Have the Southern Hemisphere storm tracks been strengthening after 1979?
Based on IPCC AR4 model simulations, storm track activity, especially over the SH, is expected to shift poleward and intensify slightly in the upper troposphere under various global warming scenarios. However, none of the AR4 simulations that we have examined show significant SH storm track increase between 1979 and 2000, even though some simulations do suggest a slight poleward shift during this period. Thus assessing whether there has been a significant trend in storm track activity between 1979 and 2000 is important to see whether AR4 climate simulations are able to reproduce past climate variability and trends, as well as to assess whether the increasing trend in storm track activity predicted by AR4 models may represent an underestimation.
Currently, we are analyzing diagnostic fields from the reanalysis datasets to examine whether the increasing trend (or its absence) is consistent with various climate diagnostics. Preliminary results based on examination of energetics show that while eddy kinetic energy (EKE) shows an increasing trend in the NCEP-NCAR reanalysis data, the EKE source terms (baroclinic and barotropic conversion) do not show a consistent trend, suggesting that the NCEP-NCAR reanalysis trend may be internally inconsistent. Other diagnostics, such as reanalysis generated precipitation, clouds, and radiation fields, are being examined and compared with available observational data. By the time of the conference we should have a more definitive assessment of whether the trend found in the NCEP-NCAR reanalysis is realistic or not.