Dry Versus Humid Heat Seasonality and Drivers in the North American Monsoon Region

Abstract The Southwest United States and northwestern Mexico are well‐known for their extreme dry heat, but these regions also experience humid heat extremes when the North American Monsoon (NAM) is active. Here, we investigate the distinct seasonality, dynamics, and drivers of dry versus humid heat in the NAM region. We find that the dry heat season can peak up to 38 days earlier than the humid heat season, with only 40%–60% overlap in the seasons on average in the core of the monsoon. Atmospheric ridges and reduced surface moisture are key drivers of extreme dry heat, consistent across space and time. In contrast, the regional dynamics of extreme humid heat are seasonally dependent. During the peak of the monsoon season, when ample moisture is available, extreme humid heat is driven by elevated dry bulb temperatures that occur alongside suppressed precipitation. However, during the shoulder months of the monsoon with lower background moisture availability, humid heat extremes are associated with elevated precipitation, soil moisture, and specific humidity. Our results identify the NAM region as possessing distinct dry and humid heat seasonalities and where the climatic drivers of humid heat depend on their timing relative to the monsoon season.