The underlying large-scale dynamical processes responsible for the development of heat waves in Victoria, southeastern Australia, in summer are presented here. Heat waves are defined as periods of at least three days and two nights for which daily maximum and minimum temperatures exceed the 90th percentile for a particular location and month, using a station daily temperature dataset. Composites of upper-level potential vorticity anomalies from the Interim ECMWF Re-Analysis (ERA-Interim) reveal that heat waves in southeastern Australia are associated with propagating Rossby waves, which grow in amplitude and eventually overturn. The process of overturning generates an upper-level anticyclone over southern Australia and an upper-level trough to the northeast, with maximum amplitudes near the tropopause. The northerly flow associated with the circulation around the surface anticyclone advects hot air from the continental interior over the southeast of Australia, leading to extreme surface temperatures. Composite rainfall shows that precipitation is enhanced in the vicinity of the upper-level trough over northeastern Australia, consistent with adiabatically forced vertical motion, destabilization of the atmosphere, and modified moisture fluxes. Heat waves in the southeast are frequently accompanied by heavy rainfall over the northeast of the continent and adjacent ocean.
The variance of a jet's position in latitude is found to be related to its average speed: when a jet becomes stronger, its variability in latitude decreases. This relationship is shown to hold for observed midlatitude jets around the world and also across a hierarchy of numerical models. North Atlantic jet variability is shown to be modulated on decadal time scales, with decades of a strong, steady jet being interspersed with decades of a weak, variable jet. These modulations are also related to variations in the basinwide occurrence of highimpact blocking events. A picture emerges of complex multidecadal jet variability in which recent decades do not appear unusual. An underlying barotropic mechanism is proposed to explain this behavior, related to the change in refractive properties of a jet as it strengthens, and the subsequent effect on the distribution of Rossby wave breaking.
Summertime heat waves in the southeastern state of Victoria, Australia, are associated with broad anticyclonic upper level potential vorticity (PV) anomalies. The current research establishes the relationship between heat waves, precipitation, and three modes of climate variability of importance for rainfall in Australia: the El Niño-Southern Oscillation (ENSO), Madden-Julian Oscillation (MJO), and Southern Annular Mode (SAM). Heat waves in Victoria in summer are more common during phases 3-6 of the MJO (when convection is enhanced over the eastern Indian Ocean, Maritime Continent, and western Pacific Ocean) and La Niña phases of ENSO. The PV-Theta (Θ) structure of the heat wave and pattern of convection varies with the phase of each mode of variability. Enhanced tropical convection results in a heat wave characterized by a monopole of anticyclonic PV, whereas suppressed convection is associated with a dipole of anticyclonic and cyclonic PV.
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