Abstract
In a changing climate, changes in rainfall variability and, in particular, extreme rainfall events are likely to be highly significant for environmentally vulnerable regions such as southern Africa. It is generally accepted that sea-surface temperatures play an important role in modulating rainfall variability, thus the majority work to date has focused on these mechanisms. However past research suggests that land surface processes are also critical for rainfall variability. In particular, work has suggested that the atmosphere-land surface feedback has been important for past abrupt climate changes, such as those which occurred over the Sahara during the mid-Holocene or, more recently, the prolonged Sahelian drought. Therefore the primary aim of this work is to undertake idealised experiments using both a regional and global climate model, to test the sensitivity of rainfall variability to land surface changes over a location where such abrupt climate changes are projected to occur in the future, namely southern Africa. In one experiment, the desert conditions currently observed over southwestern Africa were extended to cover the entire subcontinent. This is based on past research which suggests a remobilisation of sand dune activity and spatial extent under various scenarios of future anthropogenic global warming. In the second experiment, savanna conditions were imposed over all of southern Africa, representing an increase in vegetation for most areas except the equatorial regions. The results suggest that a decrease in rainfall occurs in the desert run, up to 27% of total rainfall in the regional model (relative to the control), due to a reduction in available moisture, less evaporation, less vertical uplift and therefore higher near surface pressure. This result is consistent across both the regional and global model experiments. Conversely an increase in rainfall occurs in the savanna run, because of an increase in available moisture giving an increase in latent heat and therefore surface temperature, increasing vertical uplift and lowering near surface pressure. These experiments, however, are only preliminary, and form the first stage of a wider study into how the atmosphere-land surface feedback influences rainfall extremes over southern Africa in the past (when surface i.e. vegetation conditions were very different) and in the future under various scenarios of future climate change. Future work will examine how other climate models simulate the atmosphere-land surface feedback, using more realistic vegetation types based on past and future surface conditions.
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Acknowledgements
This work was supported by a Royal Society Dorothy Hodgkin Fellowship for C. J. R. Williams. The authors wish to thank NCAS-Climate for support during this work, and in particular the NCAS-Computational Modelling Services (CMS) for support with setting up and running the model.
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Fig. S1
Mean daily 850mb geopotential height differences, DJF 1994–1997, using PRECIS: a) Des-Con. Heights in m. (JPEG 102 kb)
Fig. S1
Mean daily 850mb geopotential height differences, DJF 1994–1997, using PRECIS: b) Sav-Con. Heights in m. (JPEG 88 kb)
Fig. S2
Mean daily 850mb vector wind differences, DJF 1994–1997, using PRECIS: a) Des-Con. Winds in m sec-1. (JPEG 51 kb)
Fig. S2
Mean daily 850mb vector wind differences, DJF 1994–1997, using PRECIS: b) Sav-Con. Winds in m sec-1. (JPEG 49 kb)
Fig. S3
Mean vertically integrated (over all pressure levels) moisture flux differences, DJF 1994–1997, using PRECIS: a) Des-Con. Moisture flux in kg m-1 sec-1. (JPEG 50 kb)
Fig. S3
Mean vertically integrated (over all pressure levels) moisture flux differences, DJF 1994–1997, using PRECIS: b) Sav-Con. Moisture flux in kg m-1 sec-1. (JPEG 45 kb)
Fig. S4
Mean daily surface temperature differences, DJF 1994–1997, using PRECIS: a) Des-Con. Temperature in °C. (JPEG 109 kb)
Fig. S4
Mean daily surface temperature differences, DJF 1994–1997, using PRECIS: b) Sav-Con. Temperature in °C. (JPEG 104 kb)
Fig. S5
Mean daily surface evaporation rate differences, DJF 1994–1997, using PRECIS: a) Des-Con. Evaporation in mm day-1. (JPEG 100 kb)
Fig. S5
Mean daily surface evaporation rate differences, DJF 1994–1997, using PRECIS: b) Sav-Con. Evaporation in mm day-1. (JPEG 100 kb)
Fig. S6
Mean daily omega differences (averaged longitudinally), DJF 1994–1997, using PRECIS: a) Des-Con. Omega in m sec-1. (JPEG 68 kb)
Fig. S6
Mean daily omega differences (averaged longitudinally), DJF 1994–1997, using PRECIS: b) Sav-Con. Omega in m sec-1. (JPEG 61 kb)
Fig. S7
Mean daily 850mb geopotential height differences (Des-Con), DJF 1994–1997, using HadAM3. Heights in m. (JPEG 177 kb)
Fig. S8
Mean daily 850mb vector wind differences (Des-Con), DJF 1994–1997, using HadAM3. Winds in m sec-1. (JPEG 116 kb)
Fig. S9
Mean vertically integrated (over all pressure levels) moisture flux differences (Des-Con), DJF 1994–1997, using HadAM3. Moisture flux in kg m-1 sec-1. (JPEG 101 kb)
Fig. S10
Mean daily surface temperature differences (Des-Con), DJF 1994–1997, using HadAM3. Temperature in °C. (JPEG 173 kb)
Fig. S11
Mean daily surface evaporation rate differences (Des-Con), DJF 1994–1997, using HadAM3. Evaporation in mm day-1. (JPEG 117 kb)
Fig. S12
Mean daily omega differences (averaged longitudinally), Des-Con, DJF 1994–1997, using HadAM3. Omega in m sec-1. (JPEG 88 kb)
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Williams, C.J.R., Kniveton, D.R. Atmosphere-land surface interactions and their influence on extreme rainfall and potential abrupt climate change over southern Africa. Climatic Change 112, 981–996 (2012). https://doi.org/10.1007/s10584-011-0266-7
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DOI: https://doi.org/10.1007/s10584-011-0266-7