Synoptic components of rainfall variability and trends in southeast Australia

This work examines the contributions of different synoptic types to rainfall variability and trends in southeast Australia, with a focus on the drought of 1996–2009. Rainfall in the Mallee region is used to characterize the southeast region. The reduction in rainfall in the Mallee region during the drought is about two-thirds composed of a reduction in rainfall from cutoff lows and about one-third due to a reduction in rainfall from frontal systems. The reduction in frontal rain is mostly due to a reduction in rain per frontal system, which is associated with a reduction of baroclinicity in the southeast and south of Australia. The reduction of cutoff rainfall is mostly due to a reduction in number of the most intense cutoff systems. The frequency of cutoff systems matches changes in blocking activity in the Tasman Sea region. Blocking has undergone a weak decline in the Tasman region over the period of the drought. Analysis of synoptic system contributions to the drought indicates that causal explanations of the drought should account for variation in cutoff systems and blocking.