Circulation Features Associated with the Winter Rainfall Decrease in Southwestern Australia

Abstract

A study of atmospheric and oceanic circulation features in the wider Australian region is undertaken in an attempt to establish the cause(s) of the observed decrease in austral winter (JJA) rainfall over the southwestern portion of Western Australia. Basic regional analyses reveal that long-term mean sea level pressure (MSLP) at Perth, Western Australia, is negatively correlated with southwestern Australian rainfall in JJA over the period 1876?1989. This significant MSLP-rainfall relationship is also observed when using smoothed data series, which resolve a decadal-multidecadal signal embedded within a long-term fluctuation. The latter is punctuated by a downward (upward) rainfall (MSLP) trend over the last 50?60 years that is most pronounced since the mid-1960s. Such relationships are examined further using Southern Hemisphere gridded MSLP, sea surface temperature (SST), and cloudiness data in the Australian sector during the period 1911?1989. On decadal to multidecadal time frames (MSLP bandpass-filtered in the 7?20-year band), MSLP is out of phase between the Australian continent and the high latitudes of the Southern Ocean. Alternations in the sign of MSLP anomalies over these regions are observed during wet and dry extremes in southwestern Australian JJA rainfall. This is suggestive of changes in the longwave pattern, and thus a propensity for modulation of frontal activity, in the Australian region. Some coherent variations are also seen in SST and cloudiness fields, with perhaps an indication that cloud patterns are more organized across the southwest in wet winters. The long-term MSLP analyses (low-pass-filtered MSLP to remove frequencies less than 25 years) indicate the influence of a different forcing on the regional climate system. The dominant pattern that emerges is of MSLP anomalies that are out of phase between the south-southwest of Australia and to the southeast of New Zealand. Since 1911, the above configuration of anomalies across southern latitudes has evolved slowly from a negative/positive to a positive/negative alignment. During this period, simultaneous correlations between Darwin MSLP and southwestern Australian JJA rainfall have also changed, from coherent and significant to insignificant. This suggests that low-frequency fluctuations in the El Niño-Southern Oscillation (ENSO) phenomenon may have played a major role in this process. It would appear that southwestern Australian JJA rainfall patterns are modulated by a long-term MSLP signal with a pronounced trend in recent decades that is punctuated by decadal-multidecadal MSLP pulses. These MSLP-rainfall relationships are associated with circulation fluctuations in Australian longitudes that may be linked to low-frequency characteristics of the ENSO phenomenon. However, wider Southern Hemisphere data have not yet been analyzed to test this hypothesis further. Interestingly, MSLP fields resolved in this study bear little resemblance to 2?CO2 MSLP simulations of enhanced greenhouse conditions in any of the low- or high-resolution general circulation model (GCM) intercomparisons over the Australian region.