Role of the seasonally-oscillating Hadley Cell in interhemispheric mixing

Abstract

The contribution of the seasonally-oscillating Hadley cell to the interhemispheric mixing of trace chemical species is studied using a zonally-symmetric kinematic model of the Hadley cell flow based upon the first steady-state and first transient modes of a Fouiier fit to the observed tropical wind fields. A two box model is used as a standard of comparison. The chaoticity of the Hadley cell model flow for small perturbations is proven. Numerical experiments are performed for an array of steady state and transient mode amplitudes in order to gain further insight to the chaos/mixing of the flow. Poincar6 sections illustrate the qualitative general mixing behavior. Residence time analysis gives quantitative measures of average parcel behavior for the various cases, while transport experiments give quantitative measures of mixing and transport for the large scale motions. Transfer experiments are performed in all parameter scenarios for cases with and without a northern hemisphere source. Transfer results show remarkable similarity to the two box model solution behavior and a direct fit of box model solutions to Hadley cell transfer results reveals the mixing time-scale. All experiments show that mixing increases when steady-state amplitude decreases and/or transient mode amplitude increases. The case with the observed modal amplitudes is found to be in a region of good mixing in the parameter space. The mixing time-scale for this case is found to be about 4.5 months which is faster than observations. Further research will be necessary to determine the reasons for this seemingly over-efficient mixing. However, results definitely indicate the seasonally-oscillating Hadley cell flow to be a significant contributor to the interhemispheric mixing.