Comment on hess-2021-548

Abstract

Warming trends are responsible for an observed decrease of water discharge in Southern France (northwestern Mediterranean). Ongoing climate change and the likely increase of water demand threaten the availability of water resources over the coming decades. Drought indices like the Reconnaissance Drought Index (RDI) are increasingly used in climate characterization studies, but little is known about the relationships between these indices, water resources and the overall atmospheric circulation patterns. In this study, we investigate the relationships between the RDI drought index, water discharge and four atmospheric teleconnection patterns (TPs) for six coastal river basins in southern France, both for the historical period of the last 60 years and for a worst-case climatic scenario (RCP 8.5) reaching the year 2100. We combine Global and Regional Climate Model (CGM and RCM, respectively) outputs with a set of observed climatic and hydrological data in order to investigate the past relationships between RDI, water discharge and TPs and to project their potential evolutions in space and time. Results indicate that annual water discharge can be reduced by −49/−88 % by the end of the century under the extreme climate scenario conditions. Due to unequal links with TPs, the hydro-climatic evolution is unevenly distributed within the study area. Indeed a clustering analysis performed with the RDI time series detects two major climate clusters, separating the eastern and western part of the study region. The former indicates stronger relationships with the Atlantic TPs (e.g. the NAO and the Scand patterns) whereas the latter is more closely related to the Mediterranean TPs (MO and WeMO). The future climate simulations predict an antagonistic evolution in both clusters which are likely driven by decreasing trends of Scand and WeMO. The former provokes a general tendency of lower P in both clusters during spring, summer and autumn, whereas the latter might partly compensate this evolution in the eastern cluster during autumn and winter. However, compared to observations, representation of the Mediterranean TPs WeMO and MO in the considered climate models is less satisfactory compared to the Atlantic TPs NAO and Scand, and further improvement of the model simulations therefore requires better representations of the Mediterranean TPs.