New open access book with contributions of the project CASISAC
Posted by Dr. Birgit Hünicke, Head of the department Climate Extremes and Impacts
We are happy to announce a new open access book publication with three contributing chapters by our “Climate Extremes and Impact Group”, summarizing the results of the recently completed BMBF Project CASISAC within the frame of the SPACES II program.
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CASISAC “Changes in the Agulhas System and its impacts on Southern African Coasts” was a joint project in the “SPACES Program – Science Partnerships for the Adaptation of Complex Processes in the Earth System in the Region Southern Africa” in the BMBF Framework Programme “Research for Sustainable Development” (FONA3), coordinated by GEOMAR with project partners from Hereon, the Universities Siegen and Kiel and collaborating partners in South Africa at the Universities of Cape Town and Pretoria, the Council of Scientific and Industrial Research (CSIR) and the South African Environmental Network (SAEON).
The CASISAC project investigated the interaction between the Agulhas Current and climate in the Southeast African region. The Agulhas Current is one of the most powerful ocean currents, with a direct influence on regional climate and an important role in global ocean circulation. CASISAC was particularly interested in the impacts of global warming on the coastal zones, such as changes in sea level and waves, storm surges, precipitation and river discharge. Different socio-economic developments and potential adaptation measures for the coastal region were assessed. The results outlined the strong impact of the surrounding ocean on the land climate in Southern Africa. Through sea surface temperatures and associated air–sea fluxes, the Agulhas Current system influences regional climate in southern Africa, leading to a heterogeneous pattern of rainfall over southern Africa and to a reduction of winter precipitation in most areas under global warming conditions.
Changes in the Agulhas Current system and the regional climate also cause changes in regional sea-level and wind induced waves that deviate from global trends. Combining these oceanic changes with extreme precipitation events, global warming can considerably amplify flood impacts along the coast of South Africa if no adaptation measures are implemented.
As a follow-up of the project, our ongoing study found that simulated precipitation extremes are projected to become more intense over the South African coast in the future (Tim et al., preprint). For example, for the KwaZulu-Natal Province, the heaviest rainfall event in the future is twice as strong as the strongest extreme simulated in the historical period and the recently observed disastrous extreme event in April 2022, where days of heavy rain led to deadly floods resulting in a national state of disaster declaration.
Following three chapters of the book contain our contributions:
Zorita, E., Hünicke, B., Tim, N., & Rouault, M. (2024): Past Climate Variability in the Last Millennium. In: von Maltitz, G.P., Midgley, G.F., Veitch, J., Brümmer, C., Rötter, R.P., Viehberg, F.A., & Veste, M. (eds.): Sustainability of Southern African Ecosystems under Global Change. Ecological Studies, vol 248, Springer, Cham., doi:10.1007/978-3-031-10948-5_5
Abstract:
We review our knowledge of the climate variability in southern Africa over the past millennium, based on information provided by proxy data and by climate simulations. Since proxy data almost exclusively record past temperature and/or precipitation, the review is focused on those two variables. Proxy data identify three thermal phases in the region: a medieval warm period around year 1000 CE (common era), a Little Ice Age until about the eighteenth century, and a clear warming phase since that temperature minimum until the present period. Variations of precipitation are different in the summer-rainfall and winter-rainfall regions. In the former, precipitation tends to accompany the temperature, with warm/humid and cold/dry phases. In the winter-rainfall zone, the variations are opposite to temperature. Thus, past precipitation variations display a see-saw pattern between the summer- and winter-rainfall zones. However, climate simulations do not display these three different hydroclimatic periods. Instead, the simulations show a clearly warm twentieth century and punctuated cooling due to volcanic eruptions, with otherwise little variations during the pre-industrial period. Also, the simulations do not indicate an anticorrelation between precipitation in the summer- and winter-rainfall zones. Possible reasons for these discrepancies are discussed.
Rouault, M., Dieppois, B., Tim, N., Hünicke, B., & Zorita, E. (2024): Southern Africa Climate Over the Recent Decades: Description, Variability and Trends. In: von Maltitz, G.P., Midgley, G.F., Veitch, J., Brümmer, C., Rötter, R.P., Viehberg, F.A., & Veste, M. (eds.): Sustainability of Southern African Ecosystems under Global Change. Ecological Studies, vol 248, Springer, Cham., doi:10.1007/978-3-031-10948-5_6
Abstract:
South of 15°S, southern Africa has a subtropical climate, which is affected by temperate and tropical weather systems and comes under the influence of the Southern Hemisphere high-pressure systems. Most rainfall occurs in austral summer, but the southwest experiences winter rainfall. Much of the precipitation in summer is of convective origin forced by large-scale dynamics. There is a marked diurnal cycle in rainfall in summer. The El Niño Southern Oscillation (ENSO) influences interannual rainfall variability. In austral summer, drought tends to occur during El Niño, while above-normal rainfall conditions tend to follow La Niña. During El Niño, higher than normal atmospheric pressure anomalies, detrimental to rainfall, occur due to changes in the global atmospheric circulation. This also weakens the moisture transport from the Indian Ocean to the continent. The opposite mechanisms happen during La Niña. On top of the variability related to ENSO, the Pacific Ocean also influences the decadal variability of rainfall. Additionally, the Angola Current, the Agulhas Current, the Mozambique Channel and the southwest Indian Ocean affect rainfall variability. Over the last 40 to 60 years, near-surface temperatures have increased over almost the whole region, summer precipitation has increased south of 10°S, and winter precipitation has mostly decreased in South Africa. Meanwhile, the Agulhas Current and the Angola Current have warmed, and the Benguela Current has cooled.
Biastoch, A., Rühs, S., Ivanciu, I., Schwarzkopf, F.U., Veitch, J., Reason, C., Zorita, E., Tim, N., Hünicke, B., Vafeidis, A.T., Santamaria-Aguilar, S., Kupfer, S., & Soltau, F. (2024): The Agulhas Current System as an Important Driver for Oceanic and Terrestrial Climate. In: von Maltitz, G.P., Midgley, G.F., Veitch, J., Brümmer, C., Rötter, R.P., Viehberg, F.A., & Veste, M. (eds.): Sustainability of Southern African Ecosystems under Global Change. Ecological Studies, vol 248, Springer, Cham., doi:10.1007/978-3-031-10948-5_8
Abstract:
The Agulhas Current system around South Africa combines the dynamics of strong ocean currents in the Indian Ocean with eddy–mean flow interactions. The system includes an associated interoceanic transport towards the Atlantic, Agulhas leakage, which varies on both interannual and decadal timescales. Agulhas leakage is subject to a general increase under increasing greenhouse gases, with higher leakage causing a warming and salinification of the upper ocean in the South Atlantic. The far-field consequences include the impact of the Agulhas Current on the Benguela Upwelling system, a major eastern boundary upwelling system that supports a lucrative fishing industry. Through sea surface temperatures and associated air–sea fluxes, the Agulhas Current system also influences regional climate in southern Africa, leading to a heterogeneous pattern of rainfall over southern Africa and to a reduction of precipitation in most areas under global warming conditions. Changes in the Agulhas Current system and the regional climate also cause changes in regional sea-level and wind-induced waves that deviate from global trends. Combining these oceanic changes with extreme precipitation events, global warming can considerably amplify flood impacts along the coast of South Africa if no adaptation measures are implemented.
