Until now, it looked like rainfall in tropical southeast Africa depended on the position of the Inter-Tropical Convergence Zone, a region of cloud that migrates northwards and southwards and makes up the zone of maximum rainfall in the tropics. But sediment records from Lake Tanganyika have shown that other factors, such as the temperature of the Indian Ocean and the strength of the Indian monsoon, appear to be more significant.

"The climate dynamics of the tropics are poorly understood and sometimes difficult for climate modelers to simulate correctly," Jess Tierney of Brown University, US, told environmentalresearchweb. "Yet the tropics are very important to global climate change, because this is where the Earth receives the most radiation from sun, and this is the source of most of the planet's water vapor, a potent greenhouse gas."

Tierney says that tropical Africa is particularly important climatically because it is one of the Earth's three zones of "deep convection", where the hydrologic cycle is particularly strong and water vapour is exported to the extratropics.

Tierney and colleagues from Brown University, the NIOZ Royal Netherlands Institute for Sea Research and the University of Arizona, US, took two 8 m long cores from the bottom of Lake Tanganyika. By using proxy data from the condition of fatty acid compounds in plant leaf waxes stored in the sediments, they were able to analyse climate over the last 60,000 years.

At 1400 m deep, Lake Tanganyika is the second-deepest lake in the world after Lake Baikal. It’s also pretty large – 50 × 750 km – and is sensitive to climate changes across a significant portion of southeast tropical Africa.

"Our record is the longest and highest-resolution record of rainfall and temperature in this region to date," said Tierney. "We found that, surprisingly, climate in southeast tropical Africa seems to be sensitive to northern hemisphere forcings, both external – orbital precessional forcing – and internal - changes in ice volume, and abrupt cold snaps such as the Younger Dryas that occurred during the last Ice Age."

And during the early Holocene – roughly 9,000 years ago – climate around Lake Tanganyika did not correlate with the position of the Inter-Tropical Convergence Zone. "The zone was situated farther north than it is today, causing for example, the 'greening' of the Sahara," said Tierney. "This should have caused the southern tropics to be drier. However, we observe that it is wet at Lake Tanganyika during this time, contrary to an Inter-Tropical Convergence Zone-based prediction."

Similarly, scientists believe that the ITCZ migrated south during the brief cold interval at the end of the last ice age called the Younger Dryas, which should have made the southern tropics wet. But the team found that southeast tropical Africa was dry at this time.

"So we conclude that other factors beyond the position of the ITCZ, such as sea-surface temperatures in the Indian Ocean and the strength of the Indian monsoon, are more important in terms of controlling precipitation amount in southeast tropical Africa," Tierney explained.

The study also revealed many abrupt shifts between wet and dry climate states during the past 60,000 years, some occurring within just a few hundred years. "This implies that rainfall in southeast tropical Africa has a high sensitivity to climatic forcings, and can transition from dry to wet and vice versa very rapidly," said Tierney. "This is an aspect of tropical African hydrology that we should keep in mind in looking forward towards future changes associated with anthropogenic warming."

Tierney believes that although we do not know whether tropical East Africa will become wetter or drier in response to global warming, "we might consider that the changes that will occur could happen quickly".

Now the team plans to use similar techniques to investigate past changes in climate at other sites in East Africa and in other regions of the tropics, over both long timescales – tens of thousands of years – and the last few millennia.

The researchers reported their work in Sciencexpress.