This year’s catastrophic drought in Kenya, Ethiopia, Djibouti and Somalia is thought to be an indirect consequence of the 2010-2011 La Niña. As part of ENSO cycles such droughts come and go – and have been typical for the region for (at least) all of the Holocene up to the Last Glacial Maximum some 20,000 years ago, research shows. By nature, the pattern is irregular – and both dry and wet periods may last multiple years.
This we first learned from a publication by a German-led international research team last August in Science. Not only are droughts in the Horn of Africa (and equatorial East Africa directly to the south) a typical result of La Niña episodes, strong El Niños on the other hand are correlated with high precipitation and floodings.
One of the authors of that study, Axel Timmermann of the School of Ocean and Earth Science and Technology of the University of Hawaii, has further elaborated on the research during his Friday presentation at the American Geophysical Union meeting in San Fransisco.
The best way to envision the relation between ENSO and precipitation over East Africa is to regard the Indian Ocean as a mirror of the Pacific Ocean sea surface temperature anomalies [much like the Western Hemisphere Warm Pool creates such a SST mirror with the Atlantic Ocean too]: during a La Niña episode, waters in the eastern Pacific are relatively cool as strong trade winds blow the tropically Sun-warmed waters far towards the west. In the Indian Ocean this leads to a positive temperature anomaly in the east – and therefore relatively cool water temperatures along the tropical African east coast.
Mechanism: ENSO either intensifies or suppresses ITCZ
Over ocean stretches with a positive SST anomaly air convection is higher (as the temperature difference between the warm sea surface and the cool air higher up in the troposphere is greater), so a higher likelihood for the formation of depressions exists and more precipitation is to be expected. Negative SST anomalies may in turn favour high pressure blockades and fair weather conditions.
In tropical regions this effect is largely masked by dry and rainy seasons, as the ITCZ moves towards a northern optimum around June and back towards a southern optimum somewhere around December each year. Some tropical regions, notably East Africa, are special – as the ITCZ travels such distances here that the area is affected during two passes each year there are also two rainy seasons (October-December and March-May), and two dry seasons (in between). During May the rainy season is most pronounced further to the north in the Horn of Africa – although the ITCZ usually travels even further north in June and July – around the Tropic of Cancer solstice.
In case of an El Niño situation there is warm water of the African coast, which fuels the ITCZ when it makes its passes – leading to a wetter rainy season. Over the East African mainland trade winds would be suppressed. During La Niñas the opposite situation takes place: there is more wind – and the air is drier.
Evidence that this climate correlation was always in modern geological times was derived from sediment analysis in Lake Challa, a crater lake in the foothills of Mount Kilimanjaro, Christian Wolff of the University of Potsdam and lead author of the Science publication explains:
“During La Niña, rainfall is sparse and the winds over Lake Challa are strong. The winds enhance upwelling of nutrients, intensifying the seasonal blooms of algae. After dying and sinking, they form thick layers of light-colored sediments.”
“During El Niño events, on the other hand, rainfall is frequent and the winds are weak, resulting in thinner white layers in the sediment.”
During his presentation Timmermann went on to look at possible trends within this local East African climate variation.
He thinks there probably was an ENSO correlation with the East African climate during the Pleistocene too, but perhaps on average the region was drier then than it is now.
“Compared with this coldest time, the last 3,000 years have been wetter, but more variable with severe century-long droughts sprinkled throughout.”
Probably, as the Earth’s climate grows warmer, the general circulation intensifies, which means the ITCZ will likely carry more rain everywhere, including over East Africa and the Horn of Africa. [The 'permanent El Niño' hypothesis, from which one would conclude climate change would lead to a decrease of dry episodes in East Africa, has been largely disproven by other recent research - including Pliocene ENSO reconstructions.]
© Rolf Schuttenhelm | www.bitsofscience.org