Understanding the Increasing Hot Extremes over the Northern Extratropics Using Community Atmosphere Model

Siyu Zhao,Jiaying Zhang,Yi Deng,Na Wang

doi:10.1007/s13143-021-00264-z

Abstract

The past four decades have seen an increase of terrestrial hot extremes during summer in the northern extratropics, accompanied by the Northern Hemisphere (NH) sea surface temperature (SST) warming (mainly over 10°–70°N, 0°–360°) and CO2 concentration rising. This study aims to understand possible causes for the increasing hot extremes, which are defined on a daily basis. We conduct a series of numerical experiments using the Community Atmosphere Model version 5 model for two periods, 1979–1995 and 2002–2018. The experiment by changing the CO2 concentration only with the climatological SST shows less increase of hot extremes days than that observed, whereas that by changing the NH SST (over 10°–70°N, 0°–360°) with constant CO2 concentration strengthens the hot extremes change over mid-latitudes. The experiment with both SST and CO2 concentration changes shows hot extremes change closer to the observation compared to the single-change experiments, as well as more similar simulations of atmospheric circulations and feedbacks from cloud and radiative processes. Also discussed are roles of natural variability (e.g., Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation) and other factors (e.g., Arctic sea ice and tropical SST).

Highlights

The increase of the global mean temperature has slowed down since the late 1990s (Kosaka and Xie 2013), terrestrial hot extremes characterized by extreme high surface air temperature (SAT) have become more frequent in recent decades (Perkins-Kirkpatrick and Lewis 2020)
Our results show that the combined effects of the Northern Hemisphere (NH) sea surface temperature (SST) and CO2 can lead to more hot extremes in recent decades, similar to those observed
This study investigates possible causes for the increasing hot extremes over the northern extratropics during summer

Summary

Introduction

The increase of the global mean temperature has slowed down since the late 1990s (Kosaka and Xie 2013), terrestrial hot extremes characterized by extreme high surface air temperature (SAT) have become more frequent in recent decades (Perkins-Kirkpatrick and Lewis 2020). Ding and Wang (2005) suggested that the maintenance of the CGT pattern depends on interactions between the CGT (i.e., Rossby wave train patterns) and Indian summer monsoon (related to the west-central Asian high). Such CGT pattern was identified through a self-organizing map (SOM) analysis and tends to be linked to the increasing hot extremes over the Northern Hemisphere (NH) (Lee et al 2017)

Objectives

Results

Conclusion