For this study, precipitation and GPH data at 250 hPa from NASA’s Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA2) were used (33). The dataset covers the period from 1980 to 2016 and consists of 6-hourly data on a 1/3° × 2/3° rectangular grid. The precipitation data were smoothed using a moving average with a window size of 4 days. The two reference regions for the precipitation dipole were chosen in accordance with previous research (12), and the mean of all grid cells within these boxes was used as an index for the precipitation in SEBRA and SESA, respectively. The reference region for inferring Rossby wave activity was chosen over southern South America (see Fig. 1). The results we report below are very robust to changes in position and size of this box; it is important, however, that the reference region is not much larger than half of a typical wavelength of the wave train to still properly capture its oscillating behavior. The mean of the GPH at 250 hPa of all grid nodes within this reference thus serves as an index for the Rossby wave train. Figure 1 (A and B) shows the reference regions and the typical opposing configuration of the Rossby wave trains in the GPH anomaly fields during extreme precipitation (above the 90th percentile) in SEBRA and SESA. The precipitation itself is shown in Fig. 1 (C and D). The South American precipitation dipole is a phenomenon restricted to the austral summer from NDJF. Because some of the methods used here are easier to handle with data that have a regular time axis without jumps, all-year data were used at first. The crucial parts of the analysis are, however, limited to the NDJF data.

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