Faculty Mentor: Thomas Reichler (Atmospheric Sciences, University of Utah)

While precipitation anomalies in the Southwestern United States are moderately well predicted at seasonal timescales (Pan B. et al., 2019), the specific roles and interactions of key climate drivers for this region remain less well known (Portmann et al., 2009). In this study, five climate drivers were evaluated for their impact on the study region of California, Nevada, Arizona, Utah, and the Western half of Colorado/New Mexico. We evaluated these climate drivers against precipitation anomalies using a lead/lag correlational relationship with monthly climate driver data and Winter Precipitation data as a summed December, January, February, and March (DJFM) variable. All data came from ERA5 (Hersbach et al, 2020), the fifth generation ECMWF atmospheric reanalysis of the global climate covering the period from January 1957 to December 2024. Two of these climate drivers were identified as significant predictors of precipitation anomalies in the study region, specifically the El Niño– Southern Oscillation (ENSO) and the stratospheric Quasi-Biennial Oscillation (QBO). ENSO, commonly referred to as “El Niño,” has long been recognized as a major driver of precipitation anomalies over the region. It is a well-documented phenomenon in which changes in ocean temperatures along the Western Hemisphere tropics lead to widespread global shifts in precipitation patterns. Specifically, in the Southwestern U.S., it explains roughly 15% of all precipitation variability in the region. The stratospheric QBO measures wind speeds at 30 hPa above the tropics. It has a relatively consistent 28-month cycle, during which it shifts from westerly to easterly wind patterns. The QBO, previously not explored as a predictor of Southwestern U.S. precipitation, was also found to have significant influence on the precipitation variability over the region. Our analysis indicates that the QBO explains approximately 10% of the total variance in precipitation across the study region. Together, ENSO and the QBO account for more than 24% of the total variance in DJFM precipitation over the Southwestern U.S.. These findings not only enhance our understanding of the climate dynamics in the Southwestern U.S. but also provide valuable insights for improving future climate prediction models and preparing for potential climate-related challenges in the region.

Bibliography

Hersbach et al. (2020): The ERA5 global reanalysis, Quart. J. Roy. Met. Soc., 146(73).

Pan, B., Hsu, K., AghaKouchak, A., Sorooshian, S., & Higgins, W. (2019). Precipitation Prediction Skill for the West Coast United States. Journal of Climate, 32(1), 161–182.

Portmann, R. W., Solomon, S., & Hegerl, G. C. (2009). Spatial and seasonal patterns in climate change, temperatures, and precipitation across the United States. Proceedings of the National Academy of Sciences, 106(18), 7324–7329.