The annual cycle of precipitation in the southwestern United States is characterized by a bimodal distribution. El Niño-Southern Oscillation (ENSO) episodes influence cool-season precipitation, while the North American Monsoon (NAM) brings warm-season rainfall to the region. Because the covariability of the two seasons may result in both wet and dry water-year anomalies, we evaluated the long-term contribution of cool and warm-season precipitation to water-year totals using newly collected and/or updated tree-ring chronologies. Precipitation reconstructions back to the late 17th century, from 1663 to 2015, were produced using a total of 183 stem increment cores collected from seven ponderosa pine (Pinus ponderosa) stands located along the southern edge of the Colorado Plateau in Arizona and New Mexico. Latewood chronologies were most correlated with July- August rainfall brought by the North American Monsoon, while earlywood chronologies were related to November-February precipitation, also matching the time-series variability of the Southern Oscillation Index. Total ring-width chronologies reflected water-year precipitation, and were used to account for precipitation falling outside of either the cool or the warm season. While seasonal variability of precipitation was the main driver of regional climatic extremes, the amount of precipitation falling outside the cool and warm seasons did not play a consistent role in determining frequency and severity of droughts and pluvials. This study highlights how intra-annual paleoclimatic reconstructions help with understanding the contribution of seasonal climate to wet and dry episodes influenced by large-scale climatic modes.
Stories by Nick Miley 