Abstract

AbstractPrevious studies have recognized the societal relevance of climatic extremes on the seasonal time scale and examined physical processes leading to individual high-impact extreme seasons (e.g., extremely wet or warm seasons). However, these findings have not yet been generalized beyond case studies since at any specific location only very few seasonal events of such rarity occurred in the observational record. In this concept paper, a pragmatic approach to pool seasonal extremes across space is developed and applied to investigate hot summers and cold winters in ERA-Interim and the Community Earth System Model Large Ensemble (CESM-LENS). We identify spatial extreme season objects as contiguous regions of extreme seasonal mean temperatures based on statistical modeling. Regional pooling of extreme season objects in CESM-LENS then yields considerable samples of analogs to even the most extreme ERA-Interim events. This approach offers numerous opportunities for systematically analyzing large samples of extreme seasons, and several such analyses are illustrated. We reveal a striking co-occurrence of El Niño to La Niña transitions and the largest ERA-Interim midlatitude extreme summer events. Moreover, we perform a climate model evaluation with regard to extreme season size and intensity measures and estimate how often an extreme winter like the cold North American 2013/14 winter is expected anywhere in midlatitude regions. Furthermore, we present a large set of simulated analogs to this event, which makes it possible to study commonalities and differences of their underlying physical processes. Finally, substantial but spatially varying climatological differences in the size of extreme summer and extreme winter objects are identified.

Highlights

  • Defining weather and climate extremes is a multidimensional problem (Cattiaux and Ribes 2018), which involves at least three dimensions, namely the spatial extent, the intensity, and the duration of an extreme event

  • In the supplemental material we show that over extratropical land areas the 1979–2018 linear trends in ERA-Interim data (ERAI) typically fall within the range of the linear trends from the individual CESM-LENS ensemble members, while over tropical and subtropical oceans CESM-LENS appears to overestimate the linear trend in JJA and DJF T2m means, and likely the forced trend

  • We highlight the current lack of climatological studies on the dynamics and physics of seasonal extremes and propose a pragmatic approach for spatial pooling of seasonal extreme events

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Summary

Introduction

Defining weather and climate extremes is a multidimensional problem (Cattiaux and Ribes 2018), which involves at least three dimensions, namely the spatial extent, the intensity, and the duration of an extreme event. Along the last of those dimensions, the relevant meteorology varies greatly. For example, 10-min precipitation extremes typically occur within intense convective systems, while extreme. Denotes content that is immediately available upon publication as open access

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