Northeast Australia is a region considered to exhibit clear and consistent climate responses to El Niņo-Southern Oscillation (ENSO) events. Unusually dry conditions are associated with ENSO events and unusually wet conditions are associated with anti-ENSO events. Indices (based on a small number of stations which can be readily updated) have been developed for summer (wet season) and winter (dry season) rainfall for Queensland, northeast Australia, covering the period from 1890 to date.
Over the 108-year record period about 30% of the variance in both summer and winter Queensland rainfall can be explained by variations of ENSO (based on correlations of seasonal rainfall with the Tahiti-Darwin Southern Oscillation Index, SOI). There are, however, considerable variations in the strength of these relationships over the past century which affect rainfall variability in both space and time. High inter-annual variability of rainfall and high spatial coherence of rainfall anomalies across the State characterizes periods of strong relationship between ENSO and Queensland rainfall. Low inter-annual variability of rainfall and low spatial coherence of rainfall anomalies characterizes periods of weak relationship between ENSO and Queensland rainfall. These features are evident in both the summer wet season and winter dry season.
Based on analyses of 20-year periods, summer rainfall was most strongly related with the SOI from 1915-1934 (r = 0.85) and most weakly from 1930-1949 (r= -0.05). Winter rainfall was most strongly related to the SOI from 1945-1964 (r = 0.88) and most weakly from 1920-1939 (r = 0.21). Analyses of the most recent 20-year periods (through 1979-1998 for summer and 1978-1997 for winter) suggest that the strength of these relationships between Queensland rainfall and ENSO may once again be changing. These changes appear to have taken place about 1974/1975. The strength of the relationship between summer rainfall and ENSO has declined and this has been associated with reduced inter-annual summer rainfall variability and reduced spatial coherence of summer rainfall anomalies across Queensland. In contrast, the strength of the relationship between winter rainfall and ENSO has increased in magnitude and been associated with increased inter-annual rainfall variability and increased spatial coherence of winter rainfall anomalies. Regardless of whether this is a regional or global phenomenon, the changing relationship between northeast Australian rainfall and ENSO has important ramifications for Queensland, where ENSO has been the mainstay of seasonal climate outlooks in recent years.