Appendices for sedimentary dynamics in the Albertine Rift Valley, Equatorial East Africa

The Albertine Rift of East Africa is a prime example of active early continental rifting in the world today. Geological field surveying of onshore modern-day sedimentary depositional environments here interprets older outcrop and constructs a chronostratigraphic framework based on global glacial–interglacial climatic cyclicity for the past 1.1 million years.

Convectively coupled equatorial Kelvin waves (CCEKWs) are those types of equatorially trapped disturbances that propagate eastward and are among the most common intra-seasonal oscillations in the tropics. There exists two-way feedback between the inter-tropical convergence zone (ITCZ) and these equatorially trapped disturbances. Outgoing Longwave Radiation (OLR) was utilized as a proxy for deep convection. For CCEKWs, the modes are located over the West Atlantic, equatorial West Africa, and the Indian Ocean. The influence of other circulations and climate dynamics is studied for finding other drivers of climate within East Africa. The results show a positive relationship between Indian and Atlantic Oceans Sea Surface Temperatures and March-May rainfall over equatorial East Africa over the period of 1980 to 2010. This influence is driven by the Walker circulation and anomalous moisture influx enhanced by winds. Composite analysis reveals strong lower-tropospheric westerlies during the active phase of the CCKWs activities over Equatorial East Africa. The winds are in the opposite direction with the upper-tropospheric winds, which are easterlies. Singular Value Decomposition shows a strong coupling interaction between rainfall over equatorial East Africa and CCKWs. This study concludes that Kelvin waves are not the main factors that influence rainfall during the rainy season. Previous studies show that the main influencing factors are ITCZ, El-Nino Southern Oscillation (ENSO), and tropical anticyclones that borders the African continent. However, CCKWs are a significant factor during the dry seasons.

In this study, the performance of dynamical seasonal forecast systems is evaluated for the prediction of short rain anomalies over equatorial East Africa. The evaluation is based on observational datasets and the Asia-Pacific Climate Center (APCC) Ocean–Atmosphere coupled multi-model ensemble (MME) retrospective forecasts (hindcasts). These forecast systems have different hindcast periods; here, we have selected common years from 1982 to 2005. The ensembles of individual models and their MME mean are evaluated. Hindcasts initialized on the 1st of August from each year alone are considered, as these are the most relevant to short rain predictions. The coupled climate model ensemble reproduces the spatial distribution of mean September-October-November (SON) rainfall and seasonal climate variations over equatorial East Africa with further improvement in MME mean. Individual coupled models and MME mean also show statistically significant skill in forecasting sea surface temperatures anomalies (SSTAs) over the western and eastern parts of the equatorial Indian Ocean, giving significant correlation at 99 % confidence level for Indian Ocean dipole (IOD). Moreover, five out of ten coupled models and MME mean show statistically significant skill in predicting equatorial East Africa short rains. The fidelity of hindcasts is further measured by anomaly correlation coefficient (ACC) and four models as well as MME mean show significant skill over East Africa. It is shown that the reproduction of the observed variability in the East African region is mainly due to a realistic relationship of East African rainfall with the Indian Ocean dipole. Overall, the skill of the dynamical models is attributed to the fact that slowly evolving SSTs are the primary source of predictability and to the fact that coupled climate models produce skillful predictions of SON SST anomalies over the tropical Indian Ocean. This information opens the possibility of using readily available seasonal forecasts as skillful predictions of equatorial East Africa short rains.

Knowledge of natural long-term rainfall variability is essential for water-resource and land-use management in sub-humid regions of the world. In tropical Africa, data relevant to determining this variability are scarce because of the lack of long instrumental climate records and the limited potential of standard high-resolution proxy records such as tree rings and ice cores. Here we present a decade-scale reconstruction of rainfall and drought in equatorial east Africa over the past 1,100 years, based on lake-level and salinity fluctuations of Lake Naivasha (Kenya) inferred from three different palaeolimnological proxies: sediment stratigraphy and the species compositions of fossil diatom and midge assemblages. Our data indicate that, over the past millennium, equatorial east Africa has alternated between contrasting climate conditions, with significantly drier climate than today during the 'Medieval Warm Period' (approximately AD 1000-1270) and a relatively wet climate during the 'Little Ice Age' (approximately AD 1270-1850) which was interrupted by three prolonged dry episodes. We also find strong chronological links between the reconstructed history of natural long-term rainfall variation and the pre-colonial cultural history of east Africa, highlighting the importance of a detailed knowledge of natural long-term rainfall fluctuations for sustainable socio-economic development.

