New Research: El Niño Teleconnections in the Sahel & East Africa
Pradipta Parhi, a graduate research assistant in Columbia’s Department of Earth and Environmental Engineering published a paper in the February 2016 issue of the Journal of Climate. The study examines why two areas of Africa – the Sahel and eastern equatorial Africa – tend to experience drier- and wetter-than-normal rainy seasons, respectively, during El Niño. He is joined on the paper by advisors Alessandra Giannini of the International Research Institute for Climate and Society and Upmanu Lall of DEEE, IRI and the Columbia Water Center, as well as Pierre Gentine of DEEE and CWC.
What are the main conclusions that you and your coauthors draw from your analysis?
Scientists have known for decades now that in different tropical regions the seasonal mean rainfall responds to El Niño differently. The Sahel, for example, usually experiences drier-than-normal conditions during its rainy season (Jul-Aug-Sep), while tropical eastern Africa is wetter than normal (Oct-Nov-Dec) in an El Niño year. Analyzing both observed and reanalysis data at the daily time scale, our paper explains the physical reason for these responses. We found that the interannual variability of seasonal rainfall – both the drier response over Sahel and the wetter response over tropical eastern Africa – is mainly explained by changes in the frequency of daily rainfall events rather than by the intensity of those events. Another interesting conclusion is that the most extreme daily precipitation intensity (99.9 percentile) is not significantly different in either region during El Niño compared to a neutral year.
El Niño is one of the most-studied and well-understood climate phenomena. What has your team done differently from prior research?
From prior studies, we had a sense of why regional rainfall might respond differently to atmospheric warming caused by El Niño. We knew it would be difficult, however, to build confidence without an empirical validation of the hypothesized physical mechanisms. Analyzing sea-surface temperature, moisture budget and vertical pressure velocity fields, we provide the first empirical evidence for a coherent mechanism that explains the rainfall responses in the Sahel and eastern tropical Africa during an El Niño year. We conclude that rainfall responses depend on the phase relationship between the local rainy season and El Niño’s evolution stage.
While there have been many studies on El Niño’s influence on rainfall variability in tropical Africa, most of them used monthly data and parametric statistical methods. We instead used daily rainfall from multiple datasets (station, satellite, reanalysis) and applied robust non-parametric methods. Furthermore, instead of focusing only on seasonally-averaged rainfall, we report on multiple characteristics such as frequency and intensity of rainfall events.
During El Niño, warmer-than-average seasurface temperatures in the Pacific induce changes in atmospheric patterns around the world, known as teleconnections. What are some of the key factors that determine how a teleconnection affects a particular location?
How a local climate responds during an El Niño event is decided by two broad factors. The first is the location of the region of interest with respect to the ascending or descending branches (i.e. areas of rising or sinking air) of what’s known as the Walker Circulation (see images below, from NOAA’s ENSO blog). In the tropical climate, the large-scale climate circulations are mainly driven by latent heat released in the upper atmosphere by storms and rain clouds, also known as convection. The main center of convection is typically the tropical western Pacific Ocean due to the very warm water there. During an El Niño event, the location of this convection shifts east-ward to the central or eastern tropical Pacific Ocean. The large-scale circulations, particularly the Walker circulation, shift accordingly. The regions now under the part of the circulation with rising air will likely experience more rain than normal (due to convection), while the regions under the part of the shifted circulation with sinking air are likely to experience less rain than normal.
The second factor influencing how a location experiences El Niño is the timing of the location’s rainy season with respect to the growth or mature phase of El Niño evolution. Our current paper mainly focuses on this timing aspect. During the first few months of an El Niño’s development – known as the growth phase – atmospheric waves efficiently spread the extra heat from the upper troposphere above the Pacific Ocean to the upper troposphere in the tropical belt around the globe. The warming of the upper troposphere stabilizes the atmosphere, thus suppressing rainfall over much of the tropics. So if a location’s typical rainy season coincides with the growth phase (usually July – September) of the El Niño, a drier response is the outcome.
The other tropical oceans (Indian, Atlantic) do not warm up as fast, but as an El Niño reaches its mature phase, their sea surface temperatures experience heating. Higher sea surface temperatures lead to higher evaporation rates, which in turn provide more moisture and destabilize the atmosphere, thus increasing rainfall levels. If the rainy months of the location – particularly the ones fed by oceanic moisture, are during the mature phase (usually November-December) of the El Niño, wetter than normal conditions are expected.
Note that the teleconnection mechanisms as described above are simplistic and usually assumed to be stationary. However other components in the complex and chaotic climate system can enhance or diminish the teleconnection signals. For example, the other tropical oceans (Atlantic or Indian Ocean) have their own versions of El Niño-like quasi-oscillations, which can modify the influence of El Niño.
How can this new knowledge be useful in decision-making? What can stakeholders do with the results of your research?
Beyond advancing our scientific understanding, this knowledge can be valuable for climate risk management. In areas around the world, including the regions of Africa examined in this analysis, new innovative ideas, such as index insurance at a farmer level and risk pooling and sharing at a country level, are being tested to improve climate adaptation and resiliency. Our findings can improve climate risk pooling strategies by selecting regions and seasons more appropriately, so as to maximize the benefits of diversifying investments, which can then reduce premiums of insurance or derivative contracts.