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Atmospheric response to SST isolation on the interannual variability scale during ENSO episodes

Rildo Gonçalves de Moura, Francis Wagner Silva Correia, José Augusto Paixão Veiga, Vinicius Buscioli Capistrano, Paulo Yoshio Kubota

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ABSTRACT: This study is aimed to evaluate the impact of SST, isolated on an interannual scale, on the dynamics of atmospheric circulation, especially in the tropical region of the Pacific and Atlantic Oceans. Although there are several studies based on observations and numerical modeling, the isolated effects caused by a specific mode of variability are not yet fully known. For this purpose, the Brazilian Global Atmospheric Model (BAM) forced with an isolated interannual SST, obtained by Moura et al. 2020, was used as a surface boundary condition. To quantify the response of the atmosphere to this time scale, anomalous atmospheric fields were obtained from a reference numerical experiment, where unfiltered SST fields were used as boundary conditions. The effects of the interannual time scale on the dynamics of atmospheric circulation were evaluated for the positive and negative phases of ENSO. In general, when the atmosphere is forced by the isolated interannual variability mode, the atmospheric circulation patterns become weaker than the variability obtained by non-isolated mode. This weakening is statistically significant and independent of the positive or negative phase of ENSO. The isolated interannual mode significantly reduced the areas of precipitation and its intensity in South America, especially over the Amazon basin and northeastern Brazil. However, changes in precipitation in these regions are more evident in the positive phase of ENSO than in the negative phase. The SST filtering methodology and the use of an atmospheric general circulation model applied in this study adequately revealed the purely dynamic effects of the isolated interannual mode on the atmosphere.