Climatology of Sign Reversals of the Meridional Potential Vorticity Gradient over Africa and Australia

Abstract

A 10-yr climatology (1986?95) was performed using ECMWF gridded analyses on isentropic surfaces to identify regions where the lower-tropospheric meridional potential vorticity (PV) gradient changes sign across Africa and Australia during their respective summer seasons. While an African sign reversal has been documented, no similar study has been performed for the Australian region, which also has desert on the poleward side of open ocean. In each hemisphere, a northward decrease of PV is sufficient to produce a sign reversal. It was found that PV decreases northward in the lower troposphere across northern Australia, with the maximum reversal on the 315-K surface. It had comparable magnitude but smaller zonal extent (?3000 km) than that on the 320-K surface in Africa (?5000 km). In each region the sign reversal was associated with cyclonic PV anomalies on the equatorward side and anticyclonic anomalies on the poleward side. OLR was used as a proxy for deep convective heating in order to evaluate the total convective forcing of PV. The vertical distribution of heating was specified. In both regions the maximum total convective forcing of PV was largest on the equatorward edge of the sign reversal region. The effects of dry convection were not included in the PV budget. Dry convection, located poleward of the maximum deep convection, acts as a lower-tropospheric PV sink and produces anticyclonic PV anomalies. In both regions these anticyclonic anomalies were larger in magnitude and areal coverage than the cyclonic anomalies associated with deep convection. The potential instability implied by the sign reversal regions has traditionally been associated with the growth of easterly waves. In support of this argument, bandpass-filtered (2?6 day) meridional wind variance on the 320-K surface nearly triples from east to west along the African sign reversal. In Australia, little evidence was found of such waves in the 2?10-day meridional wind variance. Possible explanations for the lack of growing disturbances over Australia are discussed.