Palaeolimnological evidence for widespread late 18th century drought across equatorial East Africa

To better understand the potential role of environmental change in mediating human dispersals across equatorial East Africa, this study examines the biogeographic histories of ungulates, including a summary of current knowledge and fossil evidence stemming from our fieldwork in the Kenyan portion of the Lake Victoria basin. Phylogeographic and paleontological evidence indicates that vegetation changes across Quaternary climate cycles mediated ungulate distributions and dispersals via the opening and closing of biogeographic barriers in equatorial East Africa. Dispersal capabilities would have been enhanced during phases of grassland expansion and diminished during phases of grassland contraction. We propose that the distribution and dispersal of diagnostic technological markers in the archaeological record may be similarly influenced by environmental changes. The Middle Stone Age record from the Lake Victoria region provides intriguing examples of possible environmentally mediated technological dispersals.

AimWithin the last several decades, Grévy's zebra (Equus grevyi) has undergone a massive reduction in geographical range and population size, largely as the result of human impacts. To place its recent decline in a deeper prehistoric context, and to understand the factors mediating its range and abundance over geological time frames, this study examines the fossil history of Grévy's zebra in equatorial East Africa.LocationEquatorial East Africa.MethodsPresence/absence data for ungulates recovered from fossil sites spanning the last c. 400,000 years in Kenya and Ethiopia were compiled from the literature and from previously unreported palaeontological sites. Associations between Grévy's zebra and other taxa were examined using principal coordinates analysis and non‐random species pairs were identified using a Bayesian approach. Changes in rainfall were reconstructed using the average hypsodonty index of ungulate species from fossil assemblages.ResultsGrévy's zebra was common during dry phases of the Pleistocene and was found to the south and west of its historical range, coinciding with an expansion of arid grasslands. At the onset of the Holocene, Grévy's zebra was extirpated from southern Kenya and almost completely disappeared from the fossil record. Grévy's zebra was associated with several specialized grazers that became extinct by the end of the Pleistocene. These extinctions and the decline of Grévy's zebra from the Pleistocene to the Holocene are explained by increased precipitation and the consequent loss of arid grasslands at the Pleistocene–Holocene transition. Grévy's zebra is never associated with domestic livestock, unlike the widespread plains zebra.Main conclusionsGrévy's zebra thrived in equatorial East Africa during periods of the Pleistocene when environmental conditions favoured an expansion of arid grasslands. Environmental change across the Pleistocene–Holocene transition contributed to decreases in the range size and abundance of Grévy's zebra, setting the stage for the anthropogenic decline observed in recent decades. The spread of pastoralists in the middle Holocene may have additionally contributed to its prehistoric decline. Contemporary climate change warrants further consideration in planning for the long‐term survival of Grévy's zebra.

The seismic-stratigraphic record of lake-level fluctuations in Lake Challa: Hydrological stability and change in equatorial East Africa over the last 140 kyr

Centennial drought characteristics in Equatorial East Africa (EEA) is investigated using the Standardized Precipitation Evapotranspiration Index (SPEI) derived from the Climatic Research Unit (CRU) dataset. The spatial pattern of drought distribution, as well as drought duration, intensity and frequency, are analyzed for SPEI at a 3-month timescale for March-April-May (MAM) season. Rotated Empirical Orthogonal Function (REOF), Mann-Kendall method (MK), and wavelet analyses are used to study drought’s spatial pattern, trend, and periodicity. The result of the principal component analysis returned six homogenous drought sub-regions. A low drought frequency characterizes EEA (<20%). The drought in the MAM season lasts between 2.2 and 2.8 months. Overall, the result showed a weak long-term drying trend for most parts of EEA that were significant in some sub-regions and insignificant in others. An increase in drought areal extent after the 1980s could be ascribed to the increase in potential evapotranspiration (PET) and is consistent with the negative trend in SPEI value over the six sub-regions. The apparent increase is mainly attributed to the increase in moderate and severe droughts area rather than extreme drought areas. The spectral analysis further reveals that inter-annual drought variability with periodicities less than 8 years dominates in all sub-regions of Equatorial East Africa, which is associated with the critical role of El-Niño in driving the drought variations in EEA.

