Hadley Cell Expansion
As global temperatures rise, the temperature difference between the poles and the equator is likely to decrease, expanding the cell of air circulation adjacent to the equator known as the Hadley cell. One effect this has is that mid-latitude regions like the Mediterranean and the Southwestern US are likely to see an increase in sea level pressure—which corresponds to drier weather.
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Climate science at a glance
- Hadley cell expansion is one of the most well-established atmospheric responses to global warming.
- The Hadley cell has expanded toward the poles since the 1980s.[1]
- Human emissions have contributed to Hadley cell widening, especially in the Southern Hemisphere.
- Hadley cell expansion leads to crop lands in the subtropics experiencing drier climate conditions.
Background
What is the Hadley cell?
The Hadley cell, or Hadley circulation, is a worldwide tropical atmospheric circulation pattern that transports energy from the tropics to the subtropics (usually between the 20 and 40° latitude lines in both hemispheres ). The cell develops in response to intense solar heating near the equator.[2] Warm air near the equator rises and flows toward the poles and then cools off, descending, and flowing back toward the equator.
The Hadley cell controls precipitation in the subtropics and it creates a region called the intertropical convergence zone (ITCZ), which produces a band of intense and wet storms.
Why do we care about Hadley cell expansion?
Hadley cells have a major effect on weather patterns in subtropical regions, including portions of the southern United States. The key concern is that widening of the Hadley circulation would cause a poleward shift of the subtropical dry zone (which is located where air in the Hadley cell descends). This could lead to regional rainfall reductions and the drying of subtropical landmasses, which has major implications for crop productivity. Regions that face impacts in the Northern Hemisphere include southern Europe, the Mediterranean region and northern Africa. In the Southern Hemisphere, at risk regions including South Africa, central Chile, and southern and southwestern Australia.
Hadley cell expansion trends and climate change
- (Grise and Davis, 2020; Studholme and Gulev, 2018): The annual mean Hadley cell extent has shifted poleward at an approximate rate of 0.1°–0.5° latitude per decade over the last about 40 years.
- (Grise et al., 2018; Hu Y. et al., 2018; Staten et al., 2018): There is likely a larger seasonal widening in the Hadley cell for summer and autumn in each hemisphere.
- (Huang et al. 2019; D'Agostino and Lionello, 2016): Hadley cell winds may be intensifying.
Studies attribute Hadley cell expansion to climate change
- (Jebri et al., 2020, Staten et al. 2020, Grise et al., 2019): Modeling studies find that human greenhouse gas emissions contributed to the observed widening of the Hadley Cell, especially in the Southern Hemisphere.
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