Is the Projected Aridification in the Mediterranean Region a Simple "Dry-Get-Drier" Response?

Models robustly project decreases in net precipitation (precipitation minus evaporation, P–E) due to human-induced warming over the Mediterranean region, in qualitative agreement with the simple thermodynamic "dry-get-drier" scaling of the atmospheric moisture convergence (Held and Soden, 2006). This thermodynamic scaling, however, neglects changes in relative humidity and horizontal temperature gradients, which might be important in a region of large land–ocean contrasts, such as the Mediterranean. Here we explore if and to what extent the extended scaling of Byrne and O'Gorman (2015), which incorporates these gradients and is based on climatological moisture fluxes and changes in surface properties only, might better capture the thermodynamic response of the Mediterranean hydroclimate projected by the end of the 21st century by ten models in the phase 6 of the Coupled Model Intercomparison Project (CMIP6) archive.

According to the CMIP6 multi-model mean, the simple scaling for the mean thermodynamic component, in the absence of changes in atmospheric circulation and advection, causes a negative P–E tendency over the Mediterranean Sea and the surrounding land areas and a weak positive P–E tendency over northwestern Africa. This is indeed an amplified pattern of the time mean flow: there is increased moistening (drying) where the time mean flow is convergent (divergent) in the base climate.

The extended scaling, unlike the simple scaling, predicts a wettening over the ocean, in the annual mean and through the seasonal cycle. While not fully accounting for the magnitude nor the extent of the wettening due to the “full” thermodynamic adjustment of the Mediterranean hydroclimate, inclusive of thermodynamic contributions from both moisture convergence and advection, the extended scaling outperforms the simple scaling by partially capturing the overall signal. Throughout the region, differences between the simple and the extended scaling primarily arise from the contribution of the terms involving the gradients of fractional changes in near-surface relative humidity and changes in the near-surface temperature, with the term involving changes in relative humidity being negligible. Even if largely cancelling, the two gradient terms give rise to a pattern grossly characterized by moistening over the ocean and drying over neighboring land regions.

The results of this work highlight how thermodynamical changes in the Mediterranean hydrological cycle result from an interplay between different mechanisms, arising from the thermodynamical contributions from both moisture convergence and horizontal advection. While the extended scaling has been shown to be an effective approach in explaining the deviation of the global annual-mean P–E response over land from the "wet-get-wetter" paradigm, it has not been evaluated for regional studies or different seasons. Our study shows that regional studies, such as those focusing on the Mediterranean, could also benefit from the extended scaling, enhancing our understanding of future hydroclimate changes in this vulnerable region.

 

References:

• Byrne, M. P., & O’Gorman, P. A. (2015). The response of precipitation minus evapotranspiration to climate warming: Why the “Wet-get-wetter, dry-get-drier” scaling does not hold over land. Journal of Climate, 28(20), 8078–8092. https://doi.org/10.1175/JCLI-D-15-0369.1
• Held, I. M., & Soden, B. J. (2006). Robust Responses of the Hydrological Cycle to Global Warming. Journal of Climate, 19(21), 5686–5699. https://doi.org/10.1175/JCLI3990.1