ENSO and extra-tropical ocean variability drives summer drought extremes in the United States

Understanding large-scale drought patterns and the mechanisms producing extreme drought events is vital to understanding future drought risks. Here we investigate the influence of teleconnections originating in the Pacific and Atlantic Oceans on regional drought variability in the United States. For this work, the Standardized VPD Drought Index (SVDI) is calculated with Vapor Pressure Deficit (VPD), a method of drought detection based on air temperature and relative humidity, rather than precipitation deficit. This work focuses on summer months between 1980 – 2021, and SVDI was calculated with NASAs North American Land Data Assimilation System (NLDAS) data. Initially, the spatial drought characteristics were extracted from SVDI with EOF analysis. To identify extreme events, a k-means clustering technique was applied to primary principal components, multivariate ENSO index (MEI), and northern hemisphere sea surface temperature anomalies (SSTA). Additionally, to identify mechanisms driving drought variability, statistics from individual clusters (i.e. drought events) are retained for analyses using atmospheric variables (e.g. wind, HGT, and MSLP). Results show large-scale drought in the Central and Southern U.S. stem from mechanisms originating in the northern Pacific Ocean (e.g. PDO and SSTA trends) and northern Atlantic Ocean (e.g. NAO), modulating the variability in the onshore flow along the Gulf of Mexico. However, mechanisms influencing summer drought patterns in the southwestern U.S., especially the recent long-term drought patterns, originate in the equatorial Pacific Ocean and are driven by ENSO related processes.