Comparative Frequency Analysis of NASPA and Seasonal CMIP6 Precipitation in North America

Abstract This study compares the frequency spectra of seasonal precipitation during the last millenium from climate model simulations, tree-ring-based reconstructions, and gauge-based gridded observations of the twentieth century. Climate model simulations are from phase 6 of the Coupled Model Intercomparison Project (CMIP6) past1000 experiment, while tree-ring reconstructions are derived from the North American Seasonal Precipitation Atlas (NASPA). NASPA and CMIP6 model output are analyzed to understand their unique frequency biases in high-, mid-, and low-frequency ranges for both paleo-climatic millennium and recent centennial time series across North America. This was accomplished by first extracting signals from periodic ranges of 2–6, 4–15, 10–30, 20–50, and 30–110 years and then analyzing the result in Fourier space. This study reveals that the NASPA shows better alignment with observations in the frequency domain than global climate models (GCMs) even for low-frequency components. Moreover, the spatial distributions of the spectral biases indicate that there are significant disagreements between NASPA and GCMs in the east during cool seasons and in the west of North America during warm seasons for both historical centennial and preindustrial millennial periods. This is likely caused by NASPA tree-ring sensitivity, as its distribution roughly mirrors NASPA skill metrics. Notably, the spatial patterns of spectral biases differ between the modern and preindustrial eras, suggesting a changing bias through time. This study provides a new frequency-based metric to evaluate climate models and reconstructions and provides a first comparison of the two for North America.