# Variability in the width of the tropics and the annular modes

by on May 28, 2013

Kidston et al. Geophysical Research Letters, 2013.

It has been observed (Kang and Polvani 2011) that the variability in the location of the edge of the Hadley Cell ($phi_{HC}$) is correlated with the latitude of the storm-tracks and surface westerlies ($phi_{EDJ}$; EDJ is the Eddy Driven Jet), particularly in the Southern Hemisphere during Austral Summer (DJF).  This has been observed in both reanalyses and GMCs.

In this paper, the authors investigate this observation in an idealized climate model. They show that:

a) The variability in $phi_{EDJ}$ (The Annular Modes) is driven by changes in the eddy momentum flux convergence ($-partial_y bar{u^prime v^prime}$).

b) As expected from quasi-geostrophic balance*, the eddy momentum flux convergence anomalies are balanced by the coriolis torque of an anomalous meridional flow. For a poleward excursion of the EDJ, there is a poleward flow induced in the upper troposphere on the equator-ward side of the time-mean location of the EDJ.

c) If the edge of the Hadley Cell lies within the region of this flow, the zero point of the meridional flow associated with the Hadley Cell is “pulled polewards,” leading to the observed correlation.

As the climate of the model is changed to be more winter like, the separation between the edge of the Hadley cell and the EDJ increases and the magnitude of the induced meridional flow near the Hadley cell’s edge decreases.

*QG Balance. Zonal mean momentum equation

$partial_t bar{u} = -partial_ybar{u^prime v^prime} + f bar{v}$

on monthly time scales, the LHS is approximately zero, so the two terms on the RHS must approximately balance.

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