The South Atlantic Dipole: Mechanisms, variability and impacts over South America

The South Atlantic Dipole: Mechanisms, variability and impacts over South America Anita Drumond Vale Institute of Technology Sustainable Development (ITV-DS) With the collaboration of Renata Tedeschi ITV-DS Natan Nogueira Michelle S. Reboita Federal University of Itajubá 25/11/2025 1 Outline of the Presentation South Atlantic Climate Background Introducing the South Atlantic Dipole (SAD) Evolution of Scientific Understanding Physical Mechanisms Seasonal Variations and Indices Impacts on South American Climate Interactions With Other Climate Modes Climate Change Effects Model Representation and Future Projections Final Remarks 25/11/2025 2Anita Drumond - South Atlantic Dipole (SAD) From previous lectures: •Teleconnections link distant regions. •Role of the oceans in climate variability •ENSO, PDO, SAM, IOD, AMO, TAD and affect South America •What about the South Atlantic Ocean? Is there a variability mode that can influence the climate in the surrounding regions? 25/11/2025 3Anita Drumond - South Atlantic Dipole (SAD) Why studying teleconnections matters Reboita et al. (2021) 25/11/2025 4Anita Drumond - South Atlantic Dipole (SAD) Let’s review the main South Atlantic climatological features SASH SASH SPSH SPSH Trade winds SACZ 25/11/2025 5Anita Drumond - South Atlantic Dipole (SAD) And the main South Atlantic oceanic features •Benguela Current (BeC) – cold, flowing northward along the African coast •South Equatorial Current (SEC): westward flow • Brazil Current (BC) – warm, flowing southward along the Brazilian coast •Malvinas (Falklands) Current (MC): cold subpolar current flowing northward, meeting the warm Brazil Current near 35°S •Brazil–Malvinas Confluence Zone: region of strong temperature gradients •Subtropical Gyre transfers heat and momentum •South Atlantic Current (SAC): toward Africa. Ksepka and Thomas (2021) (Reboita et al., 2021, and references therein) •The South Atlantic Dipole (SAD) is the dominant mode of coupled atmosphere–ocean variability over the South Atlantic •Characterized by a northeast–southwest oriented dipole pattern with centers of action over the tropical and extratropical South Atlantic •It presents variability on intraseasonal, interannual, and interdecadal timescales 25/11/2025 6Anita Drumond - South Atlantic Dipole (SAD) What is the South Atlantic Dipole (SAD)? Correlation between SST anomalies and SVD time coefficients obtained from the SST indicating the SAD adapted from Bombardi et al. (2014) •Its investigation still faces many challenges •Few in-situ observations over the South Atlantic and surrounding countries before the 1980s (onset of the Satellite Era) 25/11/2025 7Anita Drumond - South Atlantic Dipole (SAD) Why studying the SAD makes us feel so sad sometimes? 25/11/2025 8Anita Drumond - South Atlantic Dipole (SAD) The ERA5 global reanalysis from 1940 to 2022 Maps of data counts for the conventional part of the observing system, separating by vicennium (20year time period, from left to right, except for the last column, which shows observations until December 2022) and by observable (from top to bottom). Numbers indicate the average data counts assimilated per month. White areas indicate no data assimilated. Fig1 Soci et al. (2024) •Known Impacts mainly limited to South America and Africa •Less attention compared to ENSO or the Indian Ocean Dipole •Results are dependent on the data, time period, and statistical method used 25/11/2025 9Anita Drumond - South Atlantic Dipole (SAD) Why studying the SAD makes us feel so sad sometimes? Yet, beyond these challenges, the SAD offers many opportunities for new research! Physical Processes •Nnamchi et al. (2011) explain the SAD evolution through processes that peak in winter, also showing a ocean– atmosphere coupling. •New understanding (Manta et al., 2024) –The SAD is shaped by both atmospheric and oceanic variability. –Their relative influence changes with timescale of oscillations the SAD is complex, and there is more than one way to interpret its physical drivers. 