Eastern Africa: Why the normally dry January and February are suddenly wet in 2020?

Lead Author: Eunice Koech

Contributions by ICPAC Climate Change Technical Working Group

Global Temperature Anomalies from 1880 to 2019, https://svs.gsfc.nasa.gov/4787

The globe is warming and new temperature records keep being made. July 2019 was classified as the hottest July since record keeping began 140 years ago (NOAA, 2019), and January 2020 as the hottest January in history (NOAA, 2020). These high temperatures contributed to high evaporation and high water holding capacity of the atmosphere (warm air holds more moisture). For instance, due to the high temperatures, January recorded unusually high rainfall (up to 200% more than expected) in the equatorial parts of the region, as shown below.

January 2020 rainfall expressed as a percentage of 30 years (January 1981–2010) average

Under normal conditions, the equatorial (Uganda, Kenya, southern part of Somalia, Rwanda and Burundi) and northern parts (Ethiopia, Djibouti, South Sudan, northern part of Somalia and Sudan) of Eastern Africa experience a dry season from mid December to February as the rainfall shifts to the southern part (Tanzania) of the region; following the position of the Inter-Tropical Convergence Zone (ITCZ). ITCZ is a strip of low pressure which circles the Earth near the equator where the trade winds of the northern and southern hemisphere come together. It is characterized by rising air due to converging winds, leading to convective activities which generates often vigorous thunderstorms over large areas (SKYbrary).

Inter-Tropical Convergence Zone (ITCZ) position In January (blue) and July (red). Usually, countries under the blue strip experience rainfall in January while countries under the red strip experience it in July.

The end of the October-December (OND) short rains season always occurs between the second and third week of December. However, when unusual warming occurs in the oceans, unusually wet periods happen beyond the short rainy season. This unusual warming could result in El Nino, La Nina or the Indian Ocean Dipole (IOD) phenomenon. The rainfall experienced in 2019 was associated with a strongly positive IOD and tropical cyclones that developed in the north western Indian Ocean. This led to increased rainfall during the short rains season; the third highest on record for the region overall, and the highest on record in some parts. Consequently, the region suffered adverse effects including loss of lives and livelihoods. The IOD index returned to its neutral state in the fourth week of December 2019 and the Australia Bureau of Meteorology forecasts it will remain neutral into the summer (June-August) of 2020.

Although the IOD index became neutral in December, its impacts, in terms of rainfall, were still felt in January and February. The Indian Ocean sea surface temperatures remained warm, evaporation continued and the existence of monsoon winds helped to transport moist air into our region, resulting in a lot of rainfall during the usually dry season.

Global climate systems such as the Indian Ocean Dipole (IOD) and El Nino have a “long-memory” relationship with climate patterns over Eastern Africa. They also occur as a result of continuous interaction between the surface of the ocean and the atmosphere (surrounding air) above it. This ocean and atmosphere interaction continues for a considerable period even after the phenomena (IOD) dies, and this was evident in January and February 2020.

As global warming continues, shifts in rainfall patterns are expected. Occurrence of global climate events such as strong IODs, El Nino and La Nina are also expected to be more recurrent, whilst the frequency and intensity of extreme events such as floods and droughts are likely to continue increasing. Therefore, off-season rainfall may arise, with dry and wet periods becoming common. As countries continue to honour their Paris Agreement promises, more efforts need to be made to ensure emissions are reduced and global warming remains well below 1.5 degrees Celsius above pre-industrial levels.

References

NOAA (2019). July 2019 was the hottest month on record for the planet. Polar sea ice melted to record lows. Retrieved from https://bit.ly/3aOAtCT

NOAA (2020). January 2020 was Earth’s hottest January on record. The long-term trend of above-average temperatures continues. Retrieved from https://bit.ly/339QlgH

CHIRPS: Rainfall Estimates from Rain Gauge and Satellite Observations. Retrieved from https://bit.ly/3aM3y1H

SKYbrary (2018). Inter Tropical Convergence Zone (ITCZ). Retrieved from https://bit.ly/38JTfK2