ENSO modulation of MJO and its impacts on South America: enhancement of extreme events

The changes in the Madden-Julian Oscillation (MJO) and its impacts on the South American monsoon season during different El Niño-Southern Oscillation (ENSO) states (El Niño – EN, La Niña – LN, neutral – NT) are analyzed in the global context of the MJO propagating anomalies of convection and circulation. The background ENSO-related anomalies influence several aspects of MJO (relative occurrence of phases, propagation, convection and teleconnections), and therefore modify the MJO impacts on South America (SA), such as precipitation anomalies and frequency of extreme events, as well as their temporal distribution throughout the MJO cycle. Changes include: (1) a delay in the peak of the teleconnections between central-eastern Pacific and SA, from MJOphase8 in LN to MJOphase1 in EN; (2) enhanced MJO convection in the central-east subtropical South Pacific in MJO_LNphases7+8 and a little further east in MJO_ENphases8+1, in a region efficient in generating tropics-extratropics teleconnections via Rossby wave to SA, producing rainfall anomalies over Central-East SA (CESA), especially the South Atlantic Convergence Zone (SACZ), strongest one phase earlier in LN (MJO_LNphase8) than in EN (MJO_ENphase1), and a little shifted east in the latter than in the former; (3) enhancement of the extratropical teleconnection via Rossby wave and its impacts over subtropical CESA in both EN and LN (with regard to NT), suggesting that both ENSO states generate forcing in the source region that more efficiently triggers stronger Rossby waves than forcing in NT ENSO years, indicating nonlinear ENSO effects on MJO anomalies over SA; (4) predominant increase (or reduction) in the frequency of extreme events over densely populated SA regions where both ENSO and MJO contribute in the same direction, with the greatest increase over CESA (including SACZ) during EN, in MJO_ENphase1, and over Southeast SA (SESA), in MJO_ENphase3; (4) enhanced amplitude in both states, EN and LN, of the first continental intraseasonal dipole-like mode of precipitation variability between CESA and SESA, with maximum opposite anomalies in CESA, the center with largest amplitude, in phases 1 and 4 for EN, and phases 8 and 5 for LN. Significant effects can also be observed in other regions, such as northeast and northwest SA.