Regional Climate Impacts of Future Changes in the Mid–Latitude Atmospheric Circulation: a Storyline View

Atmospheric circulation exerts a strong control on regional climate and extremes. However, projections of future circulation change remain uncertain, thus affecting the assessment of regional climate change. The purpose of this review is to describe some key cases where regional precipitation and windiness strongly depend on the mid-latitude atmospheric circulation response to warming, and summarise this into alternative plausible storylines of regional climate change. Recent research has enabled to better quantify the importance of dynamical aspects of climate change in shaping regional climate. The cold season precipitation response in Mediterranean-like regions is identified as one of the most susceptible impact-relevant aspects of regional climate driven by mid-latitude circulation changes. A circulation-forced drying might already be emerging in the actual Mediterranean, Chile and southwestern Australia. Increasing evidence indicates that distinct regional changes in atmospheric circulation and European windiness might unfold depending on the interplay of different climate drivers, such as surface warming patterns, sea ice loss and stratospheric changes. The multi-model mean circulation response to warming tends to show washed-out signals due to the lack of robustness in the model projections, with implications for regional changes. To better communicate the information contained within these projections, it is useful to discuss regional climate change conditionally on alternative plausible storylines of atmospheric circulation change. As progress continues in understanding the factors driving the response of circulation to global warming, developing such storylines will provide end–to–end and physically self-consistent descriptions of plausible future unfoldings of regional climate change.


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In the last decade consensus has started to grow on how atmospheric circulation will respond to 32 global warming [1]. On average, climate projections from multi-model ensembles indicate an over-33 all poleward shift of the mid-latitude westerlies [2], associated with a poleward expansion of the precipitation and wind speed extremes [7]; it stirs the transport of moisture from dry to wet regions 42 [8]; it drives hot extremes in summer and cold extremes in winter through the establishment of 43 persistent anticyclones [9]. As such, atmospheric circulation change can have a diverse range of 44 societal impacts. 45 Despite its potential to drive climate impacts, projections of circulation change have not yet 46 translated into high confidence statements on regional climate change [10]. This lack of confidence 47 depends on multiple causes. Zonal-mean aspects of circulation change, such as the Hadley cell 48 expansion, are not sufficient to constrain the response of regional climate over land [11,12]. At the 49 regional and seasonal scale the uncertainty in how atmospheric circulation responds to warming 50 remains large, to the extent that different models can even show opposite forced responses [13]. 51 It would be tempting to treat these uncertainties probabilistically and to take the multi-model 52 mean as a best projection, but such an approach is not supported on firm theoretical grounds [14]. 53 While the multi-model mean usually outperforms individual models in global metrics of climate, 54 this is not typically the case for regional aspects of atmospheric circulation, which can be better 55 represented in some individual climate models than in the multi-model mean [15]. Different features 56 of regional circulation change tend to be averaged out, leading to an overly smooth and possibly 57 too weak signal. 58 Given this uncertainty in the response of regional climate to global warming, the development of 59 storylines, or narratives, of climate change has been proposed as an informative way to characterise 60 and communicate future climate projections to stakeholders and policy makers [16,17]. By storyline 61 is meant a possible and physically self-consistent future unfolding of global and regional climate 62 events. In a storyline approach, multiple storylines are identified in order to span the uncertainty in 63 the future projections from multi-model ensembles. However, the focus is not placed on attributing 64 a probability to the different storylines but on understanding the driving physical factors, the chain 65 of mechanisms involved and the implications at the regional level. 66 This report aims to review the future impacts that might unfold from atmospheric circulation 67 change. Thinking in terms of storylines therefore becomes particularly useful and it naturally leads 68 to frame the problem in terms of the following question: what additional information could be gained 69 at the regional level if the response in the large-scale atmospheric circulation were known? After 70 reviewing some methodological aspects (section 2), this question will be discussed for a selection 71 of regional climate impacts associated with changes in precipitation (section 3) and windiness 72 (section 4). For each case, based on published literature, different plausible storylines of atmospheric 73 circulation change will be analysed. Conclusions are presented in section 5. 74 2 How to identify regional impacts of mid-latitude circulation change 75 Developing physically self-consistent storylines of atmospheric circulation change relies on having 76 a causal understanding of the chain of mechanisms involved. Achieving this understanding requires 77 Title Suppressed Due to Excessive Length 5 tackling two separate problems. At a global level, the challenge lies in understanding what climate 78 aspects, e.g. sea surface temperature patterns and sea ice, drive the uncertainty in the regional 79 response of atmospheric circulation. Identifying such drivers ultimately requires numerical exper-80 imentation [18]. Furthermore, at the regional level, an additional challenge lies in understanding 81 the impacts of the response of atmospheric circulation for regional climate change. This requires 82 separating the other aspects of regional climate change that directly result from energy imbalance 83 and surface warming, often called thermodynamic aspects [19]. A clean separation is generally 84 not possible. The different methods either attempt to directly quantify the regional changes due 85 to circulation, or, conversely, to quantify the thermodynamic response expected for no change in 86 circulation, and then define the dynamical part as a residual. Some of these approaches are now 87 discussed. 88 2.1 Internal variability analogs 89 The response of atmospheric circulation to global warming can resemble, or even project on, present-90 day modes of internal atmospheric variability [20]. In this case, the impacts that future changes 91 in the atmospheric circulation might have on the regional climate can be directly estimated by 92 identifying analogs of the projected circulation change in the present-day observational record. By 93 referring back to the present-day climate, any thermodynamic influence is by construction excluded. 94 This approach has been implemented in several ways.  [35] and of the energy budget [36] to understand variability and change in regional hydro-climate.

