CASE STUDY: Kenya and the 2015/2016 El Niño - ENSO amplification and forecasting failures

 Comparing CHIRPS rainfall and anomalies in ENSO and IOD strength for 3 strong El Nino events

Intensification of the hydrological cycle is a key impact of climate change, exemplified by past and continuing amplification of ENSO cycles as a result of global warming, spanning all CMIP models. Cause for this amplification is debated, with some suggesting anomalous surface winds alter Southern Ocean heat uptake patterns, while others link it to increased sea-surface temperature at the tropics, driving stronger rainfall. While the cause may be debated, the impact is not, with increasing ENSO strength having been linked to severe regional droughts and flooding worldwide.

The El Niño event of 2015/2016 was one of the most severe on record, leading to predictions of extreme flood events in Kenya. There was strong precedent for this, as the similarly strong El Niño event of 1997/1998 had devastating impacts. It resulted in 10 months of extreme heavy rainfall, with landslides, floods, proliferation of diseases, destructions of homes and livestock, and losses of thousands of lives.

 

Comparing cumulative precipitation and WASP values in Kenya for strong El Nino years

 

However, in defiance of most national and global meteorological institutions, Nairobi saw near normal rainfall in the 2015/2016 wet season. While 1997’s peak event had a return period of 15 years, 2015 peak flows only had a 1.5-year return period, meaning less widespread and long-lasting disruption. The surprisingly low precipitation during this period has been widely attributed to a weakness in the Indian Oceandipole, which was strong during the 1997 El Niño. 

While widespread flooding was limited, impacts were severely felt in some regions, especially those coastal or near the banks of Lake Victoria. Fortnam describes flood damage in Vanga, a rural coastal town, with many homes destroyed, trade severely disrupted and an increase in waterborne disease; mostly attributed to poor infrastructure. Subsistence farmers strongly felt the effects of flooding, with delayed harvests during planting season, a time when food stores run low, exacerbating food insecurity. As Kenya has little temporal buffer between rainfall and flood onset, vigilant forecasting and mitigation is paramount. Combined with strong institutional memory of extreme flooding, the nation was overprepared for disruptions, leading to experts being concerned about officials ignoring forecasts in the future.

“in 1997 everybody was a victim” 

“it just had impacts in particular zones”

 Quotes from key meteorological and management officials

This case study illustrates that a core danger brought about by anthropogenic climate change is the increasing unpredictability of weather events, partially resulting from the uncoupling of previously concurrent climate phenomena (i.e. ENSO and IOD cycles). Forecasts are central to many mitigation and adaptation strategies, but rely on accurate understanding and modelling ability – which is complicated by failures such as the East African Climate Paradox in most projections. 

 

Observed rainfall anomaly vs CMIP5 predicted future anomalies for East Africa