Characterizing the North American Monsoon in the Community Atmosphere Model: Sensitivity to Resolution and Topography
The southwest United States and northwestern Mexico experience a seasonal monsoon (known as the North American monsoon, NAM) that dominates the annual cycle of precipitation and distinguishes the dry and wet seasons. The peak of the monsoon season is in later summer with the most intense precipitation occurring in northwestern Mexico and a weakening signal through the southwestern United States (i.e., Arizona, New Mexico, and Nevada). Current understanding of the NAM and how it might respond to climate change is limited by an apparent lack of skill in current climate models, part of which is thought to be due to inadequate horizontal resolution. The purpose of this work is to test one climate model’s ability to simulate the NAM at different horizontal resolutions.
This study evaluates the NAM in the Community Atmosphere Model (version 5) using the standard 100 km and a high-resolution 25 km configuration. The low-resolution model captures a summer wet season, but the transitions are too gradual and the NAM rainfall does not extend far enough northward into northern Mexico and the southwest USA. At 25 km, the model does a better job of matching the observed northward extent, but still shows biases in the total rainfall. A detailed analysis of the circulation shows that the high-resolution model has improved moisture transport through the region. Sensitivity tests support the hypothesis that the improvement is primarily due to the better-resolved topography of the region, especially the Baja California peninsula. Having Baja better represented allows low-level flow along the Gulf of California, better matching the observed circulation.
Climate models are typically run for many years requiring relatively low resolution (of around 100 km). This coarse resolution can negatively impact regional features and phenomena. One example is the North American Monsoon, which is a societally and economically crucial for Mexico and the southwest USA. By improving the resolution, the representation of seasonal rainfall is improved. A consequence is that high-resolution is likely necessary to understand how the North American Monsoon will respond to a changing climate.