Zittis et al. (2021) Revisiting future extreme precipitation trends in the Mediterranean

Identification

• Journal: Weather and Climate Extremes
• Year: 2021
• Date: 2021-08-26
• Authors: George Zittis, Adriana Bruggeman, Jos Lelieveld
• DOI: 10.1016/j.wace.2021.100380

Research Groups

• Climate and Atmosphere Research Center, The Cyprus Institute, Nicosia, Cyprus
• Energy, Environment and Water Research Center, The Cyprus Institute, Nicosia, Cyprus
• Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany
• EURO-CORDEX initiative (Coordinated Regional Climate Downscaling Experiment)

Short Summary

This study assesses future trends in annual rainfall extremes and total precipitation in the Mediterranean using an ensemble of 33 high-resolution regional climate simulations under an RCP8.5 scenario. It projects a strong north-south gradient in extreme precipitation trends, with significant increases in 50-year and 100-year extremes across the region, despite overall decreasing total annual precipitation in the south, highlighting challenges for water resource management.

Objective

• To assess the direction, significance, and inter-model agreement of 21st-century trends of annual daily precipitation extremes (Rx1day) in the larger Mediterranean region.
• To investigate changes in 50-year and 100-year extremes between the recent past (1950–2000) and the 21st century.
• To approximate the time of occurrence of 100-year extremes during the 21st century.
• To assess changes in total annual precipitation and the contribution of daily extremes to the annual total precipitation budget, including for 32 Mediterranean cities and five precipitation classes.

Study Configuration

• Spatial Scale: Mediterranean region, subdivided into five precipitation classes and including 32 major cities, at a horizontal resolution of 0.11 degrees (approximately 12 km).
• Temporal Scale: Historical period (1951–2000) and future period (2001–2100), further divided into 21C1 (2001–2050) and 21C2 (2051–2100).

Methodology and Data

• Models used: An ensemble of 33 regional climate simulations from the EURO-CORDEX initiative, driven by six global Earth system models from CMIP5 (Coupled Model Intercomparison Project Phase 5) and nine regional climate models (RCMs).
• Data sources: Daily precipitation data from EURO-CORDEX simulations under the Representative Concentration Pathway 8.5 (RCP8.5) 'business-as-usual' emission scenario.
• Indices calculated: Annual maximum daily precipitation (Rx1day), annual total precipitation (PRCPTOT), and the ratio of Rx1day to PRCPTOT (RxTRatio).
• Statistical methods: Non-parametric Mann-Kendall test for trend significance, Sen’s Slope Estimator for trend magnitude, non-parametric one-sample Wilcoxon signed-rank test for significance of projected changes, and Generalized Extreme Value (GEV) distribution with Maximum Likelihood (ML) technique for 100-year extreme estimation, with 95% confidence intervals estimated using parametric bootstrapping.

Main Results

• A distinct north-south gradient in 21st-century daily precipitation extreme trends is projected: significant decreasing trends (up to -10 mm/decade) in the drier southern Mediterranean and Maghreb region, and less profound increasing trends in the north.
• Despite contrasting trends, 50-year daily precipitation extremes are projected to strongly increase (up to 100%) throughout the region by the second half of the 21st century, with over half of land grid cells showing statistically significant increases.
• Traditional 100-year extreme estimates (based on 1951–2000 data) underestimate 21st-century projected extremes by approximately 20-30% for wetter areas and up to 30% for drier areas (200–500 mm average annual rainfall), with a maximum difference of 36% for Class 3 (500–1000 mm/year).
• Annual total precipitation (PRCPTOT) is projected to decrease by 20-40% in most of the Mediterranean (30-45°N) by the second half of the 21st century, with increases of 10-20% in northern regions.
• The contribution of the wettest day per year to the annual total precipitation (RxTRatio) is expected to increase by 5-30% across the region, reaching 30-40% in many areas by 2051-2100, and up to 50-60% in North Africa, indicating that extreme events will constitute a larger proportion of total rainfall.
• The 100-year daily extremes are projected to occur earlier in the century (median 2050-2055) in southern Mediterranean latitudes (27.5–32.5°N) and later (median 2065-2070) in northern regions (>42.5°N).

Contributions

• Utilizes an unprecedented ensemble of 33 high-resolution EURO-CORDEX regional climate simulations, providing more robust insights into climate model uncertainties for extreme precipitation compared to previous studies.
• Confirms and quantifies the north-south Mediterranean gradient in extreme precipitation trends with high spatial detail.
• Demonstrates a "precipitation paradox" where, despite decreasing total annual precipitation, the region is projected to experience more intense and frequent extreme daily rainfall events, challenging traditional extreme value analysis and infrastructure design assumptions.
• Provides robust statistical analysis of 50-year and 100-year extreme precipitation quantiles over extended periods, highlighting the underestimation of future extremes by historical data.
• Investigates changes in extreme precipitation for specific urban environments and across different precipitation regimes, offering valuable information for regional water resource management and flood risk assessment.

Funding

• ERMIS-Floods project, funded by the European Union’s Interreg V-A Greece-Cyprus 2014–2020 Programme, Grant Agreement No. 5603.
• EMME-CARE project, funded by the European Union’s Horizon 2020 Research and Innovation Programme, Grant Agreement No. 856612.
• Co-funded by the Republic of Cyprus and the Hellenic Republic.

Citation

@article{Zittis2021Revisiting,
  author = {Zittis, George and Bruggeman, Adriana and Lelieveld, Jos},
  title = {Revisiting future extreme precipitation trends in the Mediterranean},
  journal = {Weather and Climate Extremes},
  year = {2021},
  doi = {10.1016/j.wace.2021.100380},
  url = {https://doi.org/10.1016/j.wace.2021.100380}
}