Influence of Soil Moisture on the Asian and African Monsoons. Part I: Mean Monsoon and Daily Precipitation

Abstract

Soil moisture responds to precipitation variability but also affects precipitation through evaporation. This two-way interaction has often been referred to as a positive feedback, since the water added to the land surface during a precipitation event leads to increased evaporation, and this in turn can lead to further rainfall. Various numerical experiments have suggested that this feedback has a major influence on tropical climate variability from the synoptic to the interannual timescale. In the present study, ensembles of seasonal simulations (March?September) have been performed in order to investigate the sensitivity of the Asian and African monsoon rainfall to regional soil moisture anomalies. After a control experiment with free-running soil moisture, other ensembles have been performed in which the soil water content is strongly constrained over a limited area, either south Asia or Sudan?Sahel. Besides idealized simulations in which soil moisture is limited by the value at the wilting point or at the field capacity, more realistic experiments are relaxed toward the Global Soil Wetness Project (GSWP) soil moisture climatology. The results show a different sensitivity of the Asian and African monsoons to the land surface hydrology. Whereas African rainfall increases with increasing soil moisture, such a clear and homogeneous response is not found over the Indian subcontinent. Precipitation does increase over northern India as a consequence of wetter surface conditions, but the increased evaporation is counterbalanced by a reduced moisture convergence when averaging the results over the whole Indian peninsula. This contrasted behavior is partly related to the more dynamical and chaotic nature of the Asian monsoon, for which moisture convergence is about 2 times that found over Sudan?Sahel so that water recycling has a weaker influence on seasonal rainfall. It is also due to a different response of the frequency distribution of daily precipitation, and particularly to an increased number of strong convective events with decreasing soil moisture over India. Part II of the study will investigate how soil moisture also affects the interannual variability of the Asian and African monsoons.