The Impact of Soil Moisture in the Northwest United States on Precipitation during the North American Monsoon

Throughout the literature, there is a variety of studies that attempt to understand how soil moisture (SM) influences precipitation driven by the North American Monsoon. Many focus on SM in Northwestern Mexico, Arizona, and New Mexico, where monsoon precipitation is the most prominent. However, a few studies have analyzed the impact of SM in surrounding regions on monsoon precipitation and have found that some of these relationships are worth further investigation. A previous study (Hu and Feng 2007) hypothesized a relationship between SM in the northwest United States and monsoon precipitation in the southwestern United States. They suggested that dry soil in the Northwest could lead to more surface sensible heating and higher temperatures in the area relative to when wet soil is present. This could then increase the atmospheric thickness over the Northwest and amplify the monsoon ridge over the western U.S. The amplification of the monsoon ridge could then eventually lead to a strengthening of the monsoon and an increase in precipitation for the southwest U.S. Since the hypothesis from the previous study has not yet been tested, this study attempted to determine its plausibility.

The Weather Research and Forecasting (WRF) model was used to test a variety of SM scenarios. To confirm the consistency of any patterns that arose in the simulations, 2019, which was a dry monsoon season, along with 2021 and 2022, which had wet monsoon seasons, were tested. For each monsoon season, a simulation with the original initial conditions from the North American Mesoscale (NAM) Analysis was run as a control. Then a simulation was run with the initial SM in the northwest U.S. set to field capacity (FC) and another with the SM set to wilting point (WP) to test both wet (FC) and dry (WP) SM scenarios. For all simulations, the total precipitation that fell in the interior southwest U.S. was measured as a response to the varied SM in the Northwest.

Across all three monsoon seasons tested, there was a consistent pattern that arose where the FC simulation experienced more precipitation in the interior Southwest than in the WP simulation. Since there was a positive relationship between Northwest SM and interior Southwest precipitation, the results of the WRF experiments contradict the outcome predicted by the hypothesis from the previous study. The continuing work attempts to uncover the mechanism behind the positive relationship between northwest U.S. SM and Southwest monsoon precipitation.