25/11/2025 16Anita Drumond - South Atlantic Dipole (SAD) Morioka et al. (2011) 1st SST EOF 1960-2008 (20.4%) HADISST 25/11/2025 17Anita Drumond - South Atlantic Dipole (SAD) Seasonal Variants: SASD and SAOD •The SAD presents seasonal variations (Nnamchi et al., 2017), expressed through two main modes. •South Atlantic Subtropical Dipole (SASD) Morioka et al. (2011) • Peaks in austral summer, when SASH shifts southward • Confined to subtropical latitudes •South Atlantic Ocean Dipole (SAOD) Nnamchi et al. (2011) • Peaks in austral winter, when SASH shifts northward • Extends across broader latitudes Nnamchi et al. (2011) 2nd SST EOF 1950-2008 Nogueira et al. (2025) in preparation 25/11/2025 18Anita Drumond - South Atlantic Dipole (SAD) How do we monitor SAD? •Two main Indices based on SST anomaly difference between two regions •SAD → NE pole - SW pole •+ SAD → warmer NE pole & cooler SW pole •Index choice may affect results South Atlantic Ocean Dipole Index SAODI –(Nnamchi et al., 2011) red boxes • NE pole: 10°E–20°W, 0–15°S • SW pole: 10–40°W, 25–40°S •It captures equatorial influences. South Atlantic Subtropical Dipole Index SASDI –(Morioka et al., 2011) green boxes • Focuses on subtropical variability • NE pole: 0–20°W, 15–25°S • SW pole: 10–30°W, 30–40°S Important Note • SAODI’s northern pole overlaps the Atlantic Niño region • SASDI’s southern pole is fully inside SAODI’s southern domain 25/11/2025 19Anita Drumond - South Atlantic Dipole (SAD) Strengths and Limitations of SAD Indices Strengths • Simple and widely used for monitoring SAD variability • Capture the SST dipole pattern Limitations • Sensitive to box definitions (small shifts can change index values) • SAODI partly overlaps with the Atlantic Niño region → may mix signals • Indices do not separate atmosphere–ocean coupling mechanisms • Cannot represent spatial shifts (e.g., latitudinal migration of SAD) 25/11/2025 20Anita Drumond - South Atlantic Dipole (SAD) Monitoring: how do we define extreme SAD events? • Extreme SAD events Commonly defined when SAODI or SASDI exceed ±1 standard deviation from their climatological mean. • Public time series available Both indices (SAODI, SASDI) are openly provided by the Teleconnection Index Online Tool – Federal University of Itajubá https://app-indice.streamlit.app/ (Drumond et al., 2025; in review). 25/11/2025 21Anita Drumond - South Atlantic Dipole (SAD) How Does the SAD relate with South American Climate? Relationship between SAD and the SAMS (Bombardi & Carvalho, 2011) •SAD associated with dipole in rainfall between SE and NE Brazil •Negative SAD (cool NE pole x warm SW pole) – stronger SASH SAMS (Bolivia-SE Brazil) wet season: earlier onset, longer & wetter NE Brazil wet season: Later onset, shorter & drier •Positive SAD (warm NE pole x cool SW pole) – weaker SASH SAMS: later onset, shorter & drier NE Brazil: earlier onset, longer & wetter Bombardi & Carvalho (2011) 25/11/2025 22Anita Drumond - South Atlantic Dipole (SAD) •Key process (Bombardi et al., 2014): A Rossby wave train from the extratropical Pacific. Negative SAD •Cyclonic anomalies over SE Brazil •Enhanced moisture transport from Amazon •Intensification SACZ activity Positive SAD •Anticyclonic anomalies over SE Brazil •Reduced moisture inflow from Amazon towards SE Brazil and weaker SACZ activity Bombardi et al. (2014) 25/11/2025 23Anita Drumond - South Atlantic Dipole (SAD) Methodological Sensitivity in Assessing SAD Impacts Methodological choices may shape the SAD–precipitation relationships. Example: SAODI - low skill in summer (weak N pole SST signal). Analysis based on SAODI may present low impact during Summer. Reboita et al. (2021): SAODI composites — Negative SAODI → wetter northern SA during winter and spring Carpenedo & da Silva (2022): linear correlations using SAODI — Except Summer: southern Cerrado: Negative SAODI → wetter; positive SAODI → drier. Northern Brazil: Negative SAODI → drier; positive SAODI → wetter. 25/11/2025 24Anita Drumond - South Atlantic Dipole (SAD) How Does the SAD Interact With Other Climate Modes? SAD and Other Southern Hemisphere Dipoles 25/11/2025 25Anita Drumond - South Atlantic Dipole (SAD) •SAD often occurs with other dipole patterns in SH: SIOD (South Indian Ocean Dipole) and South Pacific dipole, mainly in summer (e.g. Venegas et al., 1997; Hermes & Reason, 2005; Yu et al., 2023). •These dipoles are influenced by large-scale atmospheric wave pattern in SH circulation known as the zonal wavenumber-3/4 •These wave patterns modify subtropical anticyclones, triggerring the dipoles Regression maps of the SST anomalies (C) on the summertime indice of SAOD over 1979–1999. Yu et al. (2023) Final Remarks •Climate change: changes in ENSO regime and teleconections; expansion of SASH that shifts SACZ and SAD southward •CMIP6 models reproduce the SAD basic patterns but fail to capture some ocean–atmosphere coupling and show large uncertainty in future SAD–rainfall projections. •Key open questions: •Long-term trends and mechanisms behind SAD seasonality •Changing ENSO–SAD link •Predictability and role of ocean–atmosphere