A mechanistic understanding of the North American Monsoon (NAM) is suggested by incorporating local-and synoptic-scale processes. The local-scale mechanism describes the effect sea surface temperature (SST) in Gulf of California (GC) and how it contributes to the low-level moisture during the 2004 NAM. Before NAM onset, the strong low-level temperature inversion exists over the GC, but this inversion weakens with increasing GC SST and generally disappears once SSTs exceed 29.5 C, allowing the moist air, trapped in the MBL, to mix with free tropospheric air. This leads to a deep, moist layer that can be transported toward the NAM regions to produce thunderstorms. The synoptic scale mechanism is based on climatologies from 1983 to 2010 and explains that the warmest SSTs moving up the coast contributes to NAM convection and atmospheric heating, and consequently advancing the position of the …