Trends and low frequency variability of East Coast Lows in the twentieth century.

East Coast Lows (ECLs) are important weather systems that affect the eastern seaboard of Australia. They have attracted research interest for both their destructive nature and water supplying capability. In this paper, three objective ECL tracking methods are applied to the twentieth century reanalysis ensemble (20CR V2C) for the period of 1851–2014 to identify historical trends and variability in ECLs. While the ensemble mean is unsuitable for tracking ECLs, when all methods are applied to the full 56- member ensemble there is large agreement between tracking methods as to the low-frequency variability and trends in ECLs. The uncertainty between 56 ensemble members has dramatically decreased in recent decades. For comparison, the three tracking methods are also applied to ERA-I reanalysis dataset for the overlapping time period (1980-2009). The inter-annual variability and monthly distribution of ECLs agrees well between different reanalysis for each of tracking methods. The most recent decade has had relatively low numbers of ECLs compared to the previous century.

Figure 6: Normalized annual number of ECLs for three tracking methods. The normalization is calculated against the mean number of ECLs for 1970-2006. The three different colours are for three tracking methods, and thin solid lines are for annual ECL anomaly and dot lines are 11 year moving average, and solid lines are trend lines for 1880-1920, 1920-1975 and 1975 to 2014.