The Relationship of the Madden-Julian Oscillation with Interdecadal Variability Modes During the Monsoon Season in South America

The Madden-Julian Oscillation (MJO) has been a topic of great scientific interest due to its higher predictability and significant impact on global climate and weather, including the South American monsoon system. As a pillar of subseasonal predictability, it is important to investigate the influence of major interdecadal oscillations, such as the Interdecadal Pacific Oscillation (IPO) and the Atlantic Multidecadal Oscillation (AMO), to assess the potential modulation of MJO impacts by these oscillations. How do variations induced by these slower oscillations influence MJO teleconnections to South America (SA)? What is the frequency of MJO phases during the austral summer, and how might the interaction between the MJO and these oscillations affect the monsoon in SA? The combined impact of the MJO and low-frequency oscillations was characterized by composites of daily anomalies filtered in the 20-90 days band during the austral summer (DJF, rainy season), when the MJO is strongest. Composite anomalies of convection and circulation were analysed over the entire period from 1979 to 2020, as well as during two periods characterized by distinct combinations of opposite phases of the IPO and AMO: IPO(+)/AMO(-) (1979-1999, Period 1) and IPO(-)/AMO(+) (2000-2020, Period 2). Results indicate that during DJF, convection anomalies and the frequency of extreme events over SA are more pronounced in the Period 1 compared to Period 2, particularly in the central-east SA (CESA), the core monsoon region. In this region, increased (reduced) precipitation is observed during MJO phases 8 and 1 (4 and 5). Previous findings (Grimm, 2019) using Influence Function analysis, based on an extended vorticity equation model, and simulations, indicated a link between anomalous convection over the central subtropical South Pacific (CSSP) and the SA during phase 8 of the MJO, which may be responsible for the convection pattern in the CESA in phase 1. This anomalous convection in CSSP is stronger in Period 1 than in Period 2. Furthermore, there is a reversal in the sign of convective anomalies from reduced to enhanced precipitation in phase 6 over CESA from Period 1 to Period 2 and this may be associated with the change from reduced to enhanced convection over CSSP during phase 5, through teleconnections. Therefore, convection associated with MJO events during Period 1 is stronger (weaker) than in Period 2 in the CESA region during phases 8 and 1 (4, 5, 6 and 7). In contrast, in southern Brazil, positive convection anomalies persist from phases 3 to 5 in Period 1, whereas in Period 2 these anomalies are observed only in phases 3 and 4. Additionally, during Period 2, a reversal of the anomalies occurs in phases 1 and 2 compared to Period 1.