East coast lows, atmospheric blocking and rainfall: A Tasmanian perspective

Abstract

Although the term 'east coast low' is normally associated with intense cyclones near the east coast of mainland Australia, cutoff lows of similar type also affect Tasmania. This paper demonstrates that the cutoff low is a major source of rainfall for the agricultural districts and water catchments of eastern Tasmania. In particular, an analysis of synoptic systems and daily rainfall reveals that cutoff lows are responsible for almost 50% of April to October rainfall in parts of the northeast and a slightly lower proportion in the southeast. The other large contribution to rainfall is from frontal systems but the relative effects of the various synoptic types vary widely across the state as a result of the complex topography. Cutoff lows commonly form the cyclonic portion of a blocking dipole which can have opposing influences on Tasmanian rainfall. The high latitude anticyclone suppresses rainfall in western and southwestern Tasmania, while the cutting off of a relatively small cyclonic component equatorwards of the high frequently results in enhanced rainfall over eastern Tasmania. Results from two climate models indicate that the accurate simulation of blocking and cutoff lows remains difficult to achieve and this has implications for projections of Tasmanian rainfall on seasonal and longer time scales.