Moisture Transport from Tropical Oceans Drives Summertime Rainfall Variability over Southwest North America

Abstract Southwest North America (SWNA) is prone to hydroclimate extremes, with the west part (termed region 1) featuring a Mediterranean-type climate and the east part (termed region 2) featuring a monsoonal climate. Despite extensive research, a critical aspect of the SWNA hydroclimate—atmospheric moisture variability—is not well understood. Here, we examine the leading sources and physical processes related to SWNA summer atmospheric moisture variability with a combined Lagrangian–Eulerian approach. In a semi-Lagrangian moisture-tracking model, transport from tropical oceans is found to be a substantial source of summer atmospheric moisture variability over SWNA. The subtropical Pacific accounts for 74%/58% of atmospheric moisture variations in region 1 during wet/dry extremes of May–June. While both oceanic and land sources contribute to moisture fluctuations in region 1 during July–August and region 2 during May, the tropical Atlantic becomes the leading (83%/70% for wet/dry extremes) source for region 2 during June–August. The moisture transport variations are primarily attributed to wind anomalies, while oceanic moisture anomalies can be important. Consistent with the Lagrangian analysis, an Eulerian moisture budget analysis shows that moisture advection is critical for the growth of atmospheric moisture anomalies over SWNA. The dynamic component of moisture advection boosts atmospheric moisture over the coastal ocean (SWNA) via anomalous southerlies (easterlies), while the thermodynamic component transports moisture anomalies farther inland via climatological southwesterlies. The decline of atmospheric moisture over SWNA occurs with peak precipitation triggered by wind convergence. These results enhance our understanding of SWNA summer moisture variability, with implications for the associated rainfall predictability.