Intense east coast lows and associated rainfall in eastern Australia

Abstract

East coast lows (ECLs) are low pressure systems that occur near the east coast of Australia. But not all lows cause the same level of impact, and a small proportion of ECLs are responsible for more than half of all days with widespread rainfall above 50mm in this region. In this study, we combine analyses of cyclones at both the surface and 500hPa levels to assess the locations of cyclones responsible for widespread heavy rainfall on the east coast. We found that the majority of days with widespread totals above 100mm on the east coast occur when a low at 500hPa over inland southeast Australia coincides with a surface low located more directly over the east coast. Such events occur on about 15 days per year but are responsible for more than 50% of days with widespread heavy rainfall on the eastern seaboard of Australia. We also found that extreme rainfall was most likely when both the surface and upper cyclones were very strong, when measured using the maximum Laplacian of pressure/height. The seasonal frequency of cyclones at the surface and 500hPa were found to be only weakly correlated with each other and often had opposing relationships (albeit weak in magnitude) with both global climate drivers and indices of local circulation variability. Trends in cyclone frequency were weak over the period 1979–2019, but there was a small decline in the frequency of deep cyclone days, which was statistically significant in some parts of the southeast. Understanding which ECLs are associated with heavy rainfall will help us to better identify how future climate change will influence ECL impacts.

Linking ENSO to Synoptic Weather Systems in Eastern Australia

“…Next, we examine MSLP and Z500 anomalies composited over La Niña, El Niño and neutral ENSO years (Figure 2) to identify changes in the surface and mid-tropospheric circulations on spring cyclone days. There is little change in the frequency of cyclone days between positive, negative and neutral ENSO phases (Figure 2, number in top-right panels), consistent with earlier studies (e.g., Pepler & Dowdy, 2021b), although on average the highest frequency occurs during La Niña and the lowest during neutral phases.…”

“…These low-pressure systems are generally associated with a quasi-stationary anticyclone or blocking high to the southeast of Australia (e.g., Pook et al, 2013;Risbey et al, 2013). No strong relationship on monthly to seasonal timescales between the frequency of synoptic-scale cyclonic weather systems in eastern Australia (i.e., cut-off lows or "East Coast Lows" when they form along Australia's eastern seaboard) and ENSO has been identified in the literature (e.g., Grosfeld et al, 2021;Pepler & Dowdy, 2021b;Pepler et al, 2014). However, Risbey et al (2009) reported that blocking is more likely to occur during La Niña than El Niño.…”

The dynamics of slow‐moving coherent cyclonic potential vorticity anomalies and their links to heavy rainfall over the eastern seaboard of Australia

“…Deep cyclonic circulations are associated with deeper stratospheric intrusions of high‐magnitude PV air (Barnes et al, 2021). Moreover, the surfaceward cyclonic development of the upper‐level cyclonic circulation in composites of heavy rainfall days presumably explains the along‐shore MSLP trough reported by Black and Lane (2015) and highlights the role that deep cyclones (that extend from the surface into the midlevels) play in heavy rainfall in the ESB, as previously reported by Pepler and Dowdy (2020, 2021). The effect of the deep cyclonic circulation on the ESB is to provide poleward transport of moisture from the Tropics.…”

Negative Indian Ocean Dipole drives groundwater recharge in southeast Australia

“…Risbey et al 46 also note that the enhanced meridional thickness gradient during La Niña and negative IOD is favourable for the development of cutoff low pressure systems, which in turn increase rainfall over southeast Australia. The amount of rainfall produced by cutoff low systems increase during negative IOD and La Niña, although the frequency of these cyclones seems to be less affected 42 Here we show that the impact of La Niña and negative IOD, as well as their combined occurrences, extends to recharge events in NSW (Fig. 4).…”