Modo Meridional do Pacífico e o ENOS: relações e impactos na precipitação da América do Sul

Abstract

This study sought to characterize the meridional mode of Sea Surface Temperature (SST) in the Pacific (PMM) and its influence over the development of El Niño-Southern Oscillation (ENSO), as well as its effects independent from/combined with ENSO over the precipitation in South America (SA) for the 1951 to 2020 period using composite analysis. Primarily, PMM is associated with mostly negative (positive) anomalies over the northern and southeastern (northeastern) regions of SA during positive (negative) PMM dominant period, consistent with these patterns, changes in moisture flux divergence and vertical movement over SA are observed. Velocity potential anomalies indicate changes in the Walker circulation in response to El Niño (EN) preceded by positive PMM (PPMM) as well as more meridional patterns indicating changes in the Hadley circulation in response to EN preceded by negative PMM (NPMM). On the other hand the combined effect of PMM and ENSO in precipitation over SA presents distinct characteristics in relation to the effects of either mode separately. During the mature phase of an isolated EN, the precipitation pattern is characterized by negative anomalies over the north and tropical west coast of SA and positive anomalies over the south of northeastern SA, whereas for the scenario where EN is preceded by PPMM the pattern shows significant positive (negative) anomalies over Colombia and the northern and northeastern (southeastern) SA, for the scenario where EN is preceded by NPMM it is noticeable that the negative anomalies tend to stay more confined to the northern part of SA not causing significant negative anomalies in the central part of the Amazon, instead, a precipitation dipole occurs between the northeastern (negative anomalies) and southeastern (positive anomalies) SA. For La Niña (LN) the effects during the mature phase of the phenomenon are best characterized between the eastern Amazon and the northern and northeastern coasts of SA whereas, for the LN preceded by NPMM, positive precipitation anomalies tend to occur along the west coast of SA and northeastern Amazon with significant negative anomalies over the Brazilian northeast. Theses differences are associated to the fact that an EN when preceded by PPMM (NPMM) tend to develop over the central (eastern) equatorial Pacific, whereas LN start to develop over central equatorial Pacific when preceded by NPMM. Furthermore, when preceded by PPMM, EN tends to persist during its mature phase, indicating late decay. Thus, changes in ENSO development associated with PMM phases is responsible for the different patterns of precipitation over SA during ENSO’s development and mature period.