Abstract
A model of the tropical and subtropical atmosphere which is suitable for long-term integrations is pre-sented. The model includes both the Northern and Southern Hemispheres and is coupled to a mixed layer ocean. The atmosphere is assumed to consist of two longitudinally independent cells, one on either side of the intertropical convergence zone (ITCZ). The model equations are integrated over large grid domains so that rapid integration on a computer is possible. The size of the grid box is such that the processes acting in the domain are uniform. The grid boxes considered are 1) the boundary layer inflow region, 2) the ITCZ convective region, 3) the outflow region, and 4) a subsidence region. The mixed layer ocean has variable depth and temperature. Fluxes of heat, momentum and moisture to the mid-latitudes by longitudinally dependent eddies are parameterized. Evaporative and sensible heat fluxes from the surface are included. Condensation occurs in the ITCZ. Solar and terrestial radiative diabatic effects are predicted by use of a slab radiation model. Seasonal variations of the solar heating are included. Advection by the meridional circulation, Coriolis torques, surface friction, eddy momentum fluxes, and a solenoidal term form the dynamic effects. Radiative parameters, mid-latitude conditions, and the deep-water temperature are not predicted by the model.
Small changes in the solar constant lead to an exponential response with an e-folding time of 2.25 years. This long-time response is due to the effect of the mixed layer ocean and the fact that negative feedback conditions reduce by 84% any change in the solar beating reaching the surface. A change in the solar con-stant of ±2.63% leads to a =1.1 K and −0.96 K change in the lower level temperature respectively. The surface temperature change amplifies with height by a factor of 2 due to changes in the moisture content of the atmosphere.
An experiment in which the potential temperature gradient used to calculate the eddy flux of potential temperature to mid-latitudes was increased 2.2 10−3 K km−1 yielded a lower level temperature change of −1.4 K, which was amplified to −3.8 K, in the ITCZ and to −4.6 K in the upper layer. The ocean temperature decreased 0.9 K. The circulation changed very little.
