Influence of soil moisture on the Asian and African monsoons. Part I: Meanmonsoon and daily precipitation

Soil moisture responds to precipitation variability but also affects precip itation through evaporation. This two-way interaction has often been referr ed to as a positive feedback, since the water added to the land surface dur ing 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 fr om the synoptic to the interannual timescale. In the present study, ensembl es of seasonal simulations (March-September) have been performed in order t o investigate the sensitivity of the Asian and African monsoon rainfall to regional soil moisture anomalies. After a control experiment with free-runn ing soil moisture, other ensembles have been performed in which the soil wa ter content is strongly constrained over a limited area, either south Asia or Sudan-Sahel. Besides idealized simulations in which soil moisture is lim ited by the value at the wilting point or at the field capacity, more reali stic 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 r ainfall increases with increasing soil moisture, such a clear and homogeneo us response is not found over the Indian subcontinent. Precipitation does i ncrease over northern India as a consequence of wetter surface conditions, but the increased evaporation is counterbalanced by a reduced moisture conv ergence when averaging the results over the whole Indian peninsula. This co ntrasted behavior is partly related to the more dynamical and chaotic natur e of the Asian monsoon, for which moisture convergence is about 2 times tha t 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 numbe r of strong convective events with decreasing soil moisture over India. Par t II of the study will investigate how soil moisture also affects the inter annual variability of the Asian and African monsoons.