Heating distributions and Hadley circulation in an axisymmetric model

This thesis studies the impact of the equilibrium temperature distribution θE, on the Hadley circulation simulated by an axisymmetric model, a bi-dimensional model of the tropical atmosphere. The θE distribution, which represents the thermal forcing of the model is modulated here by two parameters, n and k, the former controlling its horizontal broadness and the latter controlling the vertical stratification of θ E . In the present study, variations in the θE distribution mimic changes in the energy input of the atmospheric system leaving as almost invariant the Equator-poles θE difference. The results give evidence that concentrated θE distributions enhance the meridional circulation and jet wind speed intensities even with a lower energy input. The meridional circulation and the subtropical jet stream widths are controlled by the broadness of horizontal θ E rather than by the vertical stratification, which is important only when the temperature distribution is concentrated at the Equator. The jet stream position does not show any dependence with n and k, except when the θE distribution is very wide and in such a case the jet is located at the mid-latitude and the model temperature clamps to forcing θE. Using n=2 and k=1 we have the formulation of the potential temperature adopted in the classical literature. A comparison with other works is performed and our results show that the model running in different configurations (equinoctial, solstitial and time- dependent) yields results similar to one another. When adiabatic forcing, corresponding to k = 0, is applied to the model, the symmetric circulation exhibits a chaotic response due to triggering of explicit convection. However the time averaged meridional circulation is still a Hadley circulation that appears more similar than that obtained in the previous experiments and in literature. Adiabatic forcing is equivalent to force the model atmosphere towards the neutral stability. In order to examine the response of the Hadley circulation to periodic changes of vertical stratification, the parameter k was set as a periodic function in such a way we can force the model atmosphere with convection that starts periodically. The response of the model was that a quasi periodic circulation is triggered by the periodic forcing, with two well defined periods, one related to the periodicity of forcing and another with half of forcing period. This periodic behavior will change when the period of k is greater than 60 days, when a chaotic behavior start periodically lasting for a while, whereas the rest of time the Hadley circulation solution is steady. Another important aspect is how the relaxation time, i.e. the velocity with which θE tends to model potential temperature θ . We will assume that the relaxation time is dependent on space and in order to investigate the effect of the space distribution of thermal relaxation time on the strength of the winter cell, the zonal wind, the temperature. A low value of relaxation time at mid-latitudes is more effective in enhancing the tropical circulation than a low value of relaxation time located in the same position of maximum heating.