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The moisture budget equation directly informs on the change in the balance between precipi-118 tation and evaporation (P-E) as, in steady state, P-E depends on the transport of moisture from 119 other regions. Part of the impact of the circulation response to warming on P-E can be estimated -120 assuming linearity -as the change in the moisture transport due to the response in the time-mean 121 winds (δv) acting on the present-day climatology of moisture (Q p ), i.e. −∇ · (δv Q p ) dz. This  Finally, experimental approaches can be used to isolate the relative impacts of atmospheric cir-135 culation changes and warming on regional climate. Regional climate models (RCMs) have been 136 particularly useful for this purpose. One such example consists in the so-called "pseudo global 137 warming" experiments [40], in which a warming signal is added to the boundary conditions driving 138 a present-day RCM simulation. By construction, the approach isolates the response of regional  In summary, different methods have different strengths and limitations. No single approach is 151 able to globally and unambiguously define the impact of future circulation changes on regional 152 climate, but confidence can be built by comparing results from different approaches.

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3 Impacts of circulation change on regional hydro-climate 154 What more could be learnt on regional hydro-climate change if the response of atmospheric circu-  The Mediterranean area has long been identified as a "hot-spot" of climate change [49], due to a 166 large projected decline in precipitation, which is of the order of 6% per degree of global warming in 167 the mean of the CMIP5 model projections [1]. Furthermore, a reduction in Mediterranean precipi-

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Title Suppressed Due to Excessive Length 9 tation since 1900 is also revealed by reconstructions from rain gauges. This has led many authors to 169 conclude that the projected precipitation decline and increase in meteorological droughts is already  Germany) more than 50% of the mean response and inter-model spread in the precipitation change 230 can be attributed to SNAO. Consistent with these findings, the projected precipitation reduction 231 in Southern Europe tends to be comparable in the two sets of CMIP5 climate models featuring the 232 smallest and largest poleward shift in the North Atlantic jet (Fig. 1c-d). 233 These apparently contrasting results can be reconciled in light of the additional warming-234 mediated processes that contribute to the response of precipitation in the warm season, particularly 235 in Southern Europe [44]. As the land warms, the soil is projected to become drier, leading to a 236 reduction in evapotranspiration and in the surface relative humidity. These local changes conse-237 quently lead to a reduction in clouds and precipitation, which may further enhance the aridity of the in part explain this mismatch [71,72,22], but it has also been speculated that the negative SNAO 251 trend could be a forced response to sea-ice loss not captured by climate models [70]. As the AMV 252 is now entering a phase reversal [73], new observations will help to evaluate the respective roles 253 played by SST variability and sea-ice loss. the Aleutian low and a stronger southerly flow on the western coast of North America [15]. In this 279 scenario, the shift in the storm-track activity [84] as well as the increased precipitation generated 280 by each storm [75] can be expected to make California more rainy under climate change (Fig. 1e).

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The alternative storyline (Fig. 1f)  appear to be important for the hydro-climate response in this region [48]. in the teleconnection of ENSO [89] have been suggested to play a role, although these analyses   Based on these results, and since changes in the stationary waves are less important than in the 341 NH [48], developing storylines of regional climate change will require accounting for the response in   Here, the focus is instead placed on discussing the uncertainty in the response, using future changes 366 in European windiness as a case study.  Finally, European windiness also depends on additional uncertainties acting on the cyclone scale.

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For a given atmospheric flow, the thermodynamic increase in the cyclone-associated precipitation 422 is expected to enhance cyclone growth and propagation speed [135], although this pathway is partly 423 balanced by the increase in atmospheric stratification [136]. The impact of enhanced latent heat 424 release on cyclone growth is unlikely to be fully resolved at the spatial resolution of current climate 425 models [137], thus highlighting the value in studies employing high-resolution models to explore sufficiently large that the magnitude, and sometimes even the direction, of these regional climate 450 trends cannot yet be anticipated, even for a specified level of global warming ( Fig. 1 and Fig. 2).

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To characterise and communicate this uncertainty, it can be useful to identify different physically 452 self-consistent storylines of how atmospheric circulation and regional climate could respond to Based on their atmospheric circulation response in the RCP8.5 scenario, the following CMIP5 479 climate models have been identified to produce the panels in Fig. 1 and Fig. 2: