Observed and projected changes in the annual cycle of Southern Hemisphere baroclinicity for storm formation

Abstract

Recent studies have shown that over the last sixty years, there have been dramatic changes in the properties of mid-latitude winter storms which have impacted on southern Australian winter rainfall. In particular, there have been large reductions and negative trends in rainfall over this period, associated with similar reductions in the growth rates of storm track modes and a preference for some storms to develop further south of the Australian continent. These changes in the properties of mid-latitude storms have been shown to be associated with major shifts in the Southern Hemisphere winter circulation over this period. In particular, there have been significant negative trends in the baroclinic instability of the mid-latitude atmospheric circulation resulting in a reduction in storm formation at these latitudes, while increases in baroclinicity further poleward have led to increased storm development. These effects have become more pronounced with time and are likely to worsen under future climate change scenarios. In this paper, we consider the observed changes in the baroclinicity of Southern Hemisphere circulation in all months. We employ a useful diagnostic of storm development related to baroclinic instability, and encapsulated in the Phillips criterion. The relationship between changes in the Phillips criterion and the implied changes in storm formation and rainfall in all months during the twentieth century is discussed. We find that there are significant negative trends in baroclinicity at mid-latitudes in all months which imply a reduction in storm formation and rainfall at these latitudes. Further poleward, we find significant positive trends associated with increased cyclogenesis and rainfall, in all months. These trends are shown generally to be statistically significant at the 99% level. Negative trends with values around -0.15 ms -1 per year occur upstream of Australia, in regions associated with storm formation. Trends further southwards can reach values of +0.1 ms -1 per year. During May to October, significant negative trends occur in the region of the subtropical jetstream; in November to April, negative trends occur about 10 degrees further south. These trends correspond to reductions of up to 7 - 8 ms -1 in the mid-latitudes, and increases of about 5 ms -1 at high latitudes, by the end of the twentieth century. This tendency for a reduction in storm development in the mid- latitudes, and a greater chance of storm development at higher latitudes, is a consistent feature of each month. Results from the miroc3_2 high and medium resolution models, which capture many of these changes during the twentieth century, have also been used to examine the projected changes in baroclinicity in SRESA1B and SRESA2 scenarios for the period 2001 to 2099. During May to October, when mid-latitude storms have the biggest impact on southern Australia, we find similar patterns of negative and positive trends in the mid- latitudes and high latitudes to those observed and simulated in the twentieth century. Consistent with this result, the projections show hemispheric reductions in rainfall in a band between 10S and 40S and increases further south. Differences in rainfall, between the periods 1980-1999 and 2080-2099, vary between -40mm to +40 mm. Over SWWA and southeastern Australia there are reductions in rainfall in all months with differences exceeding -20mm in some months, especially over SWWA.