The Seasonal and Wintertime Interannual Variability of the Split Jet and the Storm-Track Activity Minimum near New Zealand

Abstract

The seasonal and wintertime interannual variability of the split jet and the storm-track activity minimum near New Zealand (NZ) have been examined based upon the European Centre for Medium Range Weather Forecast Reanalysis (ERA-40) 1979-2001 daily data. Using the split jet index defined in this paper, the climatological variation in the split jet is closely related to the storm-track activity minimum. In austral winter, the split jet is located near NZ along with the storm-track activity minimum, which is established by the climatological Rossby wave forced by the cross-equatorial flow in the Indian Ocean. In austral spring and autumn, in contrast, both the NZ split jet and the storm-track activity minimum are less clear in the absence of the Rossby wave, because of the near-zero Indian Ocean cross-equatorial flow.However, interannual variation in the wintertime Indian Ocean cross-equatorial flow is only weakly associated with interannual variability in the split jet, the correlation being 0.14. The interannual variation in the cross-equatorial flow further east, in the Indonesian region, shows a higher correlation of 0.29. The middle and higher latitude components of the split jet structure are more strongly related to the zonally asymmetric part of the Antarctic Oscillation. An index for this has a 0.43 correlation on interannual time scales with the split jet index. Other processes must also be important for the interannual variability of the split jet and storm-track activity minimum in the NZ region. It is hypothesized that blocking in that region may be of particular importance.