Deadliest weather disaster of 2011: the East African drought

The deadliest storm of 2011 is Tropical Storm Washi, which is now being blamed for 957 deaths in the Philippines. Washi's heavy rains triggered devastating flash flooding on the island of Mindanao last Friday. However, the deadliest weather disaster of 2011 is a quiet one that has gotten few headlines--the East African drought in Somalia, Kenya, and Ethiopia. On July 20, the United Nations officially declared famine in two regions of southern Somalia, the first time a famine has been declared by the UN in nearly thirty years. Almost 30,000 children under the age of five were believed to have died of malnutrition in Somalia this summer, and the total death toll of this great drought is doubtless much higher. At least thirteen million people in East Africa are in need of food aid. However, conditions are improving. Food aid has lifted three of six provinces in Somalia out of famine. The "short rains" of the October - November rainy season were plentiful this year--too much so, since heavy rains killed 15 people in Kenya and left 80,000 homeless in early December. The flooding was worsened by the preceding drought, which killed much of the vegetation that ordinarily would have stabilized the soil and absorbed rainwater before it could run off and create destructive floods. The rains have allowed a good harvest to be planted this fall, and with continued food aid, the Somalia famine should ease by spring 2012. ReliefWeb reports that in the three Somalian provinces still experiencing famine, nearly 250,000 people face imminent starvation, though.


Figure 1. The impacts of the Horn of Africa drought on cattle in Somalia in 2006. Image credit: USGS

Meteorology of the East Africa drought
East Africa has two rainy seasons--a main "long rains" of March - June, and the "short rains" of October - November. The "short rains" failed in 2010, due to a sea surface temperature pattern featuring cooler than average waters in the western Indian Ocean, and warmer than average waters in the Eastern Indian Ocean (a negative "Indian Ocean Dipole.") When the main "long rains" in spring 2011 also failed, it brought one of the worst droughts in recorded history. The 2010 - 2011 drought was rated along with the droughts of 1983 - 1984 and 1999 - 2000 as one of the three most significant droughts of the past 60 years. It was the driest 12-month period on record at some locations in East Africa.


Figure 2. The "long rains" of March - May 2011 failed over much of East Africa, leading to drought and famine (left image.) However, the "short rains" of October - December have been up to five times higher than normal, easing the East Africa drought. Image credit: NOAA Climate Prediction Center.

The uncertain future of drought in East Africa
The climate of East Africa during the main March - June rainy season has steadily dried over the past 30 years. Since 2004, six of the past eight years have seen unusually deficient spring "long rains." This drying of the East African climate has come as the waters of the Indian Ocean have warmed significantly. A 2011 study by A. Park Williams and Chris Funk of the University of California, Santa Barbara, blames the drought in East Africa on the heating up of the Indian Ocean, which has altered the atmospheric circulation over East Africa to bring more sinking air and less moisture. The atmospheric circulation over East Africa is part of Earth's largest atmospheric circulation feature--the Walker circulation. The Walker circulation features rising air over the warmest waters of the Pacific Ocean, and compensating sinking air over over eastern tropical Africa and the eastern tropical Pacific. The Walker circulation also helps drive the El Niño/La Niña phenomena in the Eastern Pacific. Williams and Funk show that the increase in Indian Ocean temperatures in recent decades has made the Walker circulation extend farther west, resulting in more sinking air over East Africa and thus less rain. Since the increase in Indian Ocean temperature driving this change in the atmospheric circulation shows strong linkages with human-caused global warming, they conclude: "anthropogenic [human-caused] warming appears to have already significantly altered the Earth's largest circulation feature and impacted its most food insecure inhabitants." They predict that East Africa will continue to dry as global warming increases the ocean temperatures in the Indian Ocean, impacting the Walker circulation. However, eighteen of the 21 models used in the 2007 IPCC report on climate change predict more rainfall over East Africa by the end of this century. These models predict that the Walker circulation will weaken, shifting towards a more "El Niño-like" state, resulting in less sinking air (and thus more rain) over East Africa. Since there is as yet no evidence of this happening, and East African climate has gotten drier in recent years, this may be a case where the large majority of the climate models are wrong. While the models used to formulate the 2007 IPCC report do a reasonable job simulating the the current climate over most of the world, they do a poor job of simulating Africa's current climate. The models put too much precipitation in southern Africa, and displace the band of heavy thunderstorms called the Intertropical Convergence Zone (ITCZ) too far south. The 2007 IPCC report concludes, "the absence of realistic variability in the Sahel in most 20th-century simulations casts some doubt on the reliability of models". In other words, since these models do a poor job simulating the current climate of the Sahel region of Africa, we shouldn't trust their predictions for the future climate of Africa.


Figure 3. Farmers in the Horn of Africa tend their emerging crops. Image credit: USGS.

Donations sought for the East Africa famine
Weather Underground has partnered with the International Rescue Committee (IRC) to help the Horn of Africa region during the ongoing famine. With the help of the Weather Underground community, we hope to raise $10,000 that will go toward helping the refugees survive the crisis. Weather Underground will match the community's donation dollar-for-dollar up to $10,000 for a total donation of $20,000. Please visit the East Africa famine donation page to help out. Ninety cents of every dollar donated goes directly to the people in need.

References
Behera, Swadhin K., Jing-Jia Luo, Sebastien Masson, Pascale Delecluse, Silvio Gualdi, Antonio Navarra, Toshio Yamagata, 2005: Paramount Impact of the Indian Ocean Dipole on the East African Short Rains: A CGCM Study. J. Climate, 18, 4514-4530. doi: http://dx.doi.org/10.1175/JCLI3541.1

Dai A., K.E. Trenberth, and T. Qian, 2004: A global data set of Palmer Drought Severity Index for 1870-2002: Relationship with soil moisture and effects of surface warming", J. Hydrometeorol., 5, 11171130.

Sheffield, J., K. M. Andreadis, E. F. Wood, and D. P. Lettenmaier, 2009, "Global and continental drought in the second half of the 20th century: severity-area-duration analysis and temporal variability of large-scale events", J. Climate 22, pp 1962-1981.

Williams, A.P., and C. Funk, 2011, A westward extension of the warm pool leads to a westward extension of the Walker circulation, drying eastern Africa, Clim Dyn (2011) 37:2417-2435 DOI 10.1007/s00382-010-0984-y

Other posts looking back at the remarkable weather events of 2011
Tropical Storm Lee's flood in Binghamton: was global warming the final straw?
Wettest year on record in Philadelphia; 2011 sets record for wet/dry extremes in U.S.
Hurricane Irene: New York City dodges a potential storm surge mega-disaster

Wunderground releases its free iPhone and Android apps
Wunderground is proud to announce that our free Weather Underground iPhone app is now live in the iTunes store. The free Android version was released on Android Market last night. I've been having a lot of fun with the new apps; they're a great way to get weather info on the go.

I'll have a new post on Thursday.

Jeff Masters