1.4.2.6 Mid-latitude atmospheric dynamics

Mid-latitude atmospheric circulation is characterised by a band of strong westerly winds (see Figure 1.4.1), with largest velocities at an altitude of 7-12km, forming the so-called northern polar ‘jet-stream’. The jet serves as a separation of cold air masses at high-latitudes in the north from temperate air masses further south. Large meanders in the jet are referred to as planetary, or Rossby, waves. In most cases, these waves move over large distances and decline over timescales of a few days. When persisting for a prolonged time over the same location (referred to as ‘quasi-stationary’ waves) they can lead to high-impact climate extremes, including temperature extremes or heavy precipitation. An example is the record-breaking heatwave of 2021 in the North American Pacific Northwest (Bartusek et al., 2022). 

Atmospheric features such as blocks (quasi-stationary high-pressure regions that divert, or ‘block’, the large-scale atmospheric flow on timescales of several days to weeks) are intimately linked to these persistent meanders in the jet. A widely discussed effect of climate change is a poleward shift of the mid-latitude jet, although this may be season and location-dependent (Oudar et al,. 2020), and smaller than previously thought (Curtis et al., 2020) (Figure 1.4.16). 

Evidence for tipping dynamics

In climate models, the magnitude of the jet’s shift strongly depends on the reduction of the AMOC (see Chapter 1.4.2.1). Models with a strong AMOC reduction in the future tend to project a much stronger poleward shift of the jet than models with a weaker AMOC reduction, making this the largest atmospheric circulation uncertainty in regional climate change projections (Bellomo et al., 2021). 

Furthermore, it has been suggested that the mid-latitude flow might weaken, leading to more persistent and slower-moving weather patterns (Coumou et al., 2015; Kornhuber and Tamarin-Brodsky, 2021). A possible driver is Arctic amplification – namely the fact that the Arctic is warming more rapidly than the rest of the planet, partly driven by sea ice loss (see Chapter 1.2). This reduces the equator-pole temperature contrast, and could result in a weakening and enhanced meandering of the jet stream (Francis and Vavrus, 2015). While Arctic amplification is most evident during winter, such increase in waviness may also be occurring during the summer season (Coumou et al., 2018). However, evidence that the occurrence of large-amplitude atmospheric waves is increasing is debated (Screen and Simmonds, 2013; Blackport and Screen, 2020; Riboldi et al., 2020), and mechanisms which would reduce blocking in the future have also been proposed (Kennedy et al., 2016).

As part of this debate, it has been proposed that several weather extremes in recent decades were associated with a quasi-stationary, quasi-resonant wave pattern. This results from the interaction of climatological waves that are perpetually forced by orography (mountain geography) and land-sea contrasts with transient meanders of the jet stream (Petoukhov et al., 2013), given a set of favourable conditions (White et al., 2022). Petoukhov et al., (2013) also hypothesised that Arctic amplification and the associated weakened, wavier jet may provide increasingly favourable conditions for the occurrence of quasi-resonance. This can result in circulation features which accelerate regional extreme weather occurrence trends – for example, heatwave trends in Europe (Rousi et al., 2022), although the direction of causality is debated (Wirth and Polster, 2021). If recent extreme events are indeed associated with a resonance mechanism that only kicks in when the jet crosses a certain threshold in waviness, a tipping point might be involved. However, it is uncertain whether this would be associated with hysteresis and irreversibility or would just be a reversible, but abrupt, shift of the atmosphere towards enhanced large-amplitude mid-latitude waves.

More generally, there is no robust evidence that continued climate change and Arctic amplification will lead to a tipping towards a wavy-jet state, systematically higher amplitude and/or more frequent planetary waves, or blocks. Equally, there is no robust evidence that these hypothetical changes would be self-sustaining. Indeed, while a number of large changes in atmospheric dynamical features may occur under climate change, these are typically discussed as gradual changes, without explicit hysteresis or tipping behaviour. Similarly, there is no robust evidence pointing to tipping-like behaviour in the jet stream’s latitudinal location, although gradual, long-term shifts may occur. 

It should nonetheless be noted that atmospheric circulation responses to climate change are characterised by large model uncertainty and are possibly biased by the relatively low resolution of global climate models compared to, for example, weather-prediction models (Shepherd, 2019). In addition, some climate models show that tipping behaviour in atmospheric blocking, in the form of a self-sustaining, feedback-driven shift, is possible (Drijfhout et al., 2013).

Assessment and knowledge gaps

Although theoretically possible, there is thus no robust evidence for tipping point behaviour in mid-latitude atmospheric circulations in the near future. At the same time, a number of relevant physical processes are currently debated or ill-constrained. We thus evaluate, with low confidence, the mid-latitude atmosphere as not displaying tipping points. 

The mid-latitude large-scale circulation itself may, though, still affect or be affected by tipping behaviour of other components of the Earth system to which it is coupled, such as the land surface, overturning ocean circulations (e.g., Orihuela-Pinto et al., 2022) or high-latitude cryosphere. Indeed, such interactions can lead to abrupt climate shifts. A recent example is the transition to hotter and drier conditions in inner East Asia, resulting from drier soils, a strengthened land-atmosphere coupling, and a contribution from large-scale circulation anomalies (Zhang et al., 2020). Furthermore, joint non-tipping changes in mid-latitude atmospheric dynamics, the associated surface climate, and other components of the Earth system, may lead to tipping point behaviour, for example in vegetation (Lloret and Batllori, 2021). This could in turn feed back onto the atmospheric circulation.

Due to such feedbacks and interactions between the atmospheric circulation and other components of the Earth system, and due to its role in weather and climate extremes, an improved understanding of the physical processes underlying changes in mid-latitude atmospheric dynamics under recent and future climate change appears pivotal in a tipping point context. Large model uncertainty in projecting abrupt regional atmospheric circulation changes conditioned by changes in the ocean, cryosphere or land surface would lend itself eminently for a storyline approach (Zappa and Shepherd, 2017). Tipping of atmospheric circulation, and associated weather extremes, would then be conditioned by threshold behaviour in other, connected systems. 

Finally, we argue for the need to investigate whether recent, record-breaking weather extremes can be explained by the slowly changing likelihood distribution that belongs to the last decades, or whether they are signs of abruptly changing likelihood distributions. Such a shift in the distribution of extremes could be diagnosed using extreme value theory. Although a shift cannot be associated with a global tipping point, it would suggest that the extreme value distribution of (a) certain type(s) of extreme weather did witness regional tipping, whether or not reversible, in the sense of a large nonlinear change in response to a small and gradual change in forcing, potentially driven by self-sustaining feedbacks.