Ocean‐Atmosphere Trajectories of Extended Drought in Southwestern North America.

Multiyear droughts are a common occurrence in southwestern North America (SWNA), but it is unclear what causes these persistent dry periods. The ocean‐atmosphere conditions coinciding with droughts have traditionally been studied using correlation and composite methods, which suggest that cool conditions in the tropical Pacific are associated with SWNA droughts and warm conditions are associated with wet periods in SWNA. Nevertheless, the extent to which multiyear droughts are truly consistent with this paradigm remains unknown. This is, in part, because the temporal trajectory of ocean‐atmosphere conditions during these dry periods have not been sufficiently characterized. Here we examine the continuum of ocean‐atmosphere trajectories before, during, and after multiyear droughts in SWNA using observation‐based data and an ensemble of climate model simulations from the Community Earth System Model. An examination of sea surface temperature patterns at the beginning, middle, and end of SWNA droughts shows that an El Niño event tends to precede SWNA droughts, a cool tropical Pacific occurs during droughts, and central Pacific El Niño events end droughts. However, moderate El Niño events can occur in the middle of persistent droughts, so a warm tropical Pacific does not always end these dry periods. These findings are important for drought predictability and emphasize the need to improve simulations of the magnitude, life cycle, and frequency of occurrence of El Niño events. Plain Language Summary: Droughts lasting multiple years in southwestern North America (SWNA) have severe consequences for both humans and natural systems. Unusually cold tropical Pacific conditions (e.g., La Niña events) typically coincide with dry years in SWNA, and it is often assumed that persistent cold conditions in the tropical Pacific coincide with persistent droughts. Here we test this idea using climate model simulations and observation‐based data. Although a cool tropical Pacific tends to initiate SWNA droughts and a warm tropical Pacific (e.g., El Niño events) tends to terminate droughts, we show that El Niño events can occur in the middle of SWNA droughts without ending these dry periods. We also examine patterns of sea surface temperatures before, during, and after SWNA droughts. We find that El Niño events tend to precede SWNA droughts and central Pacific El Niño events often terminate droughts in both observations and model data; these El Niño events are perhaps the most consistent feature of multiyear droughts in SWNA. These findings have important implications for public perception of droughts and their causes, in addition to our scientific understanding and modeling of El Niño and its impacts. Key Points: We examine climate patterns associated with the life cycle of droughts in southwestern North America (SWNA) in observations and model dataEastern Pacific ENSO events often precede droughts, cool Pacific anomalies maintain droughts, and central Pacific ENSO events end droughtsThe state of the North Pacific and the type of El Niño event may modulate the likelihood of an El Niño event ending a drought [ABSTRACT FROM AUTHOR]