ENSO and Genesis Potential Index in Reanalysis and AGCMs

Environmental factors influencing tropical cyclone genesis are analysed using a using a genesis potential index developed by Emanuel and Nolan (2004). Four factors contribute to the genesis potential index: low-level vorticity (850hPa), relative humidity at 700hPa, the magnitude of vertical wind shear from 850 to 200hPa and PI (potential intensity). Using monthly data of both NCEP/NCAR Reanalysis and several AGCMs (atmospheric general circulation models) we calculated the genesis potential index on a latitude strip from 60 ^οS to 60 ^οN.

Composites of the anomalies of the genesis potential index are produced for El Niño and La Niña years separately. Although the genesis potential index was developed by a statistical fitting procedure based only on the mean genesis climatology of the Reanalysis, the composite anomalies predict observed interannual variations in the observed frequency and location of genesis well, for several different basins. This justifies producing composites of modified indices in which only one of the contributing factors varies, with the others set to climatology, to determine which factor is most important in causing these interannual variations in genesis frequency. Specific factors can be identified which have more influence in different regions. For example, in El Niño years, vertical shear is important to the reduction in genesis seen in the Atlantic basin, and relative humidity and vorticity are both important to the eastward shift in the mean genesis location in the western North Pacific.

The models' genesis potential index climatology and ENSO variability will be examined. The genesis potential index will be used to investigate where errors in the simulated tropical cyclone climatologies and anomalies were due to large-scale environmental factors or the models response to these factors.

References

Emanuel, K.A. and D. Nolan, 2004: Tropical cyclone activity and global climate. Proc. 26th Conference on Hurricanes and Tropical Meteorology, 240-241, Miami, FL.