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

Abstract

Purpose of Review 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 Findings 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. Summary 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.

The Future of Midlatitude Cyclones

“…The footprints of extreme precipitation in an idealized ETC (defined as 99.9th percentile) have been found to increase in size, including the size of coherent regions of extreme precipitation [48]. A broadening of the strong wind footprint in idealized ETCs has also been identified with cyclone-centric compositing [49]. Changes in the size of ETCs in idealized warming simulations appear to be sensitive to the method used to define size, with hardly any change seen in one study [47], and a large decrease in size in another [41].…”

“…The opposing response of intense ETC frequency in the NH versus the SH is consistent with the projected changes in the 850-hPa equator-to-pole air temperature gradient (i.e., a decrease in the NH and an increase in the SH [86]). For a more in-depth analysis of the regional climate impacts of changes in ETC winds and their uncertainties, the reader is directed to another review article in this issue [87].…”

“…The IPCC AR5 and the IPCC Special Report on Extremes (SREX [14]) mostly considered changes to the ETCs, and precipitation and winds separately. Nevertheless, two studies [15][16][17] cited in the AR5 regional projections chapter [18] showed cyclone-related precipitation increases with generally no increase in wind speed strength. It was also noted that due to many different ways of defining storm intensity, there is "little consensus" on how this might change.…”

Role of the mean state for the Southern Hemispheric jet stream response to CO2 forcing in CMIP6 models

“…In response to CO 2 forcing, climate models commonly predict a poleward shift of the midlatitude jet streams, particularly in the Southern Hemisphere (SH) (e.g. Kushner et al 2001, Kidston & Gerber 2010, Barnes & Polvani 2013, Ceppi et al 2014, Grise & Polvani 2016, Quantifying the magnitude of this poleward shift is important because of the associated large-scale climate impacts in terms of temperature and the hydrological cycle (Thompson 2011, Kang et al 2011, Zappa 2019. Through wind stress coupling with the ocean circulation, changes in the position of the SH jet can also have global impacts, for example via changes in overturning, air-sea carbon exchange, and Agulhas leakage (Anderson 2009, Biastoch et al 2009, Abernathey et al 2011, Durgadoo et al 2013.…”

Environmental catastrophes, climate change, and attribution

“…B; see also Ref. ). Thus, different storylines of precipitation change might be reasonably considered.…”