Synoptic Scale Conditions and Land-Atmosphere Interactions During the Western U.S. Drought of 2021 and 2022

Droughts are driven by synoptic scale features and modulated by land-atmosphere (L-A) interactions at the local scale and mesoscale. This study analyzes the 2021-2022 western U.S. drought by examining the synoptic scale drivers during the warm and cold seasons. The L-A interactions are also studied to determine how they alter the drought conditions in regions with varying vegetation and aridity.

The analysis of the synoptic scale conditions reveals the importance of ridge placement for this drought event. During the cold season, the 500 mb pattern over the West was dominated by a ridge just off the Pacific coast, acting to block low-pressure systems from entering the region. During the warm season, the most notable feature at 500 mb was a ridge over the Southwest, driving the North American Monsoon and alleviating drought conditions for the Southwest while keeping other regions in the West dry due to the limited extent of monsoon-induced precipitation.

The impact of the L-A interactions on the drought is yet to be uncovered, although a few patterns have emerged that may be important to the continued research. The patterns include latent heating being limited by soil moisture and energy availability, shifts from latent (sensible) heating to sensible (latent) heating in regions with less (more) soil moisture, and increases (decreases) in boundary layer height in regions with less (more) soil moisture. In addition, a clear signal of the North American Monsoon appears in surface variables, such as boundary layer height and CAPE, that may influence the L-A interactions in the monsoon region. The continued research uses WRF simulations to understand the mechanisms behind the L-A interactions and how they modulate the drought conditions.