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

Abstract

Cool season rainfall variability in southeastern Australia is investigated via classification and characterization of the predominant types of synoptic systems occurring in the region. These types are frontal systems, cut-off low systems, and other systems. Rainfall in the region is dominated by cut-off systems and these systems are the main influence on the interannual variability of rainfall. Both cut-off systems and frontal systems display an enhancement of thermal (thickness) gradient as rainfall increases, but the mechanisms for intensification differ. Cut-off systems intensify in the region in association with local increases in baroclinicity and the subtropical jet, whereas frontal systems tend to intensify via a confluence of subtropical and polar jets. Interannual rainfall variability is examined for groupings of years based on both clustering of continental rainfall patterns and on El Nino/Southern Oscillation (ENSO)/Indian Ocean Dipole (IOD) years. Cut-off systems exhibit consistent enhancements of thermal gradients for groupings of years in which they produce more rainfall. For ENSO/IOD groupings, the cut-off thermal gradients are consistent with the underlying sea surface temperature (SST) anomalies. Wet years in southeastern Australia are usually produced by cut-off systems, but can also be produced by frontal systems. In those cases the mid-tropospheric flow pattern is reminiscent of the negative Southern Annular Mode (SAM) pattern. The positive SAM pattern is also associated with enhanced rainfall in the southeast via local intensification of blocking and cut-off systems.

The autumn break for cropping in southeast Australia: trends, synoptic influences and impacts on wheat yield

“…The intensity of these cold-cored cyclonic systems is strongly influenced by the upper level divergence associated with the subtropical jet stream which is located equator-wards of the low and warm air advection which develops on the eastern flank. Variations in intensity of cutoff systems can be ascribed to local variations in baroclinicity, vorticity, and the strength of the subtropical jet itself (Risbey et al, 2008). Speed of movement and the tracks described by the systems are influenced by the background flow and are particularly dependent on the location and persistence of atmospheric blocking which is highly correlated with the numbers of cutoff lows in our analysis region (Pook et al, 2006).…”

What causes southeast Australia's worst droughts?

“…In contrast, there has been a complete absence of negative IOD events throughout this latest drought period, with the last negative IOD occurring in 1992 ( Figure 1a). Negative IOD events are characterized by above-average rainfall over southern regions of Australia resulting from an interaction between the tropics and temperate zone that increases moisture advection onto the region [Pook et al, 2006;Risbey et al, 2009] (see also Figure 2d). This interaction is visible as the often observed northwest cloudbands [Tapp and Barrel, 1984].…”

On the drivers of inter‐annual and decadal rainfall variability in Queensland, Australia

“…Meneghini et al, 2007;Hendon et al, 2007). Local patterns of atmospheric variability also influence seasonal rainfall in Queensland, particularly variability in the extra-tropical storm tracks due to Southern Ocean blocking (Pook and Gibson, 1999;Risbey et al, 2009a) and intense coastal depressions (Holland, 1997), as well as tropical cyclones (TCs) (Lough, 1993;Walsh and Syktus, 2003;Flay and Nott, 2007). The current understanding of how these drivers influence Queensland is summarized in the following paragraphs.…”

“…Seasonal means of B were computed from monthly values, then longitude-averaged over two bands in the Southern Ocean -120°-150°E (B 120 -150 ) and 150°E-180°(B 150 -180 ) -where blocking activity is known to influence Australian rainfall (Qi et al, 1999;Pook et al, 2006;Risbey et al, 2009aRisbey et al, , 2009b.…”