coupling •Regional impacts in South America and Africa 25/11/2025 32Anita Drumond - South Atlantic Dipole (SAD) 25/11/2025 33Anita Drumond - South Atlantic Dipole (SAD) Acknowledgements Postdoc supported by project R100603.PMP Thank you! anit[email protected] Bier, A. A. (2023). Changes in the South Atlantic Dipole and impacts on the climate of South America under the aspect of climate change. Doctoral Thesis, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, University of São Paulo, São Paulo. doi:10.11606/T.14.2023.tde-04032024-194824. Retrieved 2025-11-17, from www.teses.usp.br Bombardi, R. J., & Carvalho, L. M. (2011). The South Atlantic dipole and variations in the characteristics of the South American Monsoon in the WCRP-CMIP3 multi-model simulations. Climate Dynamics, 36, 2091-2102. Bombardi, R.J., L.M.V. Carvalho & C. Jones. 2014. Precipitation over eastern South America and the South Atlantic Sea surface temperature during neutral ENSO periods. Clim. Dyn. 42: 1553–1568. Boschat, G., Terray, P., & Masson, S. (2013). Extratropical forcing of ENSO. Geophysical Research Letters,40(8), 1605-1611. Carpenedo, C. B., & Da Silva, C. B. (2022). Influência de teleconexões na precipitação pluvial do cerrado brasileiro. Revista Brasileira de Climatologia,30, 26-46. Drumond, A. R. D. M., & Ambrizzi, T. (2005). The role of SST on the South American atmospheric circulation during January, February and March 2001. Climate Dynamics, 24(7), 781-791. Drumond, A.; Nogueira, N. C. O; Reboita, M. S.; Miguel, G. C. (2025). Teleconnection Indices: an updated version for the current climate. In review in CLIVAR Exchange. Ham, Y. et al. Inter‐Basin Interaction Between Variability in the South Atlantic Ocean and the El Niño/Southern Oscillation. Geophysical Research Letters, v. 48, n. 15, 5 ago. 2021. Hermes, J. C., & Reason, C. J. C. (2005). Ocean model diagnosis of interannual coevolving SST variability in the South Indian and South Atlantic Oceans. Journal of Climate,18(15), 2864-2882. Kayano, M.T., R.V. Andreoli & R.A.F. Souza. 2012. Relations between ENSO and the South Atlantic SST modes and their effects on the South American rainfall. Int. J. Climatol. 33: 2008–2023. References Ksepka, D. T., & Thomas, D. B. (2012). Multiple cenozoic invasions of Africa by penguins (Aves, Sphenisciformes). Proceedings of the Royal Society B: Biological Sciences, 279(1730), 1027-1032. Manta, G., Bach, E., Talento, S., Barreiro, M., Speich, S., & Ghil, M. (2024). The South Atlantic Dipole via multichannel singular spectrum analysis. Scientific Reports, 14(1), 15534. Martín-Gómez, V., Barreiro, M., & Mohino, E. (2020). Southern Hemisphere sensitivity to ENSO patterns and intensities: Impacts over subtropical South America. Atmosphere,11(1), 77. Morioka, Y., T. Tozuka & T. Yamagata. 2011. On the growth and decay of the Subtropical Dipole Mode in the South Atlantic. J. Clim. 24: 5538–5554. Morioka, Y. et al. Role of tropical SST variability on the formation of subtropical dipoles. J. Clim. 27, 4486–4507. https:// doi. org/10. 1175/ JCLI-D1300506.1 (2014). Nnamchi, H.C., J. Li & R.N.C. Anyadike. 2011. Does a dipole mode really exist in the South Atlantic Ocean? J. Geophys. Res. Atmos. 116: D15104. Nnamchi, H.C., F. Kucharski, N.S. Keenlyside, et al. 2017. Analogous seasonal evolution of the South Atlantic SST dipole indices. Atmos. Sci. Lett. 18: 396–402. Nworgu, U. C., Nnamchi, H. C., & Rosário, N. E. (2024). Divergent future change in South Atlantic Ocean Dipole impacts on regional rainfall in CMIP6 models. Environmental Research: Climate,3(3), 035002. Palastanga, V., Vera, C. S., & Piola, A. R. (2002). On the leading modes of sea surface temperature variability in the South Atlantic Ocean. Clivar Exchanges, 7(3-4), 12-15. Reboita, M. S., Ambrizzi, T., Crespo, N. M., Dutra, L. M. M., Ferreira, G. W. D. S., Rehbein, A., ... & Souza, C. A. D. (2021). Impacts of teleconnection patterns on South America climate. Annals of the New York Academy of Sciences, 1504(1), 116-153. References Rodrigues, R. R., Campos, E. J. D. & Haarsma, R. The Impact of ENSO on the South Atlantic Subtropical Dipole Mode. J. Clim. 28, 2691–2705. https:// doi. org/ 10. 1175/ JCLI-D1400483.1 (2015). Soci, C., Hersbach, H., Simmons, A., Poli, P., Bell, B., Berrisford, P., et al. (2024) The ERA5 global reanalysis from 1940 to 2022. Quarterly Journal of the Royal Meteorological Society, 150(764), 4014–4048. Available from: https://doi.org/10.1002/qj.4803 Venegas, S. A., Mysak, L. A., & Straub, D. N. (1997). Atmosphere–ocean coupled variability in the South Atlantic. Journal of Climate, 10(