Sunday 8 December 2019

Flooding and landslides kill at least sixteen in Western Uganda.

At least sixteen people have died in a series of flooding and landslide events in the Bundibugyo District of Western Uganda this weekend, according to the Uganda Red Cross. The incidents happened after weeks of heavy rains left soil in many parts of the area waterlogged, so that when the rains intensified on Saturday 7 December 2019, the ground was unable to soak up the water, causing it to accumulate on the surface and leading to a series of flash floods and landslides. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall. Several more people are still missing in the area, with rescuers still searching the landslide sites for more bodies.

Flooding in Bundibugyo District, this weekend. Uganda Red Cross.

The incidents occurred after about a month of heavy rains, in one of the area's two annual rainy seasons. This two rainy seasons per year pattern is typical in equatorial countries, with rainy seasons around the equinoxes and dry seasons around the solstices. Upland areas of East Africa have always been prone to landslides, but the problem has become worse in recent years as a rising population has led to more agriculture on hill-slopes, in many areas replacing open woodland where tree roots served to stabilise slopes, and also to more people living in harms ways. This years rains have been exceptionally heavy, and fatalities due to similar events have also been reported in other parts of Kenya, Somalia, Ethiopian, South Sudan, Sudan, Tanzania, the Democratic Republic of Congo, the Republic of Congo, Burundi and the Central African Republic.

The rains this year are thought to have been made worse by the development of a meteorological phenomenon called a Negative Indian Ocean Dipole. Indian Ocean Dipole Phases are similar to the El Niño/La Niña climatic oscillation that affect the Pacific Ocean. Under normal circumstances equatorial waters off the east coast of Africa and west coast of Indonesia are roughly similar in temperature, however during a Negative Indian Ocean Dipole Phase the waters off the coast of Indonesia become significantly warmer. As the prevailing currents in the area flow west to east, this warm water is then pushed onto the shallower continental shelf of north Australia, where it warms the air over the sea more rapidly, leading to increased evaporation (which fuels rain) and a drop in air pressure over the east Indian Ocean and west Pacific. This in turn drives air currents over the Indian Ocean to flow more strongly west to east, leading to higher rates of  cooling off the coast of Africa (where more water is drawn up from the cool sea depths) and more warming off the coast of Indonesia, fuelling a feedback cycle that tends to remain through the winter season in any year when it forms. This leads to a particularly wet winter across much of Australia, as well as a potentially damaging heatwave in the north, while much of East Africa is at risk of drought (during a Positive Indian Ocean Dipole Phase the reverse happens, with drought in Australia and flooding in East Africa).

 Areas of warming and cooling and air flow during a Negative Indian Ocean Dipole Phase. Australian Bureau of Meteorology.

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