Contrasting Short- and Long-Term Projections of the Hydrological Cycle in the Southern Extratropics

Abstract

nalysis of model output from phase 5 of the Coupled Model Intercomparison Project (CMIP5) reveals that, in the zonal mean, the near-term projections of summertime changes of precipitation in the Southern Hemisphere (SH) subtropics are very widely scattered among the models. As a consequence, over the next 50 years, the CMIP5 multimodel mean projects no statistically significant trends in the SH subtropics in summer. This appears to be at odds with the widely reported, and robust, poleward expansion of the subtropical dry zones by the end of the twenty-first century.This discrepancy between the shorter- and longer-term projections in SH summer, as shown here, rests in the recovery of the ozone hole in the coming decades, as a consequence of the Montreal Protocol. This is explicitly demonstrated by analyzing model experiments with the Whole Atmosphere Community Climate Model, version 4 (WACCM4), a high-top model with interactive stratospheric chemistry, and coupled to land, ocean, and sea ice components. Contrasting WACCM4 integrations of the representative concentration pathway 4.5 with and without trends in surface concentrations of ozone-depleting substances allows for demonstrating that stratospheric ozone recovery will largely offset the induced ?wet gets wetter and dry gets drier? projections and the accompanying poleward expansion of the subtropical dry zone in the SH. The lack of near-term statistically significant zonal-mean changes in the SH hydrological cycle during summer is of obvious practical importance for many parts of the world, and it might also have implications for the Southern Ocean and the Antarctic continent.