Nonlinear Seasonal and Long-Term Trends in a Twentieth-Century Meteorological Drought Index across the Continental United States.

Analyzing gradual trends in meteorological drought has become increasingly important as anthropogenic climate change and natural climate variability interact to complicate measurement of drought severity. Complex seasonality and long-term trends pose a limitation in understanding spatial trends in nonstationary changes of meteorological drought in the United States. This study seeks to address this issue by simultaneously analyzing recurring seasonal patterns (stationary component) and long-term drought trends (nonstationary component), with a unique focus on nonlinear trends and common regional patterns. We analyzed 696 instrumental precipitation gauges with long historical records in the continental United States, using a novel spline-based model to disaggregate a 3-month meteorological drought index (SPI) into its seasonal and long-term components. The disaggregated components for each gauge were then clustered into subregions with similar seasonality and groupings with similar long-term trends using a two-step process. Our results identify clearly defined regions based on precipitation seasonality, while long-term trends are not spatially coherent with the seasonality. Instead, these findings support prior findings of an increasingly drier western United States and an increasingly wetter eastern United States over the last century, but with more nuanced spatial and temporal patterns. The new clustering analysis based on nonstationary meteorological drought trends can contribute to informing and adapting current water management strategies to long-term drought trends. Significance Statement: This study considered 656 precipitation gauges across the continental United States to find regions with similar precipitation seasonality and then to group records with similar long-term climate trends. The study focused on 3-month average precipitation, a key indicator for drought monitoring. We identified eight regions across the United States with similar precipitation seasonality. From 1920 to the present, we found continuous drying trends throughout the western United States, continuously wetter trends in the northern plains, and an overall wetter trend interrupted by a midcentury dry period (1930–50) for much of the central Plains and Midwest. This study's use of splines, or fitted curves, allowed these nonlinear patterns, which we believe better capture the nuances and intensification of climate change effects on precipitation. [ABSTRACT FROM AUTHOR]