Severe thunderstorms in the Brisbane region and a relationship to the El Niño Southern Oscillation

The number of severe thunderstorm days in the most populated region of southeast Queensland and its relationship to El Niño Southern Oscillation (ENSO) have been examined. On a seasonal scale, ENSO has some impact in setting the background for synoptic patterns that are favourable or unfavourable for severe thunderstorm development. A climatology of severe thunderstorm days for the most populated area of southeast Queensland (inner Brisbane) was constructed using data from 1956 to 2000. Using these data and Southern Oscillation Indices (SOI) for each severe thunderstorm season, it was demonstrated that severe thunderstorm days are about 60 per cent more likely in neutral SOI years than in positive SOI years.When these data were partitioned into post and pre-1977 (approximately the year the inter-decadal Pacific Oscillation became positive and a downward trend in the SOI was observed) the pre-1977 data showed an even more impressive result with severe thunderstorm days being nearly 130 per cent more likely in neutral years than in positive SOI years. Post-1977 data showed severe thunderstorm days as being only about 23 per cent more likely in neutral years than positive SOI years. Composites of climate anomalies were constructed using the NCEP/NCAR reanalysis data for the various phases of ENSO and compared to previously published composites. The composites consist of fields of 850 hPa to 500 hPa lapse rates and sea-level pressure. Analysis of the composites shows anomalous atmospheric stability in positive ENSO years, with no conclusive results for the other composite combinations.