Influence of Topography on Tropical African Vegetation Coverage
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Clim Dyn (2016) 46:2535–2549 DOI 10.1007/s00382-015-2716-9 Influence of topography on tropical African vegetation coverage Gerlinde Jung1 · Matthias Prange1,2 · Michael Schulz1,2 Received: 17 February 2015 / Accepted: 13 June 2015 / Published online: 5 July 2015 © Springer-Verlag Berlin Heidelberg 2015 Abstract Hominid evolution in the late Miocene has long ocean–atmosphere feedbacks and a large dependence of the been hypothesized to be linked to the retreat of the tropical simulated vegetation response on the land surface/vegeta- rainforest in Africa. One cause for the climatic and vegeta- tion model. tion change often considered was uplift of Africa, but also uplift of the Himalaya and the Tibetan Plateau was sug- Keywords Global climate modelling · African gested to have impacted rainfall distribution over Africa. vegetation · Topography · Paleoclimate · Late Miocene Recent proxy data suggest that in East Africa open grass- land habitats were available to the common ancestors of hominins and apes long before their divergence and do not 1 Introduction find evidence for a closed rainforest in the late Miocene. We used the coupled global general circulation model The development of upright walking hominids in late Mio- CCSM3 including an interactively coupled dynamic veg- cene East Africa has been attributed to an environmental etation module to investigate the impact of topography on change from a densely forested environment to a more African hydro-climate and vegetation. We performed sen- open grassland habitat (e.g. Cerling et al. 2011). Against sitivity experiments altering elevations of the Himalaya and that background, the co-occurrence of important steps in the Tibetan Plateau as well as of East and Southern Africa. hominid evolution with consecutive drying and major shifts The simulations confirm the dominant impact of African in vegetation coverage from C3 dominated ecosystems to topography for climate and vegetation development of the C4 dominated grasslands, especially in East Africa, was dis- African tropics. Only a weak influence of prescribed Asian cussed (Cerling et al. 1997). In parallel climatic conditions uplift on African climate could be detected. The model and vegetation coverage changed in the tropical region of simulations show that rainforest coverage of Central Africa Central Africa throughout the Neogene (Senut et al. 2009). is strongly determined by the presence of elevated Afri- Here we first review the climate regime in Africa and its can topography. In East Africa, despite wetter conditions related vegetation at present day, as well as the develop- with lowered African topography, the conditions were not ment in the past. We hypothesize that late Neogene tectonic favorable enough to maintain a closed rainforest. A dis- uplift had an important impact on the hydro-climatic con- cussion of the results with respect to other model studies ditions and consequently also on vegetation in the African indicates a minor importance of vegetation–atmosphere or rainbelt. To test this hypothesis we performed model sen- sitivity experiments, altering the topography of Africa, as well as the Himalaya and the Tibetan Plateau. This is, to our * Gerlinde Jung knowledge, the first study on the effect of mountain uplift [email protected] in both regions that uses a comprehensive atmosphere– 1 MARUM ‑ Center for Marine Environmental Sciences, ocean general circulation model (GCM) at fine spatial reso- University of Bremen, 28334 Bremen, Germany lution coupled to a dynamic vegetation model allowing for 2 Faculty of Geosciences, University of Bremen, the simulation of climate–vegetation feedbacks and atmos- 28334 Bremen, Germany phere–ocean interactions. The relatively high resolution 1 3 2536 G. Jung et al. helps to represent mountain elevations much more realisti- 2.2 Tropical African vegetation and climate cally than with coarsely resolved models. development By the mid-Miocene rainforests extended from the West 2 Climate, vegetation and mountain uplift to the East coast over tropical Africa (Coetzee 1993; Plana 2004). East African proxy records show a shift towards a C4 2.1 Present day vegetation and climate of Africa based diet in grazers from 8.5 Ma ago (Jacobs 2004), which has been widely interpreted as shift from forest coverage to At present day, humid climate favours dense rainforests grassland environments. However, recent studies on vegeta- over large areas of Central Africa, and in a smaller block tion development of the last 10 Ma indicate the absence of in West Africa, whereas East Africa is characterized by a closed forest belt in tropical Africa during that entire time Savannah type open grassland habitats (Plana 2004; Salz- span (Bonnefille 2010). The increase in C4 plants during mann et al. 2009) with only limited patches of rainforest that time period (Brown et al. 2011; Cerling 1992; Edwards (Couvreur et al. 2008). et al. 2010) might also be disconnected from an increase in The tropical rainbelt in Africa is characterized