Variability in the width of the tropics and the annular modes


Corresponding author: J. Kidston, CCRC, University of New South Wales, Sydney, Australia. (


[1] The correlation between unforced variability in the latitude of the edge of the Hadley cell (ΦHC) and latitude of the surface westerlies (ΦEDJ) is examined using a simplified moist general circulation model (GCM) and a suite of state of the art GCMs. The correlation can be determined by the time-mean separation of the two features. When the separation is small, there is a positive correlation, and as the separation between them increases, the correlation reduces. In the simplified model, a weak negative correlation emerges at large separations.

[2] The location of the anomalous meridional mass flux associated with variations in the latitude of ΦEDJ, relative to the climatological Hadley cell position, determines the extent to which ΦHC is influenced by changes in ΦEDJ. Changes in the latitude of ΦEDJ are driven by anomalous eddy momentum flux convergence, and these are approximately balanced by the Coriolis torque on the meridional flow, as expected under quasi-geostrophic scaling. Under changing time-mean climates, the anomalous flow associated with ΦEDJ variability translates location so that it is approximately fixed relative to the time-mean ΦEDJ. This means that the influence of ΦEDJ variability on ΦHC varies as a function of the time-mean separation of the features.

[3] Initial indications are that the same causal relationship holds in a suite of state of the art GCMs and that this explains the seasonal variation in the correlation between ΦHC and ΦEDJ.