The 2019 October-December rains over East Africa were one of the wettest seasons on record, with many locations receiving more than double the climatological rainfall, leading to floods and landslides across the region. This above average rainfall led to favourable vegetation characteristics for locusts, contributing to the ongoing locust plagues across East Africa, one of the worst ever recorded. The rainfall also made a sizable contribution to a rise in Lake Victoria’s water level, which combined with above average rainfall during March and April 2020 led to record breaking water levels in Lake Victoria, flooding homes and infrastructure. A new paper describes the season and factors associated with the wet conditions, describing how these link to man-made climate change.
During October-December 2019 there was a strong positive Indian Ocean Dipole (IOD) event in the Indian Ocean, with anomalously warm Sea Surface Temperatures (SSTs) in the western Indian Ocean, adjacent to East Africa, and anomalously cool SSTs in the eastern Indian Ocean. Positive IOD events are known to be associated with enhanced rainfall over East Africa, with the positive IOD events in 1961 and 1997 leading to extremely wet conditions over East Africa. Other factors, including the presence of tropical cyclones in the western Indian Ocean, and a sub-seasonal tropical atmospheric phenomenon within the atmosphere which enhances convective activity, called the Madden-Julian Oscillation, also contributed to the above average rainfall.
Future projections from climate models indicate increasing rainfall during the short rains under future climate change, suggesting that events such as the increased rainfall during the 2019 short rains could become more frequent. Earth has already warmed by around one degree (above pre-industrial levels), and models show that under a global average warming of 1.5°C strong positive IOD events may occur twice as often. The rate of recent warming of the western Indian Ocean is one of the fastest of any tropical ocean over the last century, and climate models project that continuation of these higher rates of warming should be expected under climate change. There is strong agreement across climate models regarding these changes in Indian Ocean SSTs, increasing our confidence in increased occurrence of very wet short rains over East Africa under future climate change. Therefore, although it is generally impossible to attribute any particular event to climate change, it appears that climate change is very likely to have increased the chances of extremely wet short rains, such as that of 2019. In addition, climate change increases the local intensity of rainfall, increasing the chance of flooding and landslides for any given seasonal rainfall accumulation.
Given the adverse impacts of the enhanced rainfall, and rainfall variability, on society in East Africa, and projections of future climate change suggesting that such extremely wet seasons could become more frequent, societies should prepare and adapt for more events similar to the short rains of 2019. It will be increasingly important to use forecasts in risk management to help adapt to such events, especially for the short rains, where seasonal forecasts have been shown to be much more accurate and reliable than they are for the long rains.
This article was written by Caroline Wainwright (University of Reading).