Impact of Madden-Julian Oscillation on South America and its teleconnection with southern Africa

The impact of different phases of the Madden Julian Oscillation (MJO) on the daily precipitation amounts and frequency of extreme rainfall events in South America is assessed with unprecedented temporal and spatial coverage, and the mechanisms of this impact are analyzed. Besides, teleconnections between intraseasonal oscillations of precipitation in South America and southern Africa are detected for different seasons, with major focus on austral summer and transition seasons.

The analysis is based on daily precipitation station data over South America in the period 1979-2009, gridded to 1º, and submitted to a bandpass Lanczos filter, which retains intraseasonal oscillations in the 20-90 day band. The different phases of the MJO are determined through indices based on the first two empirical orthogonal functions of the combined fields of zonal wind at 850 hPa and 200 hPa and OLR, between 15ºS e 15ºN, after removal of the annual cycle and interannual variability. Composites of precipitation anomalies and computation of the differences in the frequency of extreme events are made for each phase, and their significance is assessed with a non-parametric test based on the hypergeometric distribution. MJO-related average anomalies of daily precipitation are greater than 5 mm/day in certain regions and phases. The associated composites of global streamfunction, velocity potential, and OLR anomalies, as well as of the continental moisture transport and its divergence give insights on the mechanisms of the impact.

The connection between precipitation in southern Africa and South America in each season is detected through correlation of filtered daily precipitation averaged over several relatively homogeneous regions in Africa with filtered precipitation in each 1° x 1° grid box with data over South America. In such correlation, lags from 0 up to 5 days are applied to the African data, in order to disclose convective anomalies over South America that could produce atmospheric perturbations associated with the precipitation anomalies over South Africa. Examples of teleconnections are given for two of such regions in South Africa. The composites of 200 hPa streamfunction anomalies associated with the beginning of positive phases of intraseasonal oscillations in rainfall over these regions (days in which precipitation anomalies start exceeding one standard deviation) exhibit wavetrains connecting both continents, with strong cyclonic anomaly centered southwest of the selected African regions, as expected. An influence function analysis based on the vorticity equation for target points in the center of these anomalies indicates that perturbations of the upper level divergence associated with MJO-related anomalous convection over South America are able to produce the atmospheric circulation anomalies associated with enhanced precipitation in those regions of South Africa. Simulations with a vorticity equation model that includes the divergence of the basic state and advection of vorticity by anomalous divergent wind, confirm the observed connection. Therefore, although internal atmospheric variability may be associated with the intraseasonal oscillation of precipitation in South Africa, the anomalous convection over South America can enhance these oscillations. Strongest contributions occur in summer and transition seasons by convection anomalies during the South American summer monsoon.