Synoptic Scale Atmospheric Processes and Heatwaves

This chapter explores how synoptic scale atmospheric processes that operate on a horizontal scale of 1000–2500 km and a vertical scale of up to 10 km, with a timescale of 5–10 days, influence heatwave development. Firstly, heatwaves are contextualised within larger modes of climate variability which set the broad ‘slow’ boundary conditions that influence the ‘faster’ synoptic scale processes discussed in this chapter. Three primary atmospheric processes crucial for heatwave development are highlighted namely blocking, advection, and the foehn effect. Blocking, a significant factor, involves persistent high-pressure systems that disrupt normal weather patterns, leading to prolonged periods of atmospheric stability and extreme heat. The nature of different blocking patterns is described. The significance of advection, the transport of heat and moisture by wind, for heatwave development is also outlined along with the importance of the interaction of local and large-scale atmospheric circulation with heat or moisture sources. The foehn effect, involving warm, dry downslope winds on the lee side of mountain ranges, is also discussed. On occasions these winds are found to cause rapid temperature increases and decreased humidity, contributing to heatwave conditions in several regions. Further to outlining the general synoptic processes associated with heatwave development, the chapter provides a geographical perspective on heatwave development, examining notable events in North America, Europe, Russia, the Indian subcontinent, Australia, East Asia, South America, and Africa. It underscores the regional variability of synoptic scale processes and their complex interactions with local climate settings. The chapter concludes by emphasizing the importance of understanding these processes for improving heatwave prediction.