Coherent Potential Vorticity Maxima and Their Relationship to Extreme Summer Rainfall in the Australian and North African Tropics

Transient synoptic-scale potential vorticity (PV) disturbances modulate rainfall over the tropics, including northern Australia and North Africa. In this research, an objective technique is used to identify coherent synoptic-scale cyclonic PV maxima. The climatology of these coherent PV maxima is most frequent in the summer hemisphere. The largest meridional flux of PV associated with these PV maxima lies along eastern Australia, whereas tropical North Africa is characterized predominantly by a zonal flux.

Composites of the PV maxima on extreme rainfall days over northern Australia show structures similar to mesoscale convective systems (MCS), i.e. the maximum of PV and diabatic heating are found in the middle troposphere and highest rainfall is often triggered ahead and equatorward of a moving composite PV maximum. The respective composite structures for extreme rainfall days in North African tropics show structures similar to easterly waves, which are shallower and weaker but move faster than those in the Australian tropics. On the days of extreme rainfall, the composites in both regions show a clear, strong and isolated PV maximum upstream of the highest rainfall in the compositing boxes. Time-lagged composites show that the PV maxima in the Australian tropics have their origins in the extratropical region of eastern Australia. In contrast, the PV maxima in North Africa have their origin in the North African tropics.