Interdecadal changes in southern hemisphere winter storm track modes

Abstract

A B S T R A C TThe interdecadal changes in southern hemisphere (SH) winter cyclogenesis have been studied using a global twolevel primitive equation instability-model with reanalysed observed July three-dimensional basic states for the periods 1949-1968 and 1975-1994. The early to mid-1970s were a time of quite dramatic reduction in the winter rainfall in the southwest of western Australia (SWWA). We find that the rainfall reduction is associated with a decrease in the vertical mean meridional temperature gradient and in the peak upper tropospheric jet-stream zonal winds near 30 • south throughout most of the SH. These changes are reflected in the properties of the leading SH cyclogenesis modes: for 1975-1994 both the fastest growing mode, and on average the 10 leading SH cyclogenesis modes that cross Australia, have growth rates which are around 30% smaller than for the corresponding modes for 1949-1968. The sensitivity of our results, to the strengths of physical parametrizations and to the choice of basic states based on different data sets, is examined.Our results suggest that a primary cause of the rainfall reduction over SWWA in the period after 1975 is the reduction of the intensity of cyclogenesis and the southward deflection of some storms.

Multidecadal and interannual changes of stationary Rossby waves

“…The SH zonal wind changes are of hemispheric extension: a decrease in the zonal wind, around 30 • S, with greater intensity in the west Australian jet and South Africa regions, and an increase, around 55 • S, a little southward of the PJ region (Figure 1(f)). These changes are consistent with the findings by Archer and Caldeira (2008), Frederiksen (2005) and Frederiksen and Frederiksen (2007), who also reported, in the SH, a weakening of the subtropical jet, whereas the PJ strengthened after the late 1970s.…”

“…In the Southern Hemisphere (SH), Frederiksen and Frederiksen (2007) found that there were quite large changes in thermal structure and atmospheric circulation, between the periods of 1949-1968 and 1975-1994. Regarding the changes in thermal structure, they found a significant warming of the SH troposphere south of 30 • S in the period [1975][1976][1977][1978][1979][1980][1981][1982][1983][1984][1985][1986][1987][1988][1989][1990][1991][1992][1993][1994], resulting in a reduction of the Equator-to-pole temperature gradient.…”

Characteristics and variability of synoptic features associated with cool season rainfall in southeastern Australia

“…This result illustrates that a flow pattern with a decreased or southward shift of the zonal mean storm tracks south of Australia is not necessarily detrimental to rainfall in Australia [cf. Karoly (2003); Frederiksen and Frederiksen (2007)]. In this case, the tendency for enhanced blocking promotes cut-off systems in the region that are very efficient at delivering rainfall to the Australian region.…”

Understanding rainfall projections in relation to extratropical cyclones in eastern Australia

“…The projected change in the occurrence frequency of strong upper-tropospheric vorticity events may potentially be related to changes in a number of different phenomena, including the closing of the split jet structure over eastern Australia (Grose et al 2012), the position and strength of the subtropical ridge (Timbal and Drosdowsky 2012), atmospheric blocking (Pook et al 2010), Rossby waves (Ndarana and Waugh 2010) and storm tracks (Hoskins andValdes 1990, Frederiksen andFrederiksen 2007). There appears to be a relatively consistent picture of poleward movement emerging from a variety of contrasting methods and phenomena, including the boundary between the…”