Reconstructing East African monsoon variability from grain-size distributions: End-member modeling and source attribution of diatom-rich sediments from Lake Chala

Ensemble simulations with a regional climate model on a large domain and 30-km resolution are used to understand why projected precipitation changes under greenhouse gas-forcing are asymmetric across seasons in equatorial East Africa, with rainfall increasing during the short rains (October through December) but not during the long rains (March through May). The model captures an accurate simulation of observed East African precipitation improving over coupled GCM simulations. Future simulations are generated by increasing atmospheric CO2 according to the RCP8.5 scenario and adding anomalies to observed SSTs as well as initial and lateral boundary conditions derived from coupled GCM simulations. In November, simulated rainfall rates increase by approximately one-third over much of equatorial East Africa by the mid-twenty-first century, and double by the end of the century. The long rains are not significantly increased. The difference in the seasonal response is attributed to differences in the background state. The East African short rains are greatest in November, more than 1 month after the autumnal equinox, when the climatological basic state is in a solstitial pattern. The well-defined heat low over southern Africa and the South Indian Ocean subtropical high to its east are intensified, leading to enhanced moisture convergence over equatorial East Africa. In contrast, the long rains are near their maximum on the vernal equinox, with a continental thermal low centered near the equator that is insensitive to twenty-first century greenhouse gas-induced changes in the subtropical atmospheric hydrodynamics.

Lowland forest collapse and early human impacts at the end of the African Humid Period at Lake Edward, equatorial East Africa

Understanding precipitation variability and extremes in Equatorial East Africa is vital for ensuring water and food security and mitigating the socioeconomic consequences of extreme events. Previous research has shown that sub-seasonal precipitation variability in this region is closely related to the wind direction, with precipitation more probable on days where the wind blows anomalously from the west, advecting moisture from the Congo basin. However, the exact nature of the westerly circulation and the conditions under which it forms are not fully understood. Here, we present a multi-decadal analysis of East African westerly winds. We use methods developed from studies of atmospheric rivers to objectively identify “westerly moisture transport events” (WMTEs), facilitating new insights into the seasonal distribution and importance of these westerlies, the regions within Eastern Africa where they occur, and the role of both the Madden-Julian Oscillation and tropical cyclones in their development. Finally, we also investigate the role of WMTEs as drivers of regional sub-seasonal precipitation variability.

Fires burning the vast grasslands and savannas of Africa significantly influence the global carbon cycle. Projecting the impacts of future climate change on fire-mediated biogeochemical processes in these dry tropical ecosystems requires understanding of how various climate factors influence regional fire regimes. To examine climate-vegetation-fire linkages in dry savanna, we conducted macroscopic and microscopic charcoal analysis on the sediments of the past 25000years from Lake Challa, a deep crater lake in equatorial East Africa. The charcoal-inferred shifts in local and regional fire regimes were compared with previously published reconstructions of temperature, rainfall, seasonal drought severity, and vegetation dynamics to evaluate millennial-scale drivers of fire occurrence. Our charcoal data indicate that fire in the dry lowland savanna of southeastern Kenya was not fuel-limited during the Last Glacial Maximum (LGM) and Late Glacial, in contrast to many other regions throughout the world. Fire activity remained high at Lake Challa probably because the relatively high mean-annual temperature (~22°C) allowed productive C4 grasses with high water-use efficiency to dominate the landscape. From the LGM through the middle Holocene, the relative importance of savanna burning in the region varied primarily in response to changes in rainfall and dry-season length, which were controlled by orbital insolation forcing of tropical monsoon dynamics. The fuel limitation that characterizes the region's fire regime today appears to have begun around 5000-6000years ago, when warmer interglacial conditions coincided with prolonged seasonal drought. Thus, insolation-driven variation in the amount and seasonality of rainfall during the past 25000 years altered the immediate controls on fire occurrence in the grass-dominated savannas of eastern equatorial Africa. These results show that climatic impacts on dry-savanna burning are heterogeneous through time, with important implications for efforts to anticipate future shifts in fire-mediated ecosystem processes.

A high-resolution geochemical record from Lake Edward, Uganda Congo and the timing and causes of tropical African drought during the late Holocene