Climate Change in the Mediterranean: Assessing Changes in Different Circulations and the Impacts on the Mediterranean Hydroclimate

Changes in the hydroclimate of the Mediterranean region by the end of the 21^st century are studied using phase 6 of the Coupled Model Intercomparison Project (CMIP6) projections. More specifically, we examine how changes in the different terms of the atmospheric moisture budget in this region, namely the moisture flux convergence due to the zonally averaged flow, and stationary and transient eddies, contribute to changes in the climatological net precipitation (precipitation minus evaporation, P – E), in the annual, seasonal, and zonal mean over land and sea.

According to the ensemble-mean of ten CMIP6 models, the climatological annual mean P – E is projected to decrease drastically by the end of the 21^st century over northern Mediterranean land regions as well as the sea. The drying is predominantly due to increased total stationary-eddy moisture flux divergence, which arises from increased divergence of the zonally averaged moisture by the zonally-anomalous circulation. For both land and sea, the annual mean pure stationary eddy term is projected to bring wetter conditions within the Mediterranean, except for northwestern Africa and the Iberian Peninsula. This wettening tendency is, however, not large enough to offset the drying caused by the zonally-anomalous circulation.

By the end of the 21^st century, the annual mean transient eddies are projected to cause increased drying over the northern Mediterranean land regions and the western Mediterranean Sea, and increased moistening over the eastern Mediterranean. The drying due to the annually and zonally averaged circulation, associated with the descending branch of the Hadley cell, is very small, and appears to be a weaker signal with respect to the others.

While increased moisture divergence due to transient eddies during winter is a contributing factor to the Mediterranean drying, it is not the main cause of year-round drying by the end of the 21^st century. In fact, there are slight increases of moisture convergence over Portugal, Spain, and northern Turkey. Rather, it is the increase in the divergent stationary eddies during summer and winter that drives the aridification phenomenon in the Mediterranean. Recent studies using CMIP5 models have reported similar results (Seager et al. 2014; Seager et al. 2019), pointing to the robustness of the projected signal.

 

References:

• Seager, R., Liu, H., Henderson, N., Simpson, I., Kelley, C., Shaw, T., Kushnir, Y., & Ting, M. (2014). Causes of increasing aridification of the mediterranean region in response to rising greenhouse gases. Journal of Climate, 27(12), 4655–4676. https://doi.org/10.1175/JCLI-D-13-00446.1
• Seager, R., Osborn, T. J., Kushnir, Y., Simpson, I. R., Nakamura, J., & Liu, H. (2019). Climate Variability and Change of Mediterranean-Type Climates. Journal of Climate, 32(10), 2887–2915. https://doi.org/10.1175/JCLI-D-180472.1