Marcus Lofverstrom, Kaustubh Thirumalai 

Understanding the response of monsoon dynamics to climatic forcing is cru- cial for anticipating future shifts in freshwater availability across the global tropics. In this regard, a recent study [1] concludes that precipitation within the core of the North American Monsoon (NAM) should be understood as “convectively enhanced orographic rainfall in a mechanically forced stationary wave, not as a classic, thermally forced tropical monsoon.” Using model simu- lations under altered surface conditions, this study suggests that interactions of the extratropical jet stream with Mexico’s Sierra Madre Occidental (SMO) orography are a first-order control on the NAM under past and future global change. While we agree that the presence of orography enhances rainfall out- side land-sea thermal gradients alone [2], we believe that their experiments and analysis are not adequately designed to refute the central importance of thermally-direct forcing in modulating past and future NAM rainfall.