A critical analysis of a month-long cyclonic anomaly in the New Zealand-Tasman Sea region using isentropic vorticity budgets

The authors investigate the processes acting during a cyclonic "blocking" anomaly that dominated the upper troposphere in the New Zealand-Tasman Sea region during the very stormy month of August 1990. The authors use European Centre for Medium-Range Weather Forecasts (ECMWF) analyses and a simple, unambiguous flux form of the isentropic vorticity equation that makes possible a robust physical interpretation of the budget and that does not suffer from the problems of strong cancellation between pairs of terms that mar the traditional, isobaric approach. The simplicity of the equation also makes tractable an analysis of the errors in each of the terms in the monthly mean budget. On the western and northern flanks of the anomaly at 310 K ( approximately 350 hPa) the tendency of the time-mean flow to advect the anomaly downstream is countered (within the limits imposed by the relatively short averaging period) by mean stretching and the repeated influx of subpolar air by the storms during the month. However, on the eastern side of the anomaly the error analysis gives confidence that the dominant mean stretching term is only partially balanced by the mean and eddy advection terms. Two regions were found, overlying the ends of surface storm tracks, where large-scale and statistically significant residuals are required to balance the isentropic vorticity budget. The authors note that the whole eastern side of the anomaly is characterised by a large-scale drag on the prevailing flow, with isentropic gradients in the drag being responsible for the vorticity residual. Interestingly, embedded within each area of significant residual a smaller region exhibiting a significant acceleration of the zonal flow in the upper troposphere is found. The authors argue that these features are caused by distinct physical processes and are not the result of systematic errors in vorticity tendencies, diabatic flux divergences, or errors in the analyzed divergent wind field. Instead, the authors conclude that during August 1990 there are several dynamically significant processes operating that are not resolved by 2.5 degree x 2.5 degree ECMWF analyses: namely, the vertical transport of horizontal momentum associated with convective activity, the quasi-horizontal transport of horizontal momentum by breaking Rossby waves, and the ageostrophic motions associated with frontogenesis.