The severe heatwave affecting Western North America, recently, has been described in the media as a heat dome, but what does this mean? And how does one form? Also, what role may climate change have played in this heatwave?
Each hemisphere of the earth’s atmosphere is divided into layers of cell systems that wrap around the planet broadly corresponding to a tropical layer, mid-latitude layer and a polar layer. Within each cell, air circulates as warm air rises, cools and then sinks back to the surface. Between the layers are narrow bands of strong winds, called jet streams, that act like ‘rivers’ of atmospheric circulation and control the location of storm tracks and fronts that bring rain (Barnes et al., 2015; Trouet et al., 2018). Typically there is a tropical jet stream at around 30° and a polar jet stream at around 60°, though they migrate, depending on the temperature gradient between the equator and the pole.
The shape of the jet stream and the strength of the winds vary as the meridional temperature gradient increases or decreases. Climate change is warming the global average temperature, but the Arctic is warming faster than lower latitudes, known as Arctic amplification (Box et al., 2019; Smith et al., 2019; Cohen et al., 2020). This reduces the meridional temperature gradient, which weakens the jet stream allowing Rossby waves, also known as planetary waves that are formed due to the Earth’s rotation, to bend it into a meandering or wavy shape (Cohen et al., 2014; Francis et al., 2015; Coumou et al., 2018). The meanders mean warm, dry air moves northwards into the peaks, while cold, moist air moves southwards into the troughs (Kornhuber et al., 2019).
Stalling and blocking
Warm, dry Rossby wave peaks in the jet stream can accentuate into enclosed high pressure systems, which intensity and stall as wave meandering slows. These stalled high-pressure systems are known as blocking highs, because they prevent new storm tracks and low pressure systems moving through with new jet stream meanders. Blocking highs over the Ural Mountains, Russia, and over Greenland have been associated with extreme weather in Asia and North America respectively (Overland et al., 2015; McLeod et al., 2016; Wang et al., 2020). Stalling or blocking has occurred more frequently in recent decades and is associated with European summer heatwaves in 2003, 2006 and 2015 (Kornhuber et al., 2019).
The heat dome discussed in relation to late June 2021’s heatwave is probably an omega blocking high, formed as an isolated Rossby wave peak in the jet stream. The intense high pressure system extends high up into the atmosphere, trapping warm air underneath like the lid on a saucepan and further intensifying the heat through both subsidence and compressional heating. The high pressure prevents clouds from forming, which also adds to the accumulation of heat.
The impact of climate change on heatwaves is clear, with many attribution studies (Christidis et al., 2014, 2016; Kam et al., 2016; Horton et al., 2016; Imada et al., 2018; Vogel et a., 2019; Ciavarella et al., 2020; Yiou et al., 2020; Vautard et al., 2020) demonstrating that climate change has increased the probability of heatwaves occurring and that their frequency, intensity and duration have risen.
However, the impact climate change has had on the system described above is still the subject of scientific debate, with competing hypotheses emerging from observations and modelling simulations and uncertainty over how physical processes are responsible (Screen et al., 2018; Blackport et al., 2020a, 2020b, 2021). This is partly due to the large natural variability acting as noise that historically has been hard to isolate from climate change signal (Francis et al., 2017). However, some studies suggest this signal is close to emerging, or recently has emerged (Mann et al., 2017; Trouet et al., 2018). Observations show that jet stream variability has increased in recent decades, coinciding with greenhouse gas emissions and Arctic amplification (Tang et al., 2014; Trouet et al., 2018).
Climate change is already making heatwaves and other extreme weather more dangerous and this will worsen as the planet continues to warm. Recent extreme temperatures in Western North America are challenging scientific views on the extent of this influence and scale of future increases. New research suggests significant increases in the probability and frequency of future record-breaking extreme weather events are projected to occur later this century (Fischer et al., 2021).
All of the citations listed in this article can be explored in the ScienceBrief on heatwaves or the ScienceBrief on Arctic amplification. There you can find more detail on the mechanisms and physical processes discussed, with insights from observations and climate modelling.
Upcoming ScienceBrief Reviews will provide a more detailed account of both of these topics and these will be published in due course.