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

Other literature type English OPEN
Nazario, Tartaglione (2016)

The time-dependent response of the Hadley circulation to a periodic forcing is explored by using 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 <i>t<sub>0</sub></i>. Some simulations were performed with different values of <i>t<sub>0</sub></i>, from 10 to 90 days, they exhibit a stronger circulation when comparing with constant thermal forcing experiment. As the period increases, a transition takes place from a stationary regime, obtained when forcing is constant, to a quasi-periodic regime, and to an intermittent regime. The stream-function response to periodic forcing is a quasi-periodic oscillation, with two main frequencies dominating, one with a period equal or close to that of forcing and another with a period that is half of the forcing period. The former is dominant for values of <i>t<sub>0</sub></i> larger than 30 days, whereas the latter is prevalent for <i>t<sub>0</sub></i> smaller than 30 days. The quasi-periodic oscillations obtained in this model might be associated with the quasi-periodic oscillations observed in the tropical regions. In this case the periodic charge and discharge of moisture in the tropical atmosphere may be linked to those oscillations. In the model, with forcing periods over 63 days the response of stream-function periodically enters in a sort of intermittent regime, with chaos appearing with high frequency oscillations, which are modulated by the slow timescale of forcing. The vertical viscosity plays a role in determining even the evolution of the Hadley circulation under the conditions established by the forcing.
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