The characteristics of Rossby wave packets in the South African domain

Abstract

Aspects of upper tropospheric Rossby wave packets (RWPs) over South Africa (SA) and adjacent oceans were explored. This study used a local finite amplitude wave activity (LWA, A) diagnostic based on isentropic primitive equations and a feature tracking algorithm to track A pockets resembling RWPs in National Centre for Environmental Predictability (NCEP) re-analysis 2 data. Results of the tracking algorithm are first presented in the context of the Southern Hemisphere, then focus is shifted to the SA domain (0-45E; 22-45S) by means of a climatology. Morphological structures of tracked A pockets on 330 K and 350 K isentropic surfaces were found to be representative of typical extratropical upper tropospheric RWPs. Seasonal variability of RWP occurrences, intensity, duration, and propagation was examined for SA domain RWPs. Results were found to potentially have relevance in terms of the predictability of severe weather phenomena over the SA domain, in so far as non-linear RWPs may be related to cut-off low pressure systems over the SA domain. Evolutionary characteristics of A pockets identified on 350 K were also examined based on potential vorticity, ambient zonal flow (uθ), and meridional wind perturbations (v′). Results showed consistency with the well-established baroclinic life cycles. The development of these RWPs was found to conform to the downstream baroclinic development of the eddy kinetic energy (EKE) framework governed by baroclinic conversion and convergence of ageostrophic geopotential fluxes. This RWP representation was compared with the A budget tendency framework, which prompted a discussion of the advantages of combining a set of diagnostics when studying RWP life cycles. Implications of these results on the forecasting of extreme weather in SA are discussed.