Insights on the Midlatitude Hydrological Cycle from Water Vapor Wave Activity Analysis

Building on the recent advent of the concept of finite-amplitude wave activity, a contour-following diagnostics for the column water vapor (CWV) is developed to understand and quantify the higher moments in the hydrological cycle in the midlatitude storm track and its peripheries. The Lagrangian nature of the diagnostics leads to a more tractable formalism for the zonally asymmetric component of the hydrological cycle, with a strong linear relation emerging between the wave activity and the wave component of precipitation minus evaporation (\widetilde{P-E}). \widetilde{P-E} measures the dry-versus-wet disparity in the hydrological cycle and it is found to increase at a super-Clausius Clapeyron rate at the poleward side of the mean storm track in response to a uniform SST warming and the meridional structure of the increase can be largely attributed to the change of the meridional stirring scale of the midlatitude Rossby waves. Further scaling for \widetilde{P-E} indicates that the rate of the wavy hydrological cycle, measured by the ratio of \widetilde{P-E} to the CWV wave activity, is subdued almost everywhere in the extratropics, implicative of an overall weakening of the zonally asymmetric circulation. Extending the CWV wave activity analysis to the transient moist regions helps reveal some unique characteristics of the atmospheric rivers in terms of transport function, minimum precipitation efficiency, and maximum hydrological cycle rate, as well as an overall weakening of the hydrological cycle rate in the atmospheric river regions under global warming.