Science Source

Moisture flux convergence in RCMs and GCMs: Implications for droughts in the southwestern United States under climate change

States the water cycle of the southwestern United States (SW) is dominated by winter storms that maintain a positive annual net precipitation
Uses four sets of regional climate model (RCM) simulations for which sufficient model output was archived to allow calculations of the mean and transient moisture flux convergence (MFC)—mean refers to averaging over time, and transient refers to the deviation from the time mean
Defines MFC, saying vertically integrated MFC is equivalent in the long-term mean to net precipitation (P-E), and in turn to river runoff—MFC can be partitioned into two components: the MFC associated with the mean flow and the transient eddies
Analyzes the control and future climate from four pairs of regional and global climate models (RCMs and GCMs), which show that the RCMs simulate a higher fraction of transient eddy moisture fluxes because the hydrodynamic instabilities associated with flow over complex terrain are better resolved
States that under global warming, this enables the RCMs to capture the response of transient eddies to increased atmospheric stability that allows more moisture to converge on the windward side of the mountains by blocking
States that as a result, RCMs simulate enhanced transient eddy moisture convergence (MFC) in the SW compared to GCMs, although both robustly simulate drying due to enhanced moisture divergence by the divergent mean flow in a warmer climate