There have been several consecutive seasons with above average rainfall in East Africa, an unusual event in this drought prone area. Then everything changed. The abundant moisture of past seasons exited the region resulting in drought conditions across Ethiopia, Somalia and Kenya. The October to December rains, known as the Deyr Rains, have failed. Pasture for cattle to graze on is running out, and crops that were planted in the hopes of a wet season have barely broken the surface, if they were lucky enough to even germinate. Many areas, especially in Somalia and Ethiopia, have received no rain at all. The prognosis isn't good. Little to no rain is coming to end the drought in the next month and a half. What happened? What went wrong? ...and when will it end?
The sea surface temperatures, cold near the Equator in the Pacific Ocean, warm near Indonesia in the Indian Ocean, and cold along the Somali coast are to blame. Meteorologists have a name for these two arrangements, El Niño Southern Oscillation and the Indian Ocean Dipole.
The cold water pattern currently in the Pacific, south of Hawaii and west of South America, is frequently associated with slightly warmer water near Indonesia. This is the "El Niño Southern Oscillation" and is currently in a La Niña phase. (An El Niño would be the opposite, warmer than normal water in the Pacific, usually with cooler than normal water near Indonesia.) This pattern alone is enough to hamper rainfall in East Africa. The warm waters near Indonesia heat the lower atmosphere. Moist air warmer than its surrounds rises and forms rain clouds. Currently because the water is much warmer than usual, this process is occurring more vigorously, dumping heavy rain. This is the culprit for the flooding that has repeatedly hit Southeast Asia over the last couple of months.
That rising air has a strong tendency to come back down over East Africa. Right now the cold water off the Somali coast is accelerating this process. By cooling the atmosphere above the water, air sinks faster that it already would be with the Indonesian air coming in. The sinking, dry air, acts like a lid on the atmosphere, preventing rain clouds from building and preventing moisture from reaching Ethiopia, Somalia and Kenya. This arrangement of sea surface temperatures in the Indian Ocean is know as the Indian Ocean Dipole and is labeled a negative pattern. (A positive pattern would be the opposite, warm water near Somalia, cold near Indonesia, and would have the opposite effect.) This process is strong enough when sea surface temperatures have small anomalies, but because the water near Indonesia is a lot warmer than average the whole cycle of rising and sinking air has been sent into overdrive.
The pattern will not change fast, the current La Niña (those colder than usual waters in the equatorial Pacific) is a moderate to strong event, and will take months to break down. The same is true for the warm waters near Indonesia. The colder than usual waters off the Somali coast however, are weak. Those could break down anytime now, and appear to be doing just that. Warming of the Somali waters may allow for slightly more rainfall, but what is really driving the drought are the warm Indonesian waters. In order to get more rainfall into East Africa, the lid on the atmosphere must be removed, and the only way to do that is for the waters near Indonesia to cool, and for the waters in the Pacific to warm.
There is also the problem that time is running out. At this point the crops are lost and there is only a month and a half left to improve pastures before the Deyr period ends, and with it the last chance of meaningful rainfall before April.