A Two-Box Model of a Zonal Atmospheric Circulation in the Tropics

A simple fixed-SST model of a zonal circulation in the tropical atmosphere has been developed that has separate boxes for the ascending and descending branches of the atmospheric circulation. This circulation resembles the Walker circulation. This is the first box model to determine the fractional widths of the warm and cold pools. The atmospheric model contains an explicit hydrologic cycle, a simplified but physically based radiative transfer parameterization, and interactive clouds. Results indicate that the intensity of the tropical circulation is crucially dependent on the amount and vertical distribution of water vapor above the cold-pool boundary layer (CPBL). In response to increasing precipitable water over the CPBL, the radiative cooling rate of the free troposphere increases. To a good approximation, subsidence warming balances radiative cooling in the subsiding branches of the circulation. If the fractional width of the cold pool (CP) does not change too much, the circulation must intensify as the subsidence rate increases. To compensate for a stronger circulation and to restore energy balance in the Walker cell, the precipitable water over the warm pool (WP) must decrease. A ‘‘moist-outflow’’ experiment shows that the Walker circulation intensifies if air is advected to the subsiding regions from lower altitudes in the WP. As the advection level decreases, air supplied to the CP becomes warmer and moister, and so the column water vapor in the CP free troposphere increases. The mechanism described above then leads to a strengthening of the circulation. This moist-outflow experiment also shows that when the authors try to moisten the atmosphere by specifying a lower advection level for water vapor, the atmosphere adjusts so as to dry out. This effect is very strong.