@phdthesis{Junginger2011,
  author    = {Junginger, Annett},
  title     = {East African climate variability on different time scales : the Suguta Valley in the African-Asian Monsoon Domain},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-56834},
  school      = {Universit{\"a}t Potsdam},
  year      = {2011},
  abstract  = {Motivation | Societal and economic needs of East Africa rely entirely on the availability of water, which is governed by the regular onset and retreat of the rainy seasons. Fluctuations in the amounts of rainfall has tremendous impact causing widespread famine, disease outbreaks and human migrations. Efforts towards high resolution forecasting of seasonal precipitation and hydrological systems are therefore needed, which requires high frequency short to long-term analyses of available climate data that I am going to present in this doctoral thesis by three different studies. 15,000 years - Suguta Valley | The main study of this thesis concentrated on the understanding of humidity changes within the last African Humid Period (AHP, 14.8-5.5 ka BP). The nature and causes of intensity variations of the West-African (WAM) and Indian Summer monsoons (ISM) during the AHP, especially their exact influence on regional climate relative to each other, is currently intensely debated. Here, I present a high-resolution multiproxy lake-level record spanning the AHP from the remote Suguta Valley in the northern Kenya Rift, located between the WAM and ISM domains. The presently desiccated valley was during the AHP filled by a 300 m deep and 2200 km2 large palaeo-lake due to an increase in precipitation of only 26\%. The record explains the synchronous onset of large lakes in the East African Rift System (EARS) with the longitudinal shift of the Congo Air Boundary (CAB) over the East African and Ethiopian Plateaus, as the direct consequence of an enhanced atmospheric pressure gradient between East-Africa and India due to a precessional-forced northern hemisphere insolation maximum. Pronounced, and abrupt lake level fluctuations during the generally wet AHP are explained by small-scale solar irradiation changes weakening this pressure gradient atmospheric moisture availability preventing the CAB from reaching the study area. Instead, the termination of the AHP occurred, in a non-linear manner due to a change towards an equatorial insolation maximum ca. 6.5 ka ago extending the AHP over Ethiopia and West-Africa. 200 years - Lake Naivasha | The second part of the thesis focused on the analysis of a 200 year-old sediment core from Lake Naivasha in the Central Kenya Rift, one of the very few present freshwater lakes in East Africa. The results revealed and confirmed, that the appliance of proxy records for palaeo-climate reconstruction for the last 100 years within a time of increasing industrialisation and therefore human impact to the proxy-record containing sites are broadly limited. Since the middle of the 20th century, intense anthropogenic activity around Lake Naivasha has led to cultural eutrophication, which has overprinted the influence of natural climate variation to the lake usually inferred from proxy records such as diatoms, transfer-functions, geochemical and sedimentological analysis as used in this study. The results clarify the need for proxy records from remote unsettled areas to contribute with pristine data sets to current debates about anthropologic induced global warming since the past 100 years. 14 years - East African Rift | In order to avoid human influenced data sets and validate spatial and temporal heterogeneities of proxy-records from East Africa, the third part of the thesis therefore concentrated on the most recent past 14 years (1996-2010) detecting climate variability by using remotely sensed rainfall data. The advancement in the spatial coverage and temporal resolutions of rainfall data allow a better understanding of influencing climate mechanisms and help to better interpret proxy-records from the EARS in order to reconstruct past climate conditions. The study focuses on the dynamics of intraseasonal rainfall distribution within catchments of eleven lake basins in the EARS that are often used for palaeo-climate studies. We discovered that rainfall in adjacent basins exhibits high complexities in the magnitudes of intraseasonal variability, biennial to triennial precipitation patterns and even are not necessarily correlated often showing opposite trends. The variability among the watersheds is driven by the complex interaction of topography, in particular the shape, length and elevation of the catchment and its relative location to the East African Rift System and predominant influence of the ITCZ or CAB, whose locations and intensities are dependent on the strength of low pressure cells over India, SST variations in the Atlantic, Pacific or Indian Ocean, QBO phases and the 11-year solar cycle. Among all seasons we observed, January-September is the season of highest and most complex rainfall variability, especially for the East African Plateau basins, most likely due to the irregular penetration and sensitivity of the CAB.},
  language  = {en}
}
@article{StoofLeichsenringJungingerOlakaetal.2011,
  author    = {Stoof-Leichsenring, Kathleen Rosemarie and Junginger, Annett and Olaka, Lydia A. and Tiedemann, Ralph and Trauth, Martin H.},
  title     = {Environmental variability in Lake Naivasha, Kenya, over the last two centuries},
  series = {Journal of paleolimnolog},
  volume    = {45},
  journal   = {Journal of paleolimnolog},
  number    = {3},
  publisher = {Springer},
  address   = {Dordrecht},
  issn      = {0921-2728},
  doi       = {10.1007/s10933-011-9502-4},
  pages     = {353 -- 367},
  year      = {2011},
  abstract  = {Lake Naivasha, Kenya, is one of a number of freshwater lakes in the East African Rift System. Since the beginning of the twentieth century, it has experienced greater anthropogenic influence as a result of increasingly intensive farming of coffee, tea, flowers, and other horticultural crops within its catchment. The water-level history of Lake Naivasha over the past 200 years was derived from a combination of instrumental records and sediment data. In this study, we analysed diatoms in a lake sediment core to infer past lacustrine conductivity and total phosphorus concentrations. We also measured total nitrogen and carbon concentrations in the sediments. Core chronology was established by (210)Pb dating and covered a similar to 186-year history of natural (climatic) and human-induced environmental changes. Three stratigraphic zones in the core were identified using diatom assemblages. There was a change from littoral/epiphytic diatoms such as Gomphonema gracile and Cymbella muelleri, which occurred during a prolonged dry period from ca. 1820 to 1896 AD, through a transition period, to the present planktonic Aulacoseira sp. that favors nutrient-rich waters. This marked change in the diatom assemblage was caused by climate change, and later a strong anthropogenic overprint on the lake system. Increases in sediment accumulation rates since 1928, from 0.01 to 0.08 g cm(-2) year(-1) correlate with an increase in diatom-inferred total phosphorus concentrations since the beginning of the twentieth century. The increase in phosphorus accumulation suggests increasing eutrophication of freshwater Lake Naivasha. This study identified two major periods in the lake's history: (1) the period from 1820 to 1950 AD, during which the lake was affected mainly by natural climate variations, and (2) the period since 1950, during which the effects of anthropogenic activity overprinted those of natural climate variation.},
  language  = {en}
}