Tuesday, 20 September 2011

East African climate

One of my 10 things to talk about that I haven't touched on very much so far is the weather, which is a bit of a surprise as I'm British, and apparently talking about the weather is the top thing that identifies us Brits. Even more so, as studying the impacts of climate change on the savannah is what pays my salary at the moment. So, it's about time that was rectified a bit and our recent trips to northern Serengeti and Tarangire have given me the ideal inspiration to do so.
Southern Serengeti is dry whilst the north is already green: Mwiba August 2011

Why do I think that the weather (or more generally, the climate) is something worth talking about (apart from because I'm British, of course)? Well, firstly it makes a huge difference to the ecology of the system - we've already talked a lot about how water availability make a huge difference, and most (though not all) of that is due to differences in rainfall. So understanding the seasons will help make sense of what's happening throughout East Africa. But also because the climate in East Africa is completely different to the climate where most of your clients come from - visitors from the north are mostly used to four seasons of different length and severity depending on quite where they live. There's winter, from about Dec - Feb, when it's cold wet (or snowy), the days are short (seriously, before I moved out here we lived in Aberdeen and in the December the sun would rise at 8.45 and set at 3.30pm) and trees loose their leaves and wildlife finds it tough. Then there's spring, March - May, when the days start to lengthen, the temperature warms up, rainfall declines somewhat, the trees grow leaves and there's a flush of invertebrate life. Summer arrives June - August, with long days (Aberdeen sunrise at 4am, set after 10pm) and madly busy breeding season for wildlife, with lower rainfall (and very little in more southern areas). Then the days start shortening, the temperature falls and September - November is Autumn (or Fall if you're not in UK), the leaves start to fall, rains pick up and the migrants all head south.
Wildebeest enjoy the green in northern Serengeti, September 2011

Wildebeest endure the dry, Tarangire September 2011
Here, of course, things are very different - in nothern Tanzania and through Kenya, there are two rainy seasons and two dry seasons with very little variation in day length (none on the equator, of course). Southern Tanzania has a single rainy season, running from November to May (or there abouts). In general, our rain comes from a process called the Inter-Tropical Convergance Zone (ITCZ) which is global band of rain that sits more or less under the sun as the eath tilts on it's axis. Essentially, wherever the sea is directly below the sun, evaporation is highest and winds bring the clouds inland to fall as rain - but as the year passes the earth tilts and so different bits of sea are closest to the sun at different times - close to the equatior the sun passes directly overhead twive a year, bringing two rainy seasons, further away (but still within the tropics) the sun is directly overhead only once a year, bring a single rainy season. Hence when the north pole is tipped towards the sun the sahel region get's it's rainy season (June - August), and when the south pole is tipped towards the sun in December - Feb), Southern Africa get's it's rainy seaon. In between, there are two rainy seasons as we have here. (lots more details here on Wikipedia if you really want to understand it). But Lake Victoria confuses the issue greately by generating its own local climate, which means that even in August and September when the rest of the region is dry, northern and western Serengeti are green and wet. What's more, these parts of Tanzania are rather far from the coast where most of the rain comes from and they show the opposite patterns to the rest of the place - in years of good rains elsewhere the coastal winds are strong, bringing rain inland. But the same wind blows the Lake Victoria rainfall further west into Uganda, and this side of Lake Victoria is drier than normal. But when the winds fail and the rains don't come to the rest of the region, Serengeti enjoys all that Lake Victoria rainfall too.

Total rainfall (from here), but seasonality might be more important
There's more variation too, of course - mountains tend to catch rain and be wetter places, whilst the areas on the inland side of the mountains tend to be drier (we say they're in the 'rain shadow'). But in general for East African ecology, I think it's the length of the dry season, rather than the total amount of rainfall, that's most important. Lots of water all at once isn't particularly helpful, but a small amount of dry season rainfall can make a huge difference to the ecology of an area - think of the coastal forests growing in areas not that different to many savannah areas, but getting enough dry season rainfall to keep things green. Which, in turn, keeps the fires out, meaning forest wins over savannah and showing yet again how the big four processes interact with one another to shape the ecology of East Africa.

Now, not only is it good to be able to chat about these seasonal differences with clients, (especially at this time of year when they might well go from somewhere dry like Tarangire straight to somewhere in wet and green in northern Serenegti) but it's impact is massive. Why do you think the Wildebeest are up in Northern Serengeti and the Mara at this time of year? That dry season rainfall is critical for providing good grazing for such large numbers of animals. In fact, the Serengeti ecosystem is a great place to explore the impacts of climate on the savanna, because it has such different seasons across relatively small areas - dry season rainfall in the north is actually equivalent to total annual rainfall in the south, where the crater highlands catch most of the rain coming from the coast, but I think I'll leave a more detailed analysis of Serengeti's climates for another post, as this is long enough already.

Now might be a good time to talk weather! Grumeti GR, July 2009

Last thing though - it's worth thinking about how you might actually impart this sort of knowledge to clients in an interesting and relevant way. It might, of course, come up easily enough if their ask you directly (and of course, if their British, they're bound to...). But otherwise it might be something to chat about when you're watching a big thunderstorm brewing on the horizon, or when your are about to take clients from a dry place to a wet one (putting them on the plane to go from Tarangire to northern Serengeti at the moment would be a good time!) or vice versa. Or even, of course, as you watch the migration unfolding in Serengeti, just to explain some of the processes involved in why they're even bothering to make these dangerous trips!

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