How Moist and Dry Intrusions Control the Local Hydrologic Cycle, in Present and Future Climates

Projected changes in the hydrologic cycle are expected to scale with the Clausius-Clapeyron relation but may locally diverge due to the uncertain response of the atmospheric general circulation. This "dynamic" component is the largest contributor to uncertainty in changes in the hydrologic cycle, both in the mean and the extremes. In this work, we examine how the component of the hydrologic cycle driven by poleward moist and equatorward dry intrusions, otherwise known as the local hydrologic cycle (LHC), contributes to the uncertain response of the total hydrologic cycle. We attribute changes in the LHC to four sources: thermodynamic, low-level convergence, anomalous meridional moisture advection, and zonal wave activity flux convergence. From our analysis of CESM LENS, we show that low-level convergence is the main dynamic contributor to the tropical and subtropical annual-mean response, consistent with a weakening of the Hadley circulation, and we show that anomalous meridional moisture advection is the primary dynamic contributor annually for the extratropics, consistent with storm track changes. Through an empirical scaling between the LHC and its associated wave activity, we achieve a clean separation between dynamic and thermodynamic contributions, revealing that subtropical expansion reduces the moist mixing length, while at the same time lengthening the dry mixing length. We find evidence for a slowdown in both wet and dry components of the LHC. Our results suggest future work should focus on how changes in wave breaking driven by subtropical expansion contribute to uncertainty in future projections of regional hydrologic cycle change.