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Summary of The Width of the Hadley Circulation in Simple and Comprehensive General Circulation Models by Frierson, Lu, and Chen, which appears in Geophysical Research Letters.

Primary arguments:
  • An idealized moist GCM and a full GCM over aquaplanet boundary conditions both show a similar expansion of the Hadley cell with increased mean temperatures. This expansion is also of similar magnitude to the expansion in coupled climate models with global warming.
  • The Hadley cell expansion in both models scales well with a scaling relation by Held (2000), which suggests that the increased static stability in the subtropics may be the key reason for the poleward shift.
Discussion:
One of the most consistent responses of climate models to global warming is a widening of the Hadley circulation. Since the downward branch of the Hadley cell is associated with many of the largest deserts on Earth, the poleward expansion of the Hadley cell also means a poleward expansion of the dry zones. In climate models, the poleward expansion of the Hadley cell is closely linked with the predicted drought in the Southwest US, the Mediterranean, and other locations in similar latitude bands.

In this paper, to get a better idea of what causes this poleward expansion, we examine simulations with two different models: the idealized moist model that we originally introduced in FHZ06, and a full general circulation model such as those used for climate prediction. We run these models over ocean-covered surfaces, with a wide range of ocean temperatures, from extremely cold surfaces (with global mean temperature of freezing), to extremely warm climates (with global mean temperatures of 35 Celsius, or 95 F). We also run over a wide range of pole-to-equator temperature differences, from 20 to 60 C degrees. This wide range of simulated climates gives us an idea of how the Hadley circulation can change in response to increases in temperature and temperature gradient.

Over the entire range of climates considered and in both models, there is an expansion of the Hadley cell with warmer temperatures, and also an expansion with increased temperature gradients. We find that the expansion scales quite well with a simple theory developed by Held (2000), which suggests that the increase in subtropical static stability may be the key ingredient causing the poleward shift. Increases in tropical static stability with global warming is expected, but only recently has our research identified that midlatitude static stability is also expected to increase (see Frierson (2006).

For our other work on the poleward shift of the midlatitude jet stream with global warming, see our other manuscripts (Lu et al and Chen et al). For some research on the effect of a convection scheme on the Hadley circulation, see Frierson 2007a. For another paper comparing the idealized GCM with the full GCM simulations, this time about midlatitude static stability, see Frierson 2007c.

The figure above shows the width of the Hadley circulation for 19 idealized GCM simulations (left) and 69 full GCM simulations (right). All simulations are over aquaplanet, fixed SST boundary conditions, with the global mean temperature on the y-axis, and the pole-to-equator temperature gradient on the x-axis. Each rectangle represents one simulation. The Hadley cell width increases with both mean temperature and the pole-to-equator gradient.


Full citation:
Frierson, D. M. W., Lu, J. and G. Chen. The Width of the Hadley Circulation in Simple and Comprehensive General Circulation Models. In press, Geophysical Research Letters.

A PDF download of the full paper can be found here.

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