The descending branches of the Hadley and Walker circulations create extensive dry zones over the subtropics and over the eastern equatorial Pacific. These dry zones are regions where the greenhouse effect in the atmosphere is substantially reduced and the outgoing long-wave radiation has its maximum value. This study investigates the role of these dry zones in determining the climate sensitivity of the tropics. I

Using NCAR CCM3, we have examined the response of the humidity and OLR over the tropical dry zones to a localized increase in the western Pacific warm-pool SST as well as to an increase in the global SST whose spatial pattern resembles the typical model response to a doubling of CO2. In response to a localized increase in the SST of the warm-pool, water vapor in both the subtropical and the equatorial eastern Pacific dry zones is found to decrease in response to the increase in the warm-pool SST. An analysis of the budget suggests that the drying is largely due to an enhanced subsidence. This result supports the notion that the water vapor in the tropical dry zones may act as a negative feedback that helps to stabilize the tropical maximum SST. However, in response to an increase in the global SST that is typical of the model response to a doubling CO2, water vapor in the dry zone actually increases, though the rate of increase is smaller than in regions with deep convection. This is because the strength of the subsidence is insensitive to global warming. Enhanced latent heating is found to be largely balanced by local changes in the adiabatic and radiative cooling. These results suggest that water vapor feedback may damp local fluctuations in the warm-pool SST, but plays a positive feedback in the global warming situation.