Assessing changes in risk of amplified planetary waves in a warming world

Abstract

Summer weather extremes are often associated with high‐amplitude atmospheric planetary waves (Petoukhov et al., 2013). Such conditions lead to stationary weather patterns, triggering heat waves and sometimes prolonged intense rainfall. These wave events, referred to as periods of Quasi‐Resonant Amplification (QRA), are relatively rare though and hence provide only a few data points in the meteorological record to analyse. Here, we use atmospheric models coupled to boundary conditions that have evolved slowly (i.e., climate), to supplement measurements. Specifically we assess altered probabilities of resonant episodes by employing a unique massive ensemble of atmosphere‐only climate simulations to populate statistical distributions of event occurrence. We focus on amplified waves during the two most extreme European heat waves on record, in years 2003 and 2015 (Russo et al., 2015). These years are compared with other modelled recent years (1987–2011), and critically against a modelled world without climate change. We find that there are differences in the statistical characteristics of wave event likelihood between years, suggesting a strong dependence on the known and prescribed Sea Surface Temperature (SST) patterns. The differences are larger than those projected to have occurred under climate change since the pre‐industrial period. However, this feature of small differences since pre‐industrial is based on single large ensembles, with members consisting of a range of estimates of SST adjustment from pre‐industrial to present. Such SST changes are from projections by a set of coupled atmosphere–ocean (AOGCM) climate models. When instead an ensemble for pre‐industrial estimates is subdivided into simulations according to which AOGCM the SST changes are based on, we find differences in QRA occurrence. These differences suggest that to reliably estimate changes to extremes associated with altered amplification of planetary waves, and under future raised greenhouse gas concentrations, likely requires reductions in any spread of future modelled SST patterns.

Heat Waves: Physical Understanding and Scientific Challenges

“…They include tropical SST anomalies (Deng et al., 2018; R. M. Horton et al., 2016), the excitation of large‐scale modes of variability by regional forcings (e.g., anthropogenic aerosols or widespread land‐cover and land‐management changes; Mahmood et al., 2014; Westervelt et al., 2018), and other mechanisms without obvious anthropogenic influences (e.g., Arctic diabatic heating by enhanced convergence of moist intrusions; Baggett & Lee, 2019). Therefore, improved understanding of the involved mechanisms is required to add robustness and elucidate whether the observed changes are caused by anthropogenic forcings (Mann et al., 2017) or internal multidecadal variability (e.g., SSTs; Huntingford et al., 2019).…”

“…Ocean‐atmosphere coupling can also affect the simulation of HWs and their trends, particularly in some tropical regions and coastal areas of open seas (Fischer et al., 2018; Gröger et al., 2021). Moreover, the simulated changes in HW‐related atmospheric circulation patterns, such as CGTs, are sensitive to the forced patterns of SST responses to anthropogenic forcings (Huntingford et al., 2019), and GCMs tend to underestimate the near‐surface impacts of CGTs (e.g., Luo et al., 2022).…”

The Problem of Diagnosing Jet Waveguidability in the Presence of Large-Amplitude Eddies

“…Our work has implications regarding earlier claims that the occurrences of extreme weather may be due to ''quasiresonant amplification'' or ''wave resonance'' (Petoukhov et al 2013;Coumou et al 2014;Petoukhov et al 2016;Stadtherr et al 2016;Kornhuber et al 2017a,b;Mann et al 2017;Huntingford et al 2018;Mann et al 2018;Kornhuber et al 2019). In these studies, strong waveguidability was argued to be a precondition for quasi-resonance and ensuing large wave amplitudes, and the analysis of waveguidability was based on the Eulerian zonal-mean background state.…”

Validation of boreal summer tropical-extratropical causal links in seasonal forecasts

“…The analysed regions (from the South Asian monsoon to the North American and Eurasian continents) are prone to suffering the effect of the ongoing anthropogenic climate change (Pfleiderer et al, 2018;Mann et al, 2018;Coumou et al, 2018Coumou et al, , 2017Huntingford et al, 2019;Turner and Annamalai, 2012). Therefore, it is critical to evaluate the model ability at…”