The Role of Seasonal Precipitation Sequences in Shaping the Climate of the United States Southwest

The climatology of the United States (US) Southwest (defined here as the states of Arizona and New Mexico) is characterised by a bi-modal pattern of precipitation seasonality across much of the region, influenced by a variety of factors including topography, geography and seasonal circulation features. The overall goal of this paper is to develop a better understanding of the spatial and temporal variability of Southwestern seasonal climate that results from these influences to help anticipate climate impacts on human and natural systems. We identify five subregions in which a summer monsoon season peak is common to all, but with differences in the relative importance of the cool season precipitation and nature of pre-monsoon precipitation. An investigation of sequences of seasonal precipitation across the subregions reveals intervals of dual (cool and monsoon) season droughts and wet periods, and those characterised by a tendency for a wet monsoon to follow a dry winter, and vice versa (called inverse conditions). This work expands on prior research by identifying multidecadal variability in seasonal precipitation sequences, supporting a prevalence of inverse precipitation conditions since about 1980, but importantly, also revealing the propensity for dual season drought and pluvial years particularly before the mid-1930s. Differences in seasonal precipitation sequences are highlighted in a comparison of two iconic droughts in the 1950s and 2010s. While the 1950s drought was characterised by dual season drought especially in the eastern subregions, the 2010s drought years were most often distinguished by dry cool seasons followed by dry springs, with warm conditions during all seasons and regions. Overall, results suggest the potential for dual season droughts in the future, along with the increasingly important role of temperature in Southwestern US droughts.