Scale-decomposed atmospheric water budget over North America as simulated by the Canadian Regional Climate Model for current and future climates

Abstract

Through its various radiative effects and latent heat release, water plays a major role in the maintenance of climate. Therefore a better understanding of climate and climate changes requires a better understanding of the hydrological cycle. In this study we investigate the scale-decomposed atmospheric water budget over North America as simulated by the Canadian Regional Climate Model (CRCM) driven by the Canadian Coupled Global Climate Model (CGCM) under current conditions for 1961–1990 and the SRES A2 scenario for 2041–2070. A discrete cosine transform is applied to the atmospheric water budget variables in order to separate small scales that are resolved exclusively by the high-resolution CRCM, from larger scales resolved by both the CRCM and low-resolution driving CGCM. The moisture flux divergence is alternatively decomposed in terms of three scales of wind and humidity to provide nine interaction terms. Statistics of these fields are calculated for winter and summer seasons, and the local statistical significance of climate-change projections is tested. The contributions of each scale band to the water budget current climatology and to its evolution in a warmer climate are investigated, addressing the issue of the potential added value of smaller scales. Results show a time variability larger than the time mean for all variables, and a significant small-scale contribution to time variability, which is even dominant in summer, both in the current and future climates. Future climate exhibits an overall intensification of the hydrological cycle in winter, and more mixed changes in summer. Relative changes in the time mean and time variability appear comparable, and the contribution of each scale band to variability changes remains overall very consistent with their contribution to current climate variability.