Modulação das oscilações de baixa frequência na interação enos-oceano índico e seus impactos na precipitação da América do Sul

Abstract

This study investigated the modulation of low-frequency oscillations in the relationship between the El Niño Southern Oscillation (ENSO) and the Indian Ocean modes, as well as their effects on precipitation over South America (SA). Climatic indices and composite analysis were used for the period 1901-2012. The main contributions are: The Interdecadal Pacific Oscillation (IPO) modulates the decay stage of ENSO and its relationship with the Indian Ocean Basin Mode (IOBM). During the positive (negative) phase of the IPO, El Niño (La Niña) events decay more slowly (rapidly) and are accompanied by the warming (cooling) of the ocean basin. The connection between these basins occurs through the anomalous Walker circulation. During the negative phase of the IPO, ENSO events in the decay stage have a faster transition to the opposite phase (La Niña to El Niño or La Niña to El Niño) in the following year, resulting in changes in zonal circulation. Precipitation in South America, especially in northeastern Brazil, is influenced by the interaction between the ENSO and the phase of the IPO. In El Niño events during the positive (negative) phase of the IPO, there is more (less) precipitation in northeastern South America. Thus, the EN (LN) and positive (negative) IOBM during POS (NEG) IPO phases prolong the scarcity of precipitation over the equatorial (subtropical) region of SA. La Niña events during the negative phase of the IPO result in an increased (decreased) rainfall in the tropical (subtropical) region of South America, associated with anomalies in the Walker circulation and wave train patterns. The Atlantic Multidecadal Oscillation (AMO) plays a role in modulating the relationship between ENSO and the Indian Ocean Dipole (IOD) during the winter and spring seasons. Combined events of El Niño-positive IOD exhibit a strengthened (weakened) connection during the cold (warm) phase of the AMO. The combination of El Niño and positive IOD causes a propagation of the tropical-extratropic wave pattern and associated with the transport of moisture from the North Tropical Atlantic, causes decreased (increased) of rainfall in the Amazon (southeastern SA). Isolated positive IOD events occur more frequently during the warm phase of the AMO. In this condition, the warmer North Tropical Atlantic combined with the wave pattern originating from the Indian Ocean generates reduced rainfall in the Amazon, central, and southeastern regions of SA. On the other hand, during El Niño years, the Equatorial Atlantic exhibits anomalously colder SST and directs moisture towards the Amazon and central-western SA. As a result, there is an increase in precipitation in the southern regions of the Amazon, SACZ, and southeastern Brazil. For LN events and Negative Indian Ocean Dipole (nIOD) events, it is not possible to separate the modulation by the AMO, as the events occur in both phases. This shows that the relationship between EN-pIOD and LN-nIOD events is not linear in relation to low-frequency modulation. In La Niña events, South America experiences increased (decreased) in precipitation in the north (southeast) and more scarcity in the SACZ region and the southern Amazon Basin. nIOD years exhibit the opposite precipitation pattern to LN. The precipitation is characterized by decrease (increase) of rainfall in northern SA (central-western SA and southern Amazon Basin). The coincident LN-nDOI events characterize the precipitation dipole with an increased (decreased) rainfall in the central-eastern (southeastern) regions of SA, emphasizing that the combination of these events is fundamental for the establishment of the precipitation dipole over SA. In summary, low-frequency oscillations such as the IPO and AMO modulate the relationship between ENSO and Indian Ocean modes on an interannual scale, impacting the precipitation regime in South America. Understanding these interactions is crucial for seasonal forecasts and monitoring extreme weather events, such as floods and droughts. This study highlights the importance of considering the complex interactions between the Pacific, Atlantic, and Indian Oceans and their impact on precipitation variability in South America.