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Atmosphere-land surface interactions and their influence on extreme rainfall and potential abrupt climate change over southern Africa
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