Abstract/Summary

Greenhouse gas (GHG) emissions affect precipitation worldwide. The response is commonly described by two timescales linked to different processes: a rapid adjustment to radiative forcing, followed by a slower response to surface warming. However, additional timescales exist in the surface warming response, tied to the time evolution of the sea surface temperature (SST) response. Here we show that in climate model projections the rapid adjustment and surface mean warming are insufficient to explain the time evolution of the hydro-climate response in three key Mediterranean-like areas, namely California, Chile and the Mediterranean. The time evolution of those responses critically depends on distinct shifts in the regional atmospheric circulation associated with the existence of distinct fast and slow SST warming patterns. As a result, Mediterranean and Chilean drying are in quasi-equilibrium with GHG concentrations, meaning that the drying will not continue after GHG concentrations are stabilised, whereas California wetting will largely emerge only after GHG concentrations are stabilised. The rapid adjustment contributes to a reduction in precipitation but has a limited impact on the balance between precipitation and evaporation. In these Mediterranean-like regions, future hydro-climate related impacts will be substantially modulated by the time evolution of the pattern of SST warming that is realised in the real world.