Interannual Variability of the Onset Date of the South American Monsoon in Observations and the CESM Large Ensemble

Most of the precipitation over continental South America is due to the South American Monsoon System (SAMS). How long the monsoon season lasts, or the onset and retreat dates of the SAMS, are crucial for water security in the area. As the Amazon rainforest is located within the region, the rainfall variability in the area also affects the Earth’s carbon budget as during drought the Amazon is less productive. A better understanding of interannual variability in the onset/retreat dates therefore has important consequences for understanding interactions in the entire Earth System.

Here, we compute the onset and retreat dates of the SAMS in observations and reanalysis data. Early and late onset and retreat dates are associated with modes of climate variability (in atmosphere and ocean), local atmospheric circulation features and vegetation activity.

Preliminary results show that although the location and intensity of the Intertropical Convergence Zone (ITCZ) has little influence on the onset date of the SAMS, it significantly affects its retreat date. Similarly, El Niño Southern Oscillation (ENSO) is connected to the SAMS retreat-date variability due to its influence on ITCZ position over the Atlantic. However, the onset date of the SAMS has a greater correlation with the Indian Ocean Dipole than with ENSO. Earlier onset of the SAMS is related to an earlier development of an upper level trough over Northeastern Brazil (“Nordeste Trough”) and to the intensity of an upper level high pressure system over the Amazon.

These results were also obtained when we examined historical runs in the Community Earth System Model Large Ensemble (CESM-LE). Although the CESM-LE somewhat underestimates the interannual variability of the SAMS onset date, it successfully reproduces its intraseasonal variability of the Nordeste Trough and the upper-level high pressure system over the Amazon.