El Niño Retrospective: Senegal, Part 1
This post is the first in a series of three articles by graduate student Catherine Pomposi about the 2015 El Niño forecast and its climate impacts in Senegal, as well current efforts to better understand climate in the Sahel.
The Complex Climate of the Sahel
Geologists travel to remote locations often. Physical oceanographers are at sea for months at a time. But what does field work look like for an atmospheric scientist? Stratospheric visits à la Felix Baumgartner? Hardly. Indeed, much of the scientific research of our Earth’s ever-changing climate system is done from behind a desk or screen, utilizing climate data from weather stations, satellites and supercomputers. Many of my colleagues may never get an opportunity to visit the part of the world whose atmospheric patterns and climate characteristics they so intensely study. Luckily for me, this hasn’t been the case.
The Sahel is is a unique area that sits between one of the driest regions of the planet, the Sahara Desert, and lush tropical rainforests of equatorial Africa (see image). It reaches across the continent, from Ethiopia in the east, to Senegal in the west, where I spent last summer conducting field work. Scientists at Senegal’s Agence Nationale de l’Aviation Civile et de la Météorologie (ANACIM) were my hosts and collaborators. The United States Agency for International Development awarded me a fellowship to conduct this work.
This opportunity served as a natural extension of my PhD research in Columbia’s Department of Earth and Environmental Sciences. My dissertation focuses on how sea surface temperature patterns in different ocean basins influence the amount of rain that falls in the Sahel over a season, as well as the distribution of rain within the season. Increased knowledge of how changes over the ocean translate to rainfall variability over land has the potential to lead to better climate forecasts — information that can help build resilience to local climate swings.
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. For example, between the 1950s-1960s, portions of the Sahel experienced summer rainfall totals upwards of 700 mm. By 1970–1980, summer totals had dropped to as little as 300 mm in some years. Such a shift was unseen anywhere else in the world, both in its spatial extent and magnitude.
Immediate impacts were felt as the drought began in the 1970s; a large portion of domestic livestock perished and millions of people were forced from rural areas into larger cities or camps where they relied on external food aid for sustenance. While the past couple of decades haven’t been as dry overall, year-to-year variability has been significant, which also presents challenges for prediction and preparation.
Since the debilitating droughts, scientists have increased their focus on understanding patterns of rainfall change in the Sahel, yet it remains one of the regions of high uncertainty in climate change projections.
Understanding precipitation shifts is critical for the region because of the reliance of the majority of the population on agricultural activities. 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.
The 2015 Seasonal Forecast in the Sahel
In June, the International Research Institute for Climate and Society issued its seasonal forecast for the upcoming summer (July, August, September). The IRI issues such forecasts monthly, updated based on current knowledge of the state of the climate system. Forecasts such as those provided by IRI and other climate centers are utilized by agencies and meteorological services worldwide, including Senegal’s ANACIM. The agencies then disseminate climate information to a large network of stakeholders regionally, including local authorities, agricultural extension workers and farmers. The ultimate goal of these efforts 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.
El Niño is the warm manifestation of the El Niño Southern Oscillation phenomenon, which is one of the most robust sources of climate variability on the planet. It plays an especially crucial role in the natural climate variability of the tropics on the year-to-year timescale. When an El Niño develops, changes in sea-surface temperatures, sea-level pressure and atmospheric circulation are communicated throughout the world, influencing temperature and precipitation patterns globally. Throughout West Africa, El Niño events have been linked with drier- than-normal conditions during the rainy season.
Could the Atlantic Ocean save the Sahel from what the Pacific has in store?
Even with the developing El Niño event, it was far from certain what the 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.
Researchers have found that warmer-than-normal conditions in waters off the west coast of Africa can help alleviate potential dry conditions in West Africa. Because the air that lies directly over a warm ocean surface also tends to be anomalously warm, and warm air can hold more moisture, the atmosphere in this area can retain more water vapor. Eventually, the air masses make their way to the continent, where the moisture will likely fall out over the land surface as monsoon rains. The increased moisture content from a warmer-than-normal North Atlantic could ensure more rainfall than if the same area of the ocean were cooler than normal.
The sea-surface temperature anomalies in mid-June 2015 did not clearly show either particularly large warm or cool anomalies off Africa’s west coast (see image below). Through the course of the summer, the small cool region could expand eastward, potentially resulting in a double-whammy of drier-than-normal conditions for the Sahel. Alternatively, we could see the warm anomaly that extends from the eastern coast of North America grow east and south, which would help improve the chances for higher than normal rainfall .
The inherent chaos of the climate system means we do not know with certainty how any season might turn out. The best we can do right now is issue seasonal forecasts that point towards the likelihood of drier or wetter, hotter or colder conditions, and continue our research efforts to understand how global sea surface temperatures affect this monsoon region. Some scientists do this by asking research questions that are directly tailored to the needs of those who might use climate information in their decision making, such as farmers, water resource managers and healthcare workers.
What happens when climate scientists and farmers sit down together? I talk more about both the questions and knowledge farmers bring to the discussion in the next post. And stay tuned to find out whether the rainy season followed the typical El Niño pattern.
