Utilizing high-resolution large-ensemble simulations to understand long-term variability and forced changes in local extreme events

Numerous detection and attribution (DA) studies have identified long-term trends in various characteristics of extreme events using general circulation models (GCMs) and attributed them to anthropogenic external forcing; however, many aspects remain poorly understood, with respect to regional extreme events that cannot be captured by GCMs and the long-term modulation of extreme events arising from non-anthropogenic factors such as multidecadal internal ocean variability. To address this challenge, we performed atmospheric GCM (AGCM)-based large-ensemble simulations and regional downscaling simulations for each ensemble member based on a regional climate model over more than 70 years to investigate the mechanisms by which large-scale and long-term climate variations modulate the frequency and intensity of topographically influenced local-scale extreme events. Through analysis of this dataset, we identified inherent multidecadal-scale signals in local, orographic precipitation over East Asia, and the results suggest that their modulation is linked to multidecadal variability in multiple ocean basins.

This approach can be extended to event attribution studies of regional-scale extreme events by adding large-ensemble simulations under non-warming conditions, enabling a quantitative assessment of the impacts of anthropogenic global warming on fine-scale orographic rainfall and heatwaves that have previously been difficult to detect. Such event attribution studies have attracted growing attention not only from scientists but also from society; however, a major bottleneck lies in their high computational cost. Recently, we have developed a new statistical framework that integrates our large-ensemble experiments with extreme-value statistics, thereby enhancing our capacity to communicate scientific information effectively to society. In this presentation, we will also introduce the newly established Weather Attribution Center Japan, which plays a key role in translating these research outcomes into societal applications.