in Senegal during the 2015 El Niño. She explains the 2015 El Niño forecast and its climate impacts in Senegal, and current efforts to better understand climate in the Sahel. Below are excerpts from each part, with links to the full posts on Medium.
By Catherine Pomposi
The Sahel’s Complex Climate and the 2015 Forecast
The need to understand the nature of precipitation and its variability in this region cannot be overemphasized. The totality of the rainfall for the entire year occurs during a single season from July-September. Known as the West African Monsoon, its variability has been challenging for scientists to understand and predict.
Farmers and pastoralists depend on the monsoon rains that occur from July to September to plan and sustain their agricultural activities for the season. The planting and cultivation of crops during the rainy season provides the majority of a farmer’s annual crop yield, and it is these crop resources that are drawn down until planting begins again the following rainy season. Any change to the rainfall can have large and lasting impacts on food security and economic infrastructure.
Forecasts such as those provided by the International Research Institute for Climate and Society and other climate centers are utilized by agencies and meteorological services worldwide, including Senegal’s. The agencies disseminate climate information to a large network of local authorities, agricultural extension workers and farmers. The ultimate goal is to increase community-level resilience to climate swings.
The summer 2015 forecast for the Sahel countries, including Senegal, was a bit bleak; the season was expected to be drier than normal, largely due to the El Niño event that was brewing in the eastern tropical Pacific Ocean. But even with the developing El Niño event, it was far from certain what the 2015 rainfall season would look like in the Sahel. The behavior of the Pacific Ocean — and particularly El Niño — is not the only oceanic driver of rainfall variability in the Sahel. Studies show that the state of the nearby Atlantic Ocean also plays a role in determining whether the rainfall season is stronger or weaker than normal. Read the full post here.
Working with and Learning from Farmers
To improve climate forecasts, scientists study the complex interactions and mechanisms within the climate system, such as the ways in which the Pacific and Atlantic Oceans may interact to produce unique rainfall years for the Sahel. But to better guide this research, scientists need to hear from potential users of climate information, such as farmers, and thus gain a better understanding of how people may use such information in their decision making.
Last June, with the 2015 seasonal forecast in hand, workers from Senegal’s Agence Nationale de l’Aviation Civile et de la Météorologie began their field visits to share the forecast with farmers in a series of workshops. After the initial visits to local communities, information continues to be distributed and exchanged between the providers and users of climate information through rural radio broadcasts and text messaging.
In addition to providing the forecast with probabilities for a wetter- or drier-than-normal season, workshop participants discuss the timing of the rains for that particular year. Timing is crucial to farmers: It influences when they should begin planting and what seed types to place in the ground (some varieties favor a longer growing season over a shorter one, for example). From a scientific perspective, the timing of monsoon rains is an even tougher question to pin down than whether or not the season will be particularly wet or dry.
I’ve expanded my own research interests to seek answers to some of the questions highlighted above. Much of the variability in West African monsoon rainfall comes from the global oceans. Prior to visiting Senegal, my research focused on how the global oceans forced drought patterns in the Sahel on long (e.g. decadal) timescales, such as during the very dry 1970s-1980s. Having now spent time with people in the area, I have a greater appreciation for the shorter-term variability and the potential for applications of research at this timescale. I am now trying to understand how the global oceans interact with one another to change precipitation patterns within a season, such as the number of times a rain event happens, the length of a season and the intensity of rain events.
Research based on needs of end-users of climate information can increase climate resilience and lead to better food security and economic livelihoods. But scientists aren’t the only ones intimately familiar with the climate. One of my favorite parts of the climate services workshops is hearing about farmers’ traditional knowledge of the monsoon system. Farmers throughout the region have made decisions relying on their own understanding of climate patterns passed down through generations.
Science-based traditional indicators include things like the behavior of plants and animals in the region, or wind patterns. When the monsoon invades the region, winds switch from coming from the north and east to coming from the south and west. It is through these changing wind patterns that moist air from the ocean is brought over land, driving the monsoon.
The resulting changes in humidity and temperature over land are felt by plants and animals, which will then respond. Some farmers note that when the fruit of the baobab tree becomes about the size of a human hand, the rains should start soon. Others note the behavior of animals. One farmer said that when lizards begin to move around more, particularly up the roofs of houses or trees, rainfall should begin soon. It is likely that some of these behaviors observed for centuries by local inhabitants are grounded in the changing humidity and temperature patterns of a monsoon climate.
Now, with the establishment of partnerships among local farmers, communities and meteorological agencies, scientific and traditional knowledge of rainfall patterns can be shared across the two groups. Read the full post here.
Looking Back at the 2015 El Niño
As the rainy season progressed, Senegal’s Agence Nationale de l’Aviation Civile et de la Météorologie continued to issue forecast updates to the communities in which it works. Now that the season (July-September 2015) is over, scientists are quantifying the accuracy and usefulness of the forecast itself.
Another important analysis is to compare how much rain fell to what was expected by scientists before the season began. Back in June, many signs were pointing toward an El Niño event developing, which usually translates to dry conditions in West Africa. In fact, the El Niño event not only developed, it continued to gain strength and is now considered one of, if not the strongest, events since 1950. So, we might be surprised to see that a map of July-September precipitation anomalies (i.e. the rainfall deviations from the long-term average) looks like this:
Looking at the Sahel, we see much of the region exhibiting above-normal precipitation. Why might the Sahel have been wetter than normal, given the El Niño event? The climate system is large and complex and nothing operates in isolation. Other ocean basins have also been shown to affect the Sahel, so perhaps their contributions helped to maintain a wet signature rather than a dry one. One possibility is that the warmer-than-usual sea-surface temperatures directly off the west coast of Africa helped to bring increased moisture supply to the continent.
Back in June, it was difficult to forecast how these sea-surface temperature anomalies were going to develop. The June sea-surface temperature map showed equal areas of warm and cool anomalies around the same region. It was the warm water, however, that persisted from July-September, likely helping alleviate some of the dry conditions we expected from El Niño. But without the help of controlled model experiments, it is difficult to make the conclusion that this warming in the Atlantic saved the region from an otherwise drier fate for the 2015 rainy season.
This year’s rainy season is a great example of the complexity of the climate system and how competing influences might work together to give us an unexpected outcome. Visiting Senegal motivates me to continue to study rainfall patterns in this semi-arid region. Indeed, I have tailored some of my research questions based on my time spent in the country. I have a better appreciation and understanding of the difficult climate conditions that exist in this semi-arid land than I would have received simply from reading climate textbooks. My trips have also allowed for me to witness first-hand the power of climate information and its influence on lives and livelihoods, as well as the ability of science to truly serve society. Read the full post here.