For 20 000 years, climate variability in East
Africa has been following a pattern that is evidently a remote effect of the
ENSO phenomenon (El Niño Southern Oscillation) known as El Niño/La Niña. During
the cold phase of La Niña, there is marginal rainfall and stronger winds in
East Africa, while the El Niño warm phase leads to weak wind conditions with
frequent rain.
Moreover, during the coldest period of the
last ice age about 18 000 to 21 000 years ago, East Africa’s climate was
relatively stable and dry. This result was published by an international group
of researchers from Potsdam, Switzerland, the United States, the Netherlands
and Belgium in the latest issue of the journal “Science” (Vol. 333, No.6043,
05.08.2011).
ENSO with its warm phase (El Niño) and its cold phase (La Niña) is actually
known as a climate phenomenon in the Pacific with global tele-connections. Using
the example of climate variability in East Africa, the study shows the long-term
impact of this phenomenon in this sensitive region. The sediments in Lake
Challa in southeastern Kenya, at the foot of Mount Kilimanjaro, serve as a
climate archive.
Sample cores that were drilled here show a
pattern of stripes, so called annual laminae. Each individual layer holds
information about the climate.
“A
comparison of temperature measurements in the tropical Pacific over the last
150 years shows a strong correlation between ENSO cycles and the rhythms of
droughts and floods in East Africa.”
That is to say, two climate patterns are superimposed here. The basic rhythm is
provided by the annual rainy seasons, which are linked to the so-called
Intertropical Convergence Zone (ITCZ). This refers to the band of clouds near
the equator, which is formed by sunlight and excessive evaporation. With the
sun high in June on the northern hemisphere and in December in the southern
hemisphere, this band of clouds and the rain associated with it moves north- or
southwards, respectively. This seasonal event is apparently superimposed by the
ENSO phenomenon.
The length of the drill core allowed the tracking of climate variations to the
last ice age. Bio-geochemical proxy data and careful seismological exploration
of the sediment layers brought together a very good agreement with the
thicknesses of individual layers. It was found that even in the last glacial,
when the world was about 5 degrees colder globally and thus the energy budget
of the tropics system was smaller due to lower temperature, the ENSO phenomenon
was seen as a superimposed pattern, albeit weaker.
In comparison, the last 3000 years were
warmer and more volatile, with intensive dry and drought periods, as this year,
and massive rainy periods, which frequently lead to flooding. Climate models
show that this trend towards an increase in extreme dry and wet phases
continues on in a warming world.
The study was funded by the DFG German Research Foundation, the ESF European
Science Foundation and the U.S. National Science Foundation NSF.
Christian Wolff et
al.: „Reduced Inter-annual Rainfall Variability in East Africa During the Last
Ice Age”, Science, Vol. 333, No.6043, 05.08.2011
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