Whether Arctic amplification has contributed to a wavier circulation and more frequent extreme weather in midlatitudes remains an open question. For two to three decades starting from the mid-1980s, accelerated Arctic warming and a reduced meridional near-surface temperature gradient coincided with a wavier circulation. However, waviness remains largely unchanged in model simulations featuring strong Arctic amplification. Here, we show that the previously reported trend toward a wavier circulation during autumn and winter has reversed in recent years, despite continued Arctic amplification, resulting in negligible multidecadal trends. Models capture the observed correspondence between a reduced temperature gradient and increased waviness on interannual to decadal time scales. However, model experiments in which a reduced temperature gradient is imposed do not feature increased wave amplitude. Our results strongly suggest that the observed and simulated covariability between waviness and temperature gradients on interannual to decadal time scales does not represent a forced response to Arctic amplification.

INTRODUCTION

Rising global temperatures are expected to increase the severity of certain types of extreme events such as heatwaves, droughts, and floods (1–3), primarily for well-established thermodynamical reasons. However, potential changes in weather extremes related to atmosphere dynamics, particularly over the midlatitudes, are far less certain (4, 5). It has been proposed that the faster warming of the Arctic compared to the rest of world—so-called Arctic amplification—is altering the atmospheric circulation and contributing to an increase in extreme weather in the midlatitudes (6). One hypothesis proposed by Francis and Vavrus suggests that the reduced equator-to-pole temperature gradient weakens the predominant westerly wind, which, in turn, causes larger-amplitude waves in the midlatitude circulation (7, 8), hereafter referred to as a “wavier” circulation. A wavier circulation has been linked to increased occurrence of extreme midlatitude weather, with the types of extremes favored by amplified waves varying by location (9). The link between Arctic amplification and a wavier midlatitude circulation remains controversial because of numerous studies arriving at often conflicting conclusions (10–13).

The purported evidence used to support the link between Arctic amplification and a wavier circulation stems primarily from observational analyses. The acceleration of Arctic warming for two to three decades starting from the mid-1980s coincided with a trend toward a wavier midlatitude circulation, particularly in autumn and winter (7, 8). Furthermore, longitudes where there was a strong decrease in the meridional temperature gradient coincided with longitudes with increasing waviness (8). However, the metrics used to measure waviness have been questioned (14, 15), and alternative metrics show that statistically robust trends are limited to few regions and seasons (14–18) and often only when more recent, short-term, trends are considered (18–20). The absence of statistically robust signals could be because Arctic amplification has only recently become of sufficiently large magnitude to have a detectable effect on the midlatitude circulation (8, 12); thus, the effect is difficult to detect amidst the large internal atmospheric variability (21). Regardless of their statistical significance or not, the coincidence of observed trends in waviness and Arctic amplification may not mean that the relationship is causal.

Evidence for a causal response will likely have to come from theoretical arguments and modeling experiments. Basic theoretical arguments do not provide any unambiguous support for an increase in wave amplitudes under reduced meridional temperature gradients and zonal wind speeds (4, 22). A decrease in wave amplitude was found in response to Arctic amplification in experiments with a highly idealized model but which retained the essential physics required in the hypothesis proposed by Francis and Vavrus (23). This decrease in wave amplitude occurred because of weaker synoptic variability (22, 24) in midlatitudes and despite a mean reduction in the zonal wind speed. Numerous studies have used more complex climate models to test whether we might expect to see a wavier circulation in the future. In contrast to the Francis and Vavrus hypothesis, models forced with increasing greenhouse gas concentrations show a small decrease in waviness, albeit with substantial intermodel spread (18, 25, 26). In addition to the effect of reduced variability, this waviness decrease may be partly attributed to an increase in the meridional temperature gradient aloft, which tends to oppose the midlatitude circulation response to a decreased meridional temperature gradient near the Earth’s surface (10, 18, 25). Model experiments forced with Arctic amplification in isolation find only a weak response in waviness compared to internal variability (27–29). Other aspects of the large-scale circulation also show only weak responses, compared to internal variability, in model experiments forced with observed sea ice loss (30–32). Overall, model simulations do not support a causal link between Arctic amplification and increased waviness and, instead, suggest that the observed increase in waviness was a result of internal variability and is unlikely to continue. It is possible that the models are wrong because of deficiencies in simulating the relevant processes, but direct evaluation of the models’ capability in reproducing the observed links between Arctic amplification and waviness has not been undertaken.

Despite substantial scientific uncertainty, the Francis and Vavrus hypothesis has become a regular narrative in media reporting of extreme weather events (33–35). This widespread media reporting is likely a major reason why there is high public belief that if Arctic warming continues, it will have major effects on midlatitude weather (33). Some scientists argue that the possible effects of Arctic amplification on the circulation have been overstated in the public discourse and distracted from other more certain and no less concerning consequences of climate change (36).

Previous work examining changes in waviness in response to Arctic amplification has focused on either only observations (7, 8, 19, 20) or only models (23, 25–29) or compared recently observed trends to future model projections (18), making fair model-observation comparisons difficult. Here, we attempt to reconcile the divergent conclusions of previous studies by making “like-for-like” comparisons between observations and models. First, we update the observed waviness trends to the end of 2018 to examine whether the previously reported increases (7, 8, 18, 19) have continued and compare them to the range of simulated trends from a multimodel large ensemble. Next, we examine the correspondence between Arctic amplification and waviness as manifested in interannual to decadal variability in both observations and models. Last, we perform controlled model experiments to determine the direction of causality in simulated relationships between Arctic amplification and waviness.