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Shifts in the synoptic systems influencing southwest Western Australia

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Abstract

A self-organising map is used to classify the winter circulation affecting southwest Western Australia (SWWA) into 20 different synoptic types. The changes in the frequency of these types and their links to observed rainfall are analysed to further understand the significant, prolonged, rainfall drop observed in this region since 1975. The temporal variability of the different synoptic types link well with the observed rainfall changes. The frequency of the troughs associated with wet conditions across SWWA has declined markedly since 1975 while the frequency of the synoptic types with high pressure over the continent, associated with dry conditions, has increased. Combining the frequency of the synoptic systems with the amount of observed rainfall allows a quantitative analysis of the rainfall decline. The decreased frequency of the troughs associated with very wet conditions accounts for half of the decline. Reductions in the amount of rainfall precipitating from each system also contribute to the decline. Large-scale circulation changes, including increases in the mean sea-level pressure and a decrease in the general baroclinicity of the region have been associated with the rainfall decline. These changes are suggested to be linked to increasing levels of greenhouse gases. Due to the strong link between the number of trough types and the rainfall over SWWA, the shifts in the frequency of these synoptic types could be used as a tool to assess simulated rainfall changes, particularly into the future.

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Acknowledgements

This research was supported by the Indian Ocean Climate Initiative (IOCI). Thank you to Debbie Hudson, who provided guidance in the early stages of this project and to Vaughan Barras, who provided helpful suggestions.

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Authors and Affiliations

  1. Bureau of Meteorology Research Centre, GPO Box 1289, Melbourne, VIC 3001, Australia

    Pandora K. Hope, Wasyl Drosdowsky & Neville Nicholls

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  1. Pandora K. Hope
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  2. Wasyl Drosdowsky
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  3. Neville Nicholls
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Correspondence to Pandora K. Hope.

Appendix: calculation of the relative contributions to the rainfall changes

Appendix: calculation of the relative contributions to the rainfall changes

For each synoptic type, the rainfall differences plotted in Fig. 10 (Δ) can be broken up into the contribution from the change in the number of occurrences and the mean daily rainfall. The raw values for these calculations are in Table 1. If the change in the number of occurrences is of a different sign to the change in rainfall then the component with the same sign as Δ is counted as contributing 100%. If both components are of the same sign then the proportions are calculated as:

$$\begin{aligned} {\rm Contribution}_{f} &+ {\rm Contribution}_{r}= \Delta \\ (f_{2}-f_{1})\bar{r} &+ (r_{2}-r_{1})\bar{f} = \Delta\\ \frac{{(f_{2}-f_{1})\bar{r}}}{{\Delta}}&+\frac{{(r_{2}-r_{1})\bar{f}}}{{\Delta}} = 1\\ {\rm Proportion}_{f} &+ {\rm Proportion}_{r} = 1 \end{aligned} $$

where f is the number of occurrences divided by 2 and r refers to the rainfall. Subscript 2 refers to the later period (1976–2003), while subscript 1 refers to the early (1958–1975) period. The overbar corresponds to the mean value over the full period. The proportions are expressed as a percentage in Table 2. The contributions towards Type C1 cannot be calculated as the total change is near zero.

Table 2 The percentage proportion of the contribution from the change in the average number of occurrence (Proportion f ) and the average daily rainfall (Proportion r ) to the total rainfall anomaly, Δ (1976–2003 minus 1958–1975)
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Hope, P.K., Drosdowsky, W. & Nicholls, N. Shifts in the synoptic systems influencing southwest Western Australia. Clim Dyn 26, 751–764 (2006). https://doi.org/10.1007/s00382-006-0115-y

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