Southern Hemisphere Sensitivity to ENSO Patterns and Intensities: Impacts over Subtropical South America

Abstract

El Niño flavors influence Subtropical South American (SSA) rainfall through the generation of one or two quasi-stationary Rossby waves. However, it is not yet clear whether the induced wave trains depend on the El Niño pattern and/or its intensity. To investigate this, we performed different sensitivity experiments using an Atmospheric General Circulation Model (AGCM) which was forced considering separately the Canonical and the El Niño Modoki patterns with sea surface temperature (SST) maximum anomalies of 1 and 3 °C. Experiments with 3 °C show that the Canonical El Niño induces two Rossby wave trains, a large one emanating from the western subtropical Pacific and a shorter one initiated over the central-eastern subtropical South Pacific. Only the shorter wave plays a role in generating negative outgoing longwave radiation (OLR) anomalies over SSA. On the other hand, 3 °C El Niño Modoki experiments show the generation of a large Rossby wave train that emanates from the subtropical western south Pacific and reaches South America (SA), promoting the development of negative OLR anomalies over SSA. Experiments with 1 °C show no impacts on OLR anomalies over SSA associated with El Niño Modoki. However, for the Canonical El Niño case there is a statistically significant reduction of the OLR anomalies over SSA related to the intensification of the upper level jet stream over the region. Finally, our model results suggest that SSA is more sensitive to the Canonical El Niño, although this result may be model dependent.

The dominant modes of the long rains interannual variability over Tanzania and their oceanic drivers

“…IEMI has an advantage over EMI as proposed by Ashok et al (2007), since it accounts the effect of SSTA over the western tropical Pacific on capturing EMI and could equally monitor the weak CP ENSO events better (Yeh et al, 2009). The IEMI has been used to examine the impacts of the CP ENSO on rainfall over subtropical South American (e.g., Martín-G omez et al, 2020). Subsequently, same delineation was applied in the current study to determine the sensitivity of rainfall to SSTA over the CP ENSO.…”

Drought metrics and temperature extremes over the Okavango River basin, southern Africa, and links with the Botswana high

“…Evidence has been provided that these anomalous ENSO cases tend to be associated with regional circulation features (the Angola Low, the Botswana High) not being impacted to the same extent as during other ENSO events and thus the expected rainfall response is muted (Reason & Jagadheesha, 2005; Blamey et al, 2018; Driver et al, 2019). Blamey and Reason (2023) showed that even for seasons with the expected drier and hotter (wetter and cooler) conditions during El Niño (La Niña), there is a wide spread in the severity of these impacts over southeastern Africa; this spread is likely to also be the case as well for the ORB further suggesting the need to improve understanding and monitoring of the regional circulation features (Angola Low, Botswana High).…”

A late 1980s shift in El Niño influence on rainfall over Uruguay during austral spring

“…Such wave train, previously reported in Grimm and Ambrizzi (2009), Barreiro (2017) and Martín‐Gómez et al . (2020) was subsequently characterized through ITSA. A detailed study of this index showed that the centres of this wave have a strong positive trend and interannual variability related to ENSO and SAM during P1, while during P2 they vary jointly and are mainly linked to warming of the tropical Pacific Ocean.…”

“…In the results above it was possible to identify a short‐wave pattern associated with anomalous equatorial Pacific warming that emanates from the eastern basin and reaches SESA. This cyclonic–anticyclonic circulation dipole has been found instrumental for precipitation over SESA also in previous studies as it can modulate the meandering of the jet (e.g., Barreiro, 2017; Martín‐Gómez et al ., 2020). Thus, it is important to understand whether the centres that compose this dipole pattern show a similar evolution or if they evolve independently, answering to different processes.…”

“…On the other hand, when the equatorial Pacific Ocean warming occupies the eastern basin only one Rossby wave emerges from the central Pacific generating a regional circulation over SESA dominated by two cyclonic circulation anomalies located in the eastern Pacific and western Atlantic Ocean centred in 45°S. This configuration shifts the jet northward disfavouring rainfall over southern Uruguay and promotes it in the northern region (Barreiro, 2017; Martín‐Gómez et al ., 2020).…”