Why does it rain in Australia during El Niño?

Variability of weather systems explains different precipitation patterns during El Niño as revealed from a comprehensive climatological analysis

Every 3-8 years, a significant increase in water temperature in the equatorial Pacific Ocean causes global changes in temperature and rainfall - El Niño. Though Peru and western North America expect above-average rainfall during El Niño, the densely populated southeastern Australia fears drought conditions and a high risk of bushfires. For example, the last El Niño event in 2015/2016 was classified as the third strongest El Niño event since 1965 and was characterized by long lasting heat waves, a very early start of the bushfire season and a record-breaking drought. Still, El Niño is not necessarily associated with reduced rainfall in southeastern Australia shown by close to average rainfall during the strong El Niño event of 1997/1998. So, why does it rain in Australia during El Niño? Are large-scale atmospheric flow patterns and embedded weather systems responsible for this rainfall variability between similarly strong El Niño events?

In collaboration with Prof. Michael Reeder and Dr. Shayne McGregor from Monash University in Melbourne, these questions were investigated in our research group "Large-scale dynamics and predictability" [1]. By clustering monthly rainfall anomalies between June and November, a total of four monthly rainfall patterns could be found in southeastern Australia (Fig. 1): wet southeastern Australia (Cluster 1), dry southeastern Australia (Cluster 2), wet East Coast (Cluster 3) and wet South Coast (Cluster 4). Nearly 50% of the El Niño months considered are assigned to the dry pattern (Cluster 2), which reflects the general dry conditions expected in southeast Australia during El Niño.

 

 

 

Abbildung 1. Clustermittel der monatlichen Niederschlagsanomalien (in mm month-1) in Südostaustralien. Die Box zeigt die Region, welche für die Clusteranalyse der Niederschlagsanomalien verwendet wurde. Statistisch signifikante Anomalien sind gepunktet. Anomalien, die zusätzlich innerhalb des Clusters robust sind, können an schraffierten Flächen erkannt werden.

In order to advance the meteorological understanding driving these patterns, objectively identified weather systems are used [2]. The focus is on cut-off lows, extratropical cyclones, atmospheric blocking, moisture transport and warm conveyor belts (WCBs), which represent the main-ascending, precipitating part of extratropical cyclones. During dry El Niño conditions in southeastern Australia, a blocking high pressure system is centred over the southern part of the continent (Fig. 2b). This suppresses the occurrence frequency of rain-bringing WCBs, cut-off lows, and extratropical cyclones. In contrast, composites of the unusually wet Cluster 1 show a high frequency of blocking high pressure systems southeast of Australia and an increased frequency of cut-off lows along the South Coast and WCBs more inland. In addition, enhanced moisture transport from the Tropics across the continent supports rain-bringing WCB activity in southeastern Australia (Fig. 2a). It is the change in frequency of WCBS and cut-off lows that explains the wet conditions but not the change in rainfall intensity.

To return to the initial question, wet conditions during El Niño occur in southeastern Australia on monthly time-scales due to changes in the activity of weather systems. The results of this study highlight that these weather systems may override the effect of El Niño which is important when aiming for skilful seasonal forecast in this region.

Abbildung 2. Schematische Zusammenfassung der Strömungsanomalien und der Wettersystemanomalien für den nassen Cluster 1 (a) und den trockenen Cluster 2 (b) während El Niño. Die rote durchgezogene (blaue gestrichelte) Linie zeigt die clustergemittelte Position der positiven (negativen) Geopotentialanomalie auf 500 hPa. Regionen mit statistisch signifikanten und robusten Frequenzanomalien sind markiert (positiv: +, negativ: -): Cut-off Zyklonen (blau), WCBs (rosa) und blockierende Hochdrucksysteme (gelb).

 

Um auf die Ausgangsfrage zurückzukommen: Die nassen Bedingungen während El Niño treten im Südosten Australiens auf monatlichen Zeitskalen aufgrund von Veränderungen in der Aktivität der Wettersysteme auf. Die Ergebnisse dieser Studie zeigen, dass diese Wettersysteme den Effekt von El Niño überlagern können, was wichtig ist, wenn eine gute Saisonvorhersage in dieser Region anstrebt wird.

Research Group:         Large-scale dynamics and predictability

Authors:                      Seraphine Hauser, Julian Quinting, Christian Grams

Date:                           27 April 2020

[1] Hauser, S., C. M. Grams, M. J. Reeder, S. McGregor, A. H. Fink, J. F. Quinting (2020) A weather system perspective on winter-spring rainfall variability in southeastern Australia during El Niño. Quarterly Journal of the Royal Meteorological Society. doi: 10.1002/qj.3808. Accepted.

[2] Sprenger, M., Fragkoulidis, G., Binder, H., Croci-Maspoli, M., Graf, P., Grams, C. M., Knippertz, P., Madonna, E., Schemm, S., Škerlak, B. and Wernli, H. (2017) Global climatologies of Eulerian and Lagrangian flow features based on ERA-Interim. Bulletin of the American Meteorological Society, 98, 1739–1748.