Large scale and sub-regional connections in the lead up to summer heat wave and extreme rainfall events in eastern Australia Ghyslaine Boschat • Alexandre Pezza • Ian Simmonds • Sarah Perkins • Tim Cowan • Ariaan Purich Received: 27 November 2013 / Accepted: 10 June 2014 Ó Springer-Verlag Berlin Heidelberg 2014 Abstract Australia has been exposed to a vast array of extreme weather regimes over the past few years, and the frequency and intensity of these events are expected to increase as a result of anthropogenic climate change. How- ever, the predictability of extreme droughts, heat waves (HWs), bushfires and floods, is still hampered by our inability to fully understand how these weather systems interact with each other and with the climate system. This study brings new insight into the regional and large scale dynamics of some extreme events in Australia, by describing and comparing the climate signature of summer HWs and extreme rainfall events which have occurred in the states of Victoria and Queensland respectively, during 1979–2013. Our analyses highlight the importance of mid-latitude dynamics operating during HWs, in contrast with more tropical interactions at play during extreme rainfall events. A ‘common’ blocking high pressure system is observed over the Tasman Sea during the two types of extreme events, and may explain why some southeastern HWs (only about 25 %) occur in close succession with floods in Queensland. However, our results suggest that there is no dynamical link between these two types of events, since the HW-related anticyclone evolves as part of a baroclinic wave train, whereas in the case of rainfall events, this structure emerges as an equivalent barotropic response to tropical convection. Sub-regional surface temperatures and air–sea fluxes also suggest that distinct processes may be operating in the lead up to these two events. Indeed, HWs tend to occur when the wave train propagates from the south Indian to the Pacific Ocean, inducing a quasi-stationary blocking high system over the Tasman Sea. This anticyclonic anomaly can then advect hot dry air towards the southern Victorian coast, where it produces HW conditions. On the other hand, extreme rainfall events mostly occur when the background conditions correspond to a La Nin ˜a state. The convection induced in the western Pacific can trigger a tropical–extratropical telecon- nection over Queensland. This may generate an anticyclonic anomaly over the Tasman Sea, able to divert air parcels over a warm and humid area where conditions are, this time, favorable for more extreme rainfall along the Queensland coast. Keywords Heat wave  Extreme rainfall  Sea surface temperature  Synoptic climatology  Blocking  Australian climate 1 Introduction Considered the driest inhabited continent on Earth, Aus- tralia’s climate is harsh and vulnerable to the devastating effects of weather extremes. Whether it be droughts, pro- longed periods of extreme high temperatures [heat waves (HWs)], bushfires, cyclones or floods, these events share the common ability to cause severe strain on the popula- tion’s health and resources, especially when they occur simultaneously or in succession across the country. In southeastern Australia, where the country’s population and agricultural production are highly concentrated, G. Boschat (&)  A. Pezza  I. Simmonds School of Earth Sciences, University of Melbourne, Melbourne, VIC 3010, Australia e-mail: ghyslaine.boschat@unimelb.edu.au S. Perkins ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney, NSW, Australia T. Cowan  A. Purich CSIRO Marine and Atmospheric Research, Aspendale, VIC, Australia 123 Clim Dyn DOI 10.1007/s00382-014-2214-5