The influence of changes in synoptic regimes on north Australian wet season rainfall trends

[1] Precipitation over the north of Australia mainly falls during the wet season and is associated with the Australian monsoon. Recent studies have shown that precipitation in the north and northwest of Australia during the wet season has increased over the past 50 years. In previous work, daily radiosonde data at a single site (Darwin) were used to identify five distinct wet season regimes, each associated with a characteristic synoptic circulation pattern and rainfall probability distribution. Here the five regimes are used to decompose the 50-year precipitation trend at Darwin from 1957/1958–2007/2008 into two contributions; that due to changes in the regime relative frequency of occurrence, and that due to changes in the within-regime precipitation. Over the entire wet season from September to April, the within-regime precipitation does not change significantly for any of the regimes. However, the relative frequency of occurrence decreases significantly for the driest regimes, and increases significantly for one of the wettest regimes, suggesting that changes in the large-scale circulation are a more important contributor to the precipitation trends than are thermodynamic changes. During December to March, the largest contributions to the total precipitation trend come from changes in all three of the wettest regimes. During November and April, when the average precipitation is lower, there is a large relative contribution to the precipitation trend from the increase in frequency of a wet regime and a decrease in frequency of the dry regimes. This contributes to the significant lengthening of the north Australian wet season.