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

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 Climatic Change (2012) 112:981-996 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.

A Diagnostic Evaluation of Precipitation in CORDEX Models over Southern Africa

“…Previous studies demonstrate that RCMs, while showing some improvements over the driving GCMs in simulating precipitation over Southern Africa, also show some persistent biases (Arnell et al 2003;Haensler et al 2011) such as having unrealistic rain day frequency and rainfall intensity (Tadross et al 2006). RCM performance is found to be dependent on the internal physics (e.g., hydrostatic versus nonhydrostatic; Hewitson et al 2004;Tadross et al 2006), dynamics, and atmosphere-land surface feedbacks associated with soil moisture and vegetation cover (Tadross et al 2005;Williams and Kniveton 2012). Dependence of the simulated climate change on the driving model, time slice, season, and location has been apparent.…”

Evolution and current state of our understanding of the role played in the climate system by land surface processes in semi-arid regions

“…Williams and Kniveton (2012) surmise that land-atmosphere coupling will also play a role in future climate development over Southern Africa.…”

Vegetation dynamics and responses to climate anomalies in East Africa

“…25 For instance, recent studies have reported a strong land-atmosphere coupling in West Africa, whereby vegetation dynamics play a significant role in regulating the West African monsoon and therefore rainfall distribution (Hales et al, 2006;Xue et al, 2012;Zheng and Eltahir, 1998). In South Africa, Williams and Kniveton (2012) reported increases and decreases in annual rainfall, based on idealized scenarios of expanding savanna and desert cover, respectively. Recent studies on the climatic impacts of tropical deforestation have consistently shown increased warming and reduced evapotranspiration and precipitation 30…”