LAUSR

This study examines changes to daily weather patterns associated with the occurrence of the precipitation deficits that contribute to seasonal drought events, with durations from 3–8 months in the Northeast US, for the period 1980–2018. Our over-arching question is the extent to which seasonal droughts are due to a shift in frequency to drier patterns as compared to a suppression of precipitation during wetter patterns. Seasonal drought events are defined using the 3-month Standardized Precipitation Evapotranspiration Index (SPEI). The changes to daily weather are considered with respect to twenty characteristic weather patterns for the region, identified by applying Self-Organizing Map (SOM) analysis to daily reanalysis 500-hPa geopotential height and 900-hPa horizontal wind anomalies. This allows an analysis of the drought-related changes to the structure and frequency of the weather patterns that determine the production of precipitation. Twenty-two seasonal Northeast US drought events are identified. The precipitation deficits associated with these droughts is, on average, ∼70% due to suppression of precipitation in wet patterns and ∼30% due to a change in frequency from wetter to drier patterns. Nearly 80% of the precipitation deficits associated with seasonal drought are associated with changes to only eight of the twenty daily weather patterns, with the relative importance of those patterns changing seasonally. Key features of seasonal drought weather in the cool season are flattening of ridges or shifting in trough location eastward; and, in the warm season, weakening of troughs along with ridge-building, or westward shifts in trough location.