Abstract
In boreal summer, circumglobal Rossby waves can promote stagnating weather systems that favor extreme events like heatwaves or droughts. Recent work highlighted the risks associated with amplified Rossby wavenumber 5 and 7 in triggering simultaneous warm anomalies in specific agricultural breadbaskets in the Northern Hemisphere. These type of wave patterns thus pose potential risks for food production, as well as human health, and other impacts. The representation of such summertime wave events and their surface imprints in general circulation models (GCMs) has not been systematically analyzed. Here we validate three state-of-the-art global climate models (EC-Earth, CESM, and MIROC), quantify their biases and provide insights into the underlying physical reasons for the biases. To do so, the ExtremeX experiments output data were used, which are (1) historic simulations (1979–2015/2016) of a freely running atmosphere with prescribed ocean, and experiments that additionally nudge toward the observed (2) upper-level horizontal winds in the atmosphere, (3) soil moisture conditions, or (4) both. The nudged experiments are used to trace the sources of the model biases to either the large-scale atmospheric circulation or surface feedback processes. We show that while the wave position and magnitude is represented well compared to ERA5 reanalysis data. During high amplitudes (> 1.5 s.d.) wave-5 and wave- 7 events, the imprint on surface variables temperature, precipitation and sea level pressure is substantially underestimated: typically, by a factor of 1.5 in correlation and normalized standard deviations (n.s.d.) for near-surface temperature and mean sea level pressure. As for the precipitation, it’s still a factor of 1.5 for n.s.d. but 2 for correlation. The correlations and n.s.d. for surface variables do not improve if only the soil moisture is prescribed, but considerably increased when the upper-level atmosphere circulation is nudged. The underestimation factors are corrected almost entirely. When applying both soil moisture prescription and the nudging of upper-level atmosphere, both the correlation and n.s.d. values are quite similar to only atmosphere component is nudged experiments. Hence, the near-surface biases can be substantially improved when nudging the upper-level circulation providing evidence that relatively small biases in the models’ representation of the upper-level waves can strongly affect associated temperature and rainfall anomalies.
Highlights
The past decade has witnessed a series of unprecedented boreal summer weather extreme events around the globe such as the 2010 Russian heatwave, 2012 North American heatwave, and the record breaking heatwaves of 2015, 2018 and 2019 in Europe (Barriopedro et al 2011; Kornhuber et al 2019; Krzyżewska and Dyer 2018; Wang et al 2014; Huntingford et al 2019; Xu et al 2021)
This paper aims at addressing following questions: 1: Can models capture the key characteristics of high-amplitude circumglobal waves in summer? 95 2: What are the near-surface temperature, rainfall, and mean sea level pressure anomalies from such waves and how do they compare to observations? 3: Do potential model biases originate from the atmospheric circulation or land surface-feedbacks?
The wave amplitudes, regardless of wave numbers and models, are reasonably well reproduced with errors ranging from 5% to 12% in the differences of the model climatology compared to ERA5 data
Summary
The past decade has witnessed a series of unprecedented boreal summer weather extreme events around the globe such as the 2010 Russian heatwave, 2012 North American heatwave, and the record breaking heatwaves of 2015, 2018 and 2019 in Europe (Barriopedro et al 2011; Kornhuber et al 2019; Krzyżewska and Dyer 2018; Wang et al 2014; Huntingford et al 2019; Xu et al 2021). Some of these events happened simultaneously with other types of extremes such as the persistent 45 Russian heatwave and Pakistan flood in 2010 July and August (Lau and Kim 2012; Martius et al 2013). In summer wave-5 (Ding and Wang 2005; Kornhuber et al 2020) and wave-7 (Kornhuber et al 2019, 2020) have preferred phase positions and thereby favor simultaneous extremes in major bread-basket regions (Kornhuber et al 2020)
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