Five centuries of US West Coast drought: Occurrence, spatial distribution, and associated atmospheric circulation patterns

The U.S. West Coast drought commencing in 2012 developed in association with a large, persistent high-pressure ridge linked to internal atmospheric variability. This study places the occurrence, spatial patterns, and associated circulation features of West Coast drought into a paleoclimate context through a synoptic dendroclimatology approach linking atmospheric circulation to surface hydroclimate patterns. Spatial reconstructions of upper atmosphere pressure patterns and cool-season drought show that West Coast-wide drought, although relatively rare compared to north-south dipole drought, has occurred periodically since 1500 Common Era and is consistently associated with a strong ridge centered along the Pacific Northwest coast. Atmospheric blocking is also linked to north-dry dipole droughts, while south-dry and wider Western droughts indicate La Nina-type patterns. The transition latitude between the northern and southern sides of the western precipitation dipole, important for California hydroclimate patterns, has had frequent year-to-year fluctuations but remained centered on 40 degrees N over the past five centuries.