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

Abstract

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 nearsurface 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.

The Southern Hemisphere semiannual oscillation and circulation variability during the Mid-Holocene

“…The southwesterlies cool the western side of NZ but allow warming to the east as a result of a föhn effect. The opposite is true however in the FGOALS model, where the position of the low anomaly is further east, causing air to be drawn from the north and warming over NZ (see Salinger and Mullan, 1999;Kidson, 2000;Kidston et al, 2009, for more details on the influence of flow regimes on NZ temperatures). The importance of this result is that, despite FGOALS and HadCM3 UB both showing a tendency toward enhanced wavenumber 3 activity, the mean climatological positions of those anomalies result in completely different effects on NZ climate.…”

“…Unrotated EOFs were used, as the Southern Annular Mode (SAM, see below) pattern is most clearly identified in the unrotated EOF analysis, and for consistency with other studies (e.g. Thompson and Wallace, 2000;Kidston et al, 2009). The three leading modes of SH interannual variability (with the seasonal cycle removed) have been calculated from the NCEP/NCAR reanalysis data for monthly mean Sea Level Pressure (SLP) and can be seen in Fig.…”

“…The SAM varies on monthly, seasonal and interannual time-scales (see Kidson, 1999;Kidston et al, 2009;Jones et al, 2009, and references therein) and has been shown to be sensitive to changes in atmospheric greenhouse-gas concentrations and SH ozone depletion Miller et al, 2006;Perlwitz et al, 2008). Therefore, the SAM may show some sensitivity to the changes in seasonal insolation during the Mid-Holocene relative to today.…”

The Southern Annular Mode: a comparison of indices

“…polewards of 40 • S) in the Southern Hemisphere (Rogers and van Loon, 1982). The alternating, near zonallysymmetric structure is centred at approximately 50 • S. SAM also varies seasonally with a strong zonally symmetric pattern in summer that deteriorates and becomes weaker in winter and spring (J. M. Kidston et al, 2009). The fundamental mechanism driving SAM circulation is the non-uniform heating of the Earth and the subsequent energy transport and atmospheric circulations that occur (King and Turner, 2007).…”

Precipitation delivery trajectories associated with extreme river flow for the Waitaki River, New Zealand

“…In particular, the climate of New Zealand is influenced by both the Southern Annular Mode and the Southern Oscillation (Gordon, 1986;Kidston et al, 2009). At the same time, the presence of a substantial topographic barrier (the Southern Alps; peaks > 3000 m) orientated near-perpendicular to the prevailing westerly circulation results in strong climatic variation over short distances, resulting in a complex combination of large and small scale influences on the climate across the South Island.…”