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Soil moisture–precipitation feedback on the North American monsoon system in the MM5‐OSU model

In this study, the Pennsylvania State University/National Center for Atmospheric Research fifth generation Mesoscale Model (MM5) linked to the Oregon State University (OSU) land‐surface scheme, is used to assess the strength of soil moisture–precipitation feedback in the region of influence of the North American monsoon (NAM). Two control simulations are made with external forcing taken from the National Centers for Environmental Prediction re‐analysis, and with a nested horizontal resolution of 30 km, for the period 1 June to 30 September in wetter than average (1999) and drier than average (2000) monsoon seasons. These two model runs are then repeated with a prescribed precipitation rate anomaly in July over the entire NAM region, and comparisons made between atmospheric and land‐surface states in the two control runs and the two runs with anomalous precipitation. The results show that size and importance of soil moisture–precipitation feedbacks in the NAM region have substantial interannual variability, and that the resulting behaviour has a strong dependency on the intensity of the prescribed precipitation anomaly. It is also shown that a marked precipitation anomaly in the NAM region results in modified soil moisture, rainfall, and surface temperature, which persist for about one month, and that a precipitation anomaly within the NAM region not only has an impact on soil moisture locally, but also causes a remote, downwind soil moisture anomaly one month later. Analysis of the modelled response to the soil moisture anomaly indicates that not only land–atmosphere interactions, but also the large‐scale atmospheric circulation act together to determine the modified precipitation and soil moisture fields in the NAM system. Copyright © 2004 Royal Meteorological Society