Assessment of Future Precipitation Changes in Mediterranean Climate Regions from CMIP6 ensemble

Previous studies have indicated a large model disagreement in the future projections of precipitation changes over the regions featuring Mediterranean climate. Many of these highly populated regions have been experiencing major droughts in the recent decades, raising concerns about future precipitation changes and their impacts. Here we investigate precipitation projections across five Mediterranean climate regions in the CMIP6 ensemble, and study their respective model agreements on the sign of future precipitation changes. We focus on the period 2050-2079 relative to 1970-1999, and consider two climate change scenarios (ssp2-4.5 and ssp5-8.5) over the Mediterranean Basin (MED), California (CAL), the central coast of Chile (SAA), the Cape Province area of South Africa (SAF) and southwest Australia (AUS).

The CMIP6 ensemble mean suggests that annual mean cumulative precipitation will decrease over all the regions studied with the exception of northern California. In most cases, this decline is primarily attributed to a reduction in winter precipitation, except over the Mediterranean Basin, where the most significant decrease occurs in autumn. The model agreement on the sign of future precipitation changes is generally high over the regions and seasons where the ensemble mean indicates the precipitation decline in the future, and low over the regions showing the precipitation increase or no change. Specifically, the model agreement is low in southern California during all seasons, in northern Mediterranean during winter and autumn, and in southwest Australia during austral summer and autumn. CMIP6 ensemble means also indicate that the consecutive dry days (CDD) will increase in the future in all regions, but again the model agreement on this increase is low over southern and central California, the southern Mediterranean, and parts of southwest Australia. Similarly, the ensemble mean of consecutive wet days (CWD) indicate a decrease in all regions, with weak model agreement on the sign of future changes over CAL, northeast AUS and part of the MED region. The ensemble mean maximum one-day precipitation increases over all the regions, the most over the parts of southwest Australia and the Mediterranean.

We conclude that despite substantial improvements to the new CMIP6 generation of models, the intermodel differences in future projections of precipitation changes continue to be high across parts of California, the Mediterranean Basin and southwest Australia. Impact studies need to account for these uncertainties and consider the whole intermodel range of projected precipitation changes.