Expansion of the Hadley Cell under Global Warming: Winter versus Summer

A scaling relationship is introduced to explain the seasonality in the outer boundary of the Hadley cell in both climatology and trend in the simulations of phase 3 of the Coupled Model Intercomparison Project (CMIP3). In the climatological state, the summer cell reaches higher latitudes than the winter cell since the Hadley cell in summer deviates more from the angular momentum conserving state, resulting in weaker upper-level zonal winds, which enables the Hadley cell to extend farther poleward before becoming baroclinically unstable. The Hadley cell can also reach farther poleward as the ITCZ gets farther away from the equator; hence, the Hadley cell extends farther poleward in solstices than in equinoxes. In terms of trend, a robust poleward expansion of the Hadley cell is diagnosed in all seasons with global warming. The scaling analysis indicates this is mostly due to an increase in the subtropical static stability, which pushes poleward the baroclinically unstable zone and hence the poleward edge of the Hadley cell. The relation between the trends in the Hadley cell edge and the ITCZ is also discussed.