Looking back on the 2015 Rainy Season

As the rainy season progressed, Senegal’s Agence Nationale de l’Aviation Civile et de la Météorologie (ANACIM) continued to issue forecast updates to  the communities in which they work. Now that the season (July-September) has concluded,  scientists are quantifying the accuracy and usefulness of the forecast itself.

One way in which ANACIM and partner organizations test the application of the forecast is through experimental crop beds in some of the local villages, where they compare yields from beds where the scientific seasonal forecast information was and wasn’t applied in management decisions. This serves as a good research tool and one way to show how climate information can be meaningful to the sectors most sensitive to climate variability.

Another important analysis is to compare how much rain fell to what was expected by scientists (see my first post) before the season began. Back in June 2015, many signs were pointing towards an El Niño event developing, which usually translates to dry conditions in West Africa. In fact, the El Niño event only continued to gain strength and is now considered one of, if not the, strongest event since 1950. So, we might be surprised to see a July-September precipitation anomaly map (i.e. the deviations from the long term average) that looks like this:

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Figure 1: July-September precipitation anomaly (1979-2000 baseline; in mm/day) from NOAA CPC CAMS_OPI dataset. Data accessed through https://iridl.ldeo.columbia.edu/maproom/

If we focus on the Sahel region (outlined by the box), then 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 possibility is that the warmer-than-usual SSTs directly off the west coast of Africa (see map below) helped to bring increased moisture supply to the continent.

seas sst anomsst scaleFigure 2July-September sea surface temperature anomaly (compared to 1971-2000 climatology) from NCEP OISST version 2 dataset. Data accessed through https://iridl.ldeo.columbia.edu/maproom/

Back in June, it was difficult to forecast how these SST anomalies were going to develop. The June SST map in my first post 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. 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.

Having visited Senegal motivates me to continue to study rainfall patterns in this semi-arid region. Indeed, some of my research questions have been tailored based on my time spent in the country. I have a better appreciation and understanding of the difficult climate conditions that exist in this unique land region 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 power of science to truly serve society.

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Dr. Ousmane Ndiaye and me at the end of my visit to Senegal. Photo credit: Jillian Pomposi

The 2015 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. As I continue my studies of the monsoon far from the Sahel, I’ll be trying to continue to disentangle how the major ocean basins work together to force changes in the atmosphere that then translate to changing precipitation over land.

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Sunset over the Sahel. Photo credit: Catherine Pomposi