Abstract. Mercury is a global pollutant due to its long lifetime in the atmosphere. Its hemispheric transport patterns and eventual deposition are therefore of major concern. For the purpose of global atmospheric mercury chemistry and transport modelling the ECHMERIT model was developed. ECHMERIT, based on the global circulation model ECHAM5 differs from most global mercury models in that the emissions, chemistry (including general tropospheric chemistry and mercury chemistry), transport and deposition are coupled on-line to the GCM. The chemistry mechanism includes an online calculation of photolysis rate constants using the Fast-J photolysis mechanism, the CBM-Z tropospheric gas-phase mechanism and aqueous-phase chemistry based on the MECCA mechanism. Additionally, a mercury chemistry mechanism that incorporates gas and aqueous phase mercury chemistry is included. A detailed description of the model, including the wet and dry deposition modules, and the implemented emissions is given in this technical report. First model testing and evaluation show a satisfactory model performance for surface ozone and mercury mixing ratios (with a mean bias of 1.46 nmol/mol for ozone and a mean bias of 13.55 fmol/mol for TGM when compared with EMEP station data). Requirements regarding measurement data and emission inventories which could considerably improve model skill are discussed.
[1] The Volta region is a climate-sensitive semiarid to subhumid region in West Africa. To investigate the impact of expected global climate change on regional water availability, regional climate modeling was performed. Two time slices (1991-2000 and 2030-2039) of the ECHAM4 scenario IS92a were dynamically downscaled with MM5 to a spatial resolution of 9 km. The quality of MM5 simulations in reproducing regional climate was assessed using reanalysis data for initial and boundary conditions. Although an underestimation of coastal rainfall was detected, sufficient accuracy in the Volta Basin could be achieved. The regional climate simulations show an annual mean temperature increase of 1.2-1.3°C in the Volta region. This temperature change significantly exceeds interannual variability. A mean annual change in precipitation from À20% to +50% ($ À150 to +200 mm) is simulated, with a spatial mean increase of 5% ($45 mm). In the rainy season, rainfall predominantly increases, whereas a strong decrease is found for April, which is connected to a delay in the onset of the rainy season. In addition, interannual variability in the Volta region increases in the early stage of the rainy season. The climate change signals in infiltration excess and evapotranspiration show a nonlinear response to precipitation change. Aridity, expressed by the de Martonne aridity index, does not change significantly. The change signal in precipitation predominantly lies within the range of interannual variability. In contrast, the decrease in April exceeds interannual variability in the Sahel region.Citation: Jung, G., and H. Kunstmann (2007), High-resolution regional climate modeling for the Volta region of West Africa,
The Benguela Current, located o the west coast of southern Africa, is tied to a highly productive upwelling system 1 . Over the past 12 million years, the current has cooled, and upwelling has intensified 2-4 . These changes have been variously linked to atmospheric and oceanic changes associated with the glaciation of Antarctica and global cooling 5 , the closure of the Central American Seaway 1,6 or the further restriction of the Indonesian Seaway 3 . The upwelling intensification also occurred during a period of substantial uplift of the African continent 7,8 . Here we use a coupled ocean-atmosphere general circulation model to test the e ect of African uplift on Benguela upwelling. In our simulations, uplift in the East African Rift system and in southern and southwestern Africa induces an intensification of coastal low-level winds, which leads to increased oceanic upwelling of cool subsurface waters. We compare the e ect of African uplift with the simulated impact of the Central American Seaway closure 9 , Indonesian Throughflow restriction 10 and Antarctic glaciation 11 , and find that African uplift has at least an equally strong influence as each of the three other factors. We therefore conclude that African uplift was an important factor in driving the cooling and strengthening of the Benguela Current and coastal upwelling during the late Miocene and Pliocene epochs.
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