Observed increases in radius as Southern Hemisphere surface cyclones evolve

Recently, some regional or short-term studies have indicated that surface cyclones in the NH increase in size as they develop. We explore the extent to which this is true for SH systems and extend the considerations in both the space and time domain. We have applied a state-of-the-art cyclone tracking scheme to the global 'reanalyses' produced by the National Centers for Environmental Prediction for the four-decade period 1958-1997. We particularly focus on cyclones which halfway through their lifetime are located in the 30-50oS and 50-70oS latitude bands in both winter and summer.

This comprehensive study shows that the radius of SH cyclonic systems does indeed increase as they evolve to maturity. This result holds for the two baroclinic domains considered and in both winter and summer. In winter, the mean radius of cyclones in the two latitude belts peak is reached at about day 6 after first identification, while the scale increase is about 20% over the period. In the mean, the summer peak occurs a little later.

As has been pointed out in other studies, these findings over the SH represent a significant departure from an assumption built into current theories of cyclone development.