Dynamics of the Hadley circulation in an axisymmetric model undergoing stratification periodic forcing

Abstract. The time-dependent response of the Hadley circulation to a periodic forcing is explored via a simplified nonlinear axisymmetric model. Thermal forcing towards a given equilibrium potential temperature drives the model atmosphere. The vertical stratification of this temperature is forced to become periodically neutral with a period t₀. Simulations performed with values of t₀ ranging from 10 to 90 days exhibit stronger circulation compared to the results of a constant thermal forcing experiment. As the period increases, a transition occurs first from a stationary regime, obtained when forcing is constant, to a periodic (and possibly quasi-periodic) regime, and then to an intermittent regime, albeit one with a strong periodic component. The stream-function response to periodic forcing is generally a periodic oscillation, with two main frequencies dominating: one with a period equal or close to the forcing period and another with a period that is half of the forcing period. The former is dominant for values of t₀ larger than 30 days, whereas the latter is prevalent for t₀ smaller than 30 days. The periodic oscillations obtained in this model may be associated with the periodic oscillations observed in the tropical regions. In this case the periodic charge and discharge of moisture in the tropical atmosphere, with consequent change of stratification, may be linked to those oscillations. In the model, at forcing periods of over 63 days the response of the stream function periodically enters into a quasi-intermittent regime, exhibiting high-frequency chaotic oscillations that are modulated by the slow timescale of forcing. Sensitivity experiments for model parameters and configuration were performed to check whether results obtained are still valid under different conditions. Although for small changes of parameters the results are still valid, when parameters depart from the prescribed ones, we can observe change in the Hadley circulation dynamics.