Intraseasonal teleconnections between South America and South Africa

Teleconnections of climate anomalies between various parts of the tropics and extratropics is a well-established feature of the climate system. Building on previous work showing that a teleconnection exists between the South American monsoon system and interannual summer rainfall variability over southern Africa, this study considers intraseasonal variability over these landmasses. Observed daily gauge precipitation data for 1970-1999 were gridded to 1° resolution for South America and 2.5° for South Africa. At each grid point, a bandpass Lanczos filter was used to isolate intraseasonal oscillations in the 20-90 day band. For each season, the filtered precipitation for the grid boxes over South Africa is correlated with filtered precipitation in the grid boxes over South America. The results shown here represent the best correlations for different climatic regimes such as the winter-rainfall dominated southwestern Cape, the all season rainfall South Coast and the summer-rainfall dominated Limpopo region. In performing the correlations, lags from 0 up to 5 days are applied to the South African data, in order to investigate convection anomalies over South America that could produce atmospheric perturbations associated with precipitation anomalies over South Africa. The significance assessment of the correlations is made with the Student's test using a reduced number of independent members in the series. NCEP re-analyses are used to composite intraseasonal anomalies in 200 hPa streamfunction, OLR, 500hPa omega, and vertically integrated moisture flux associated with precipitation anomaly above one standard deviation in the filtered series (positive phases) of the South African selected regions.

The possible origin of the atmospheric circulation anomalies associated with those positive phases is searched with influence functions (IFs) of a vorticity equation model with a divergence source. The model is linearized about a realistic basic state and includes the divergence of the basic state and the advection of vorticity by divergent wind. The IF for a given target point indicates the tropical/subtropical regions in which the anomalous upper-level divergence is most efficient in producing streamfunction anomalies around the target point. Regions in South America with upper-level anomalous divergence associated with anomalous convection are indicated by the precipitation correlation patterns or by anomaly composites of OLR or omega.

It is shown that teleconnections exist between the intraseasonal variability of South African daily rainfall and that over various areas of South America, with the latter leading by one to five days, for both winter and summer, involving regions with strong rainfall in these seasons. During the summer, the mechanisms involve both a modulation of the local Walker cell as well as extra-tropical Rossby wave trains. For winter, the latter mechanism is more important. While in summer tropical convective anomalies over South America play an important role, in winter the subtropics become more important. In both cases, these modulations lead to regional changes in circulation over southern Africa that are favourable for the dominant synoptic rainfall-producing weather systems such as cut-off lows and tropical extratropical cloud bands.