Hemispheric-Scale Seasonality of the Southern Annular Mode and Impacts on the Climate of New Zealand

The seasonality of the southern annular mode (SAM) and the resulting impacts on the climate variability of New Zealand (NZ) are investigated. As with previous studies, during summer the SAM is found to be largely zonally symmetric, whereas during winter it exhibits increased zonal wavenumber 2-3 variability. This is consistent with seasonal variations in the mean state, and the authors argue that the seasonal cycle of near-surface temperature over the Australian continent plays an important role, making the eddy-driven jet, and hence the SAM, more zonally symmetric during summer than winter. During winter, the SAM exhibits little variability over the South Pacific and southeast of Australia. Dynamical reasons for this behavior are discussed. For the NZ region this seasonality implies that fluctuations in the SAM are associated with a zonal wind speed anomaly during summer but a more meridional wind speed anomaly during winter. This behavior is well captured by temperature and rainfall station data, which serves to corroborate the seasonal changes seen in the large-scale analysis. Moreover, the mode of climate variability that corresponds to a fluctuation of the zonal wind speed is well correlated with the SAM during the summer only and exhibits less variance during the winter. This is consistent with the notion that the seasonality of the SAM significantly impacts modes of climate variability in the region.