Role of Austral Summer Upper-Level Jets in Modulating ENSO response on Precipitation and Temperature in South America
Authors: Collazo Soledad , García-Herrera Ricardo , Barriopedro David 
Published: July 2023

The upper-level jet stream is a crucial component of atmospheric circulation, as it propels synoptic systems and their associated extreme weather events. In addition, the El Ni&#241;o-Southern Oscillation (ENSO) is the most significant driver of global interannual variability, exerting its influence on temperature and precipitation patterns worldwide through teleconnections. This study aims to analyze the modulation exerted by upper-level jets on the South American summer temperature and precipitation under different ENSO phases, utilizing the ERA5 reanalysis dataset from 1979 to 2022. For this purpose, a multiparametric daily characterization of the jet streams is first conducted. The next step involves applying the Partitioning Around Medoids clustering technique, using these parameters to identify the synoptic weather types of the zonal wind in the upper troposphere. We quantify the frequencies of these patterns during different ENSO phases and analyze the composites of temperature and precipitation over South America, considering the joint influence of jet stream patterns and ENSO. An analysis of the jet parameters shows a significant trend of the polar front jet (PFJ) intensifying and shifting southward. The upper-level jet patterns obtained are strongly influenced by the presence or absence of the subtropical jet (STJ), while the PFJ is found to be consistently present. When the STJ is absent (about 45% of summer days), a blocking pattern over the Atlantic Ocean and an active South American Convergence Zone are detected as the main features of the geopotential height anomaly field at 500 hPa. Warm (cold) anomalies associated with this circulation have been observed in southern South America (Brazil). Precipitation anomalies are less robust but are significant in southeastern South America, with drier conditions, and in northeastern Brazil, with wetter conditions. This jet configuration occurs slightly more frequently during the La Ni&#241;a phase. Composites of the maximum temperature anomalies under this upper-level atmospheric circulation pattern (without STJ) and discrimination according to the ENSO phase reveal that the synoptic signal is dominant over the low-frequency variability signal. In other words, the absence of the STJ leads to warm (cold) conditions in southern South America (Brazil), regardless of the ENSO phase. The same analysis of precipitation shows significant results only in localized areas of the continent. Finally, we examine the impact of the jet stream on warm spells (WS). Our findings indicate that the occurrence of WS in many regions of South America, including most of Argentina, Uruguay, central and northern Chile, and eastern Brazil, is influenced by the upper-level circulation patterns. This study provides a better understanding of the complex interactions between atmospheric circulation patterns and large-scale climate drivers.
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Acknowledge: The SAFETE project, which has received funding from the European Union&#8217;s Horizon 2020 research and innovation programme under the Marie Sk&#322;odowska-Curie grant agreement No 847635