8.2.3.2 Soil Moisture Feedbacks in Climate Models

A key role of the land surface is to store soil moisture and control its evaporation. An important process, the soil moisture-precipitation feedback, has been explored extensively since the TAR, building on regionally specific studies that demonstrated links between soil moisture and rainfall. Recent studies (e.g., Gutowski et al., 2004; Pan et al., 2004) suggest that summer precipitation strongly depends on surface processes, notably in the simulation of regional extremes. Douville (2001) showed that soil moisture anomalies affect the African monsoon while Schär et al. (2004) suggested that an active soil moisture-precipitation feedback was linked to the anomalously hot European summer in 2003.

The soil moisture-precipitation feedback in climate models had not been systematically assessed at the time of the TAR. It is associated with the strength of coupling between the land and atmosphere, which is not directly measurable at the large scale in nature and has only recently been quantified in models (Dirmeyer, 2001). Koster et al. (2004) provided an assessment of where the soil moisture-precipitation feedback is regionally important during the Northern Hemisphere (NH) summer by quantifying the coupling strength in 12 atmospheric GCMs. Some similarity was seen among the model responses, enough to produce a multi-model average estimate of where the global precipitation pattern during the NH summer was most strongly affected by soil moisture variations. These ‘hot spots’ of strong coupling are found in transition regions between humid and dry areas. The models, however, also show strong disagreement in the strength of land-atmosphere coupling. A few studies have explored the differences in coupling strength. Seneviratne et al. (2002) highlighted the importance of differing water-holding capacities among the models while Lawrence and Slingo (2005) explored the role of soil moisture variability and suggested that frequent soil moisture saturation and low soil moisture variability could partially explain the weak coupling strength in the HadAM3 model (note that ‘weak’ does not imply ‘wrong’ since the real strength of the coupling is unknown).

Overall, the uncertainty in surface-atmosphere coupling has implications for the reliability of the simulated soil moisture-atmosphere feedback. It tempers our interpretation of the response of the hydrologic cycle to simulated climate change in ‘hot spot’ regions. Note that no assessment has been attempted for seasons other than NH summer.

Since the TAR, there have been few assessments of the capacity of climate models to simulate observed soil moisture. Despite the tremendous effort to collect and homogenise soil moisture measurements at global scales (Robock et al., 2000), discrepancies between large-scale estimates of observed soil moisture remain. The challenge of modelling soil moisture, which naturally varies at small scales, linked to landscape characteristics, soil processes, groundwater recharge, vegetation type, etc., within climate models in a way that facilitates comparison with observed data is considerable. It is not clear how to compare climate-model simulated soil moisture with point-based or remotely sensed soil moisture. This makes assessing how well climate models simulate soil moisture, or the change in soil moisture, difficult.