Interhemispheric Exchange by Seasonal Modulation of the Hadley Circulation

Abstract

Observations of long-lived tracers indicate that the time required to mix tropospheric air between the Northern and Southern Hemisphere extratropics is on the order of 1 yr. Mixing within the extratropical troposphere is known to be quite efficient, with timescales of a few months, in large part due to effective mixing by Rossby waves and extratropical cyclones. The mechanisms for transport through the Tropics, however, are not well understood, and general circulation models have reproduced the observed interhemispheric mixing time with varying degrees of success. A simple two-mode kinematic model of the zonally symmetric Hadley circulation is developed to investigate whether the seasonal oscillation of the Hadley cells could be responsible for a significant part of the interhemispheric mixing. The model explains 78% of the variance in the observed tropical climatological mean-meridional circulation. The flow has two adjustable parameters, α, which determines the strength of the time-mean part of the flow, and ?, which determines the strength of seasonal oscillation. Lagrangian trajectories in the model are shown to be chaotic in the formal sense. The mixing properties of the flow are examined for a range of αs and ?s around the observed values. The mixing timescale decreases as the amplitude of the seasonal cycle increases, and, in this parameter range, generally decreases as the strength of the time-mean part of the flow increases. For observed values of α and ?, the mixing time is on the order of 4 months. Given that time is also needed to mix tracers into the Tropics from the midlatitudes of one hemisphere, and out of the Tropics into the midlatitudes of the other, this timescale is not inconsistent with a 1-yr interhemispheric exchange time. The rapid 4-month mixing time within the Tropics indicates that the seasonal cycle of the Hadley circulation alone may account for a large part of tropical mixing. The residence times of particles in a given hemisphere roughly follow an exponential distribution, and the bulk transport can be well represented by a simple box model, providing further evidence that the mixing in the model is fast and efficient.