The 1941 Project: An Extreme Annual Precipitation Anomaly in the Preradiosonde Era

The wettest year in recorded history for the state of New Mexico was 1941, when a statewide annual average of 26.25 inches of precipitation fell compared to the average of approximately 14 inches. Annual indices of the Pacific Decadal Oscillation (PDO) and El Niño Southern Oscillation (ENSO) in 1941 were both extremely positive (+1.99 and +1.54 for the Niño 3.4 region, respectively). This is consistent with the historical tendency for above average precipitation across the southwestern United States in such conditions. With that said, these large precipitation anomalies occurred during transition season months (e.g. May and September) that do not fit the typical seasonality associated with strong ENSO- or PDO-related continental climate anomalies in the more recent historical record. The overarching goal of this project is to explore why the 1941 precipitation anomaly was nearly twice the seasonal average for the state of New Mexico. Given upper air measurements were not common in the United States until the late 1950s, I used a relatively new large-scale data set generated from a 20th century “reanalysis” from NOAA’s Earth Science Research Laboratory Physical Science Division. The reanalysis is based on historical surface data that were assimilated by a modern atmospheric general circulation model with a consistent analysis scheme that yields reconstructed three-dimensional weather information.

The only other year that comes close to the 1941 PDO anomaly occurred in 1987 with an annual average index of +1.82. That year was also coupled with an annual ENSO anomaly of +1.95 for the Niño 3.4 region. Could these combined and unique climate patterns be used to signal future high-impact events across New Mexico and improve forecaster situational awareness of potential heavy rainfall periods?