by a grassland, but rather indicating a replacement of a similar, strong precipitation regime over Central Africa, with two but C3 dominated ecosystem (Feakins et al. 2013). distinct rainy seasons (Pokam et al. 2012) and a region- On a global scale a drying and cooling trend from the ally diverse pattern of rainfall seasonality over East Africa mid to late Miocene was detected. This trend was sug- (Nicholson 1996). The seasonality in Northern Africa is gested to have been induced by a changing atmospheric strongly determined by the intra-annual movement of the CO2 concentration (Pound et al. 2012). Additional pro- Intertropical Convergence Zone (ITCZ) that separates the cesses that might have likely influenced African long-term moist monsoonal inflow from the Atlantic Ocean from dry climate evolution in the Neogene are, amongst others, the north-eastern trades (Nicholson 2000). At higher altitudes restriction of the Indonesian Seaway and the closing of the the precipitation regime is influenced by the mid-tropo- Tethys Ocean and shrinkage of the Paratethys. The shrink- spheric African Easterly Jet (AEJ) which plays a role in age of the Tethys started in the late Oligocene and was a moisture and energy supply for atmospheric disturbances rather continuous process that probably lead to a drying (Cook 1999; Wu et al. 2009b) and the upper level Tropi- of East, North and Central Africa (Zhang et al. 2014). The cal Easterly Jet. The Tropical Easterly Jet forms as a con- restriction of the Indonesian throughflow in the Pliocene sequence of the heat low over the Tibetan Plateau and also has been suggested to have led to a cooling of the central impacts the development of disturbances in the zonal flow Indian Ocean and due to the link between Indian Ocean of the AEJ (Hulme and Tosdevin 1989). In East Africa the sea surface temperature and East African rainfall also to a Somali or Findlater Jet which is the cross-equatorial flow at drying of Eastern Africa (Cane and Molnar 2001). Moun- the southern edge of the monsoon trough and channelled by tain uplift was also suggested to have played a role in Afri- the East African topography, influences moisture transport can climate evolution. Asian uplift was discussed to have to the Indian monsoon region (Findlate 1969; Slingo et al. strengthened the cross-equatorial flow of the Somali Jet 2005; Vizy and Cook 2003). In Southern Africa the posi- along the East African coast that transports moisture to the tion of the Congo Air Boundary separates air masses com- Indian subcontinent (Chakraborty et al. 2002; Fluteau et al. ing from the Indian Ocean and the westerly flow of Atlantic 1999). Also Neogene uplift of the African continent might origin (Nicholson 2000). have caused drying and vegetation changes especially over The distribution of rainfall in tropical Africa is Eastern Africa as suggested by various model experiments strongly altered by regional topography. The East African (Kaspar et al. 2010; Prömmel et al. 2013; Sepulchre et al. rift system with its plateaus and mountains acts in many 2006; Slingo et al. 2005; Sommerfeld et al. 2014). ways on the rainfall regime of the region: it supports Superimposed on that secular drying trend, climate convective heating, enhances convection through oro- variability on shorter time scales is also evident in proxy graphically forced lifting of air, and alters regional pre- records. Some proxy studies indicate variable climate and cipitation patterns by lee and luv effects due to blocking vegetation conditions both geographically and through time or channelling of the low-level flow (Levin et al. 2009). (Kingston 2007) with periodically occurring and intensify- Therefore, most of the moisture from the Indian Ocean is ing arid periods since the late Miocene (Maslin et al. 2014). rained out on the way, when it is lifted over the Southern The diversification of rainforest trees in East Africa and the and Eastern African Plateaus (Meehl 1992), while most origins of endemic species was also found to have occurred of the moist Congo air is blocked by the East African stepwise, during times of repeated isolation and reconnec- Rift and does not reach the coastal zone of East Africa tion between West/Central and East African rainforests (Nicholson 1996). prior to the Pleistocene (Couvreur et al. 2008). Moreover 1 3 Influence of topography on tropical African vegetation coverage 2537 arid and humid conditions varied largely through time and study demonstrates even the existence of an Andean-type space in Central Africa during the Neogene. In the Congo mountain range along the southern margin of the Asian basin, thick mid-Miocene sand deposits indicate semi-arid block before the collision of India with Asia during the conditions in today’s humid tropical region (Senut et al. early Cenozoic (Ding et al. 2014). Despite this controversy 2009). One late Miocene period that might have been char- there is strong evidence that most likely substantial uplift acterized by some variation also in African climate is the occurred in most parts of the Tibetan Plateau and the Hima- Messinian Salinity Crisis.