Abstract

The mechanisms leading to the occurrence of extreme weather and climate events are varied and complex. They generally encompass a combination of dynamical and thermodynamical processes, as well as drivers external to the climate system, such as anthropogenic greenhouse gas emissions and land-use change. Here we present the ExtremeX multi-model intercomparison experiment, which was designed to investigate the contribution of dynamic and thermodynamic processes to recent weather and climate extremes. The numerical experiments are performed with three Earth System Models: CESM, MIROC, and EC-Earth. They include control experiments with interactive atmosphere and land surface conditions, and experiments where either the atmospheric circulation, soil moisture or both are constrained using observation-based values. The temporal evolution and magnitude of temperature anomalies during heatwaves is well represented in the experiments with constrained atmosphere. However, mean climatological biases in temperature and precipitation are not substantially reduced in any of the constrained experiments, highlighting the importance of error compensations and tuning in the standard model versions. The results further reveal that both atmospheric circulation patterns and soil moisture conditions substantially contribute to the occurrence of heat extremes. Soil moisture effects are particularly important in the tropics, the monsoon areas and the Great Plains of the United States.

Highlights

  • 15 Weather and climate extremes strongly affect society, human health, and ecosystems; they need to be accurately simulated in numerical weather predictions and climate projections (e.g. Seneviratne et al, 2012)

  • Across climate models of the fifth phase of the Coupled Model Intercomparison Project (CMIP5) consistent biases can be found in the mean climatology of the lower atmosphere and land surface, for example

  • We present the new "ExtremeX" multi-model experiment in which the contribution of thermodynamic and dynamic processes to recent extreme events is investigated in three Earth System Models (ESMs)

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Summary

Introduction

15 Weather and climate extremes strongly affect society, human health, and ecosystems; they need to be accurately simulated in numerical weather predictions and climate projections (e.g. Seneviratne et al, 2012). 25 in a warming climate is the identification of the respective contribution of thermodynamic (thermal structure, water vapor and precipitation, land-atmosphere interactions) and dynamic (large-scale circulation) processes to their changes in occurrence and intensity (e.g. Pfahl et al, 2017; Shepherd, 2014; Trenberth et al, 2015; Wehrli et al, 2018, 2019; Zappa et al, 2015). Better isolating these contributions would help inform further model development as well as research on the attribution and projection of changes in weather and climate extremes (Vautard et al, 2016). – What is the relative contribution of the land surface and the atmospheric circulation to warm spells globally and how do the contributions vary regionally?

Design of the model intercomparison project
Methods
Constraining of atmospheric circulation
Reference data sets
Data analysis
Model descriptions
Model for Interdisciplinary Research on Climate version 5 (MIROC5)
European Community Earth System Model Version 3 (EC-Earth3)
Global and regional biases in surface temperature and precipitation
Location and sign of seasonal biases
Findings
Conclusions and outlook
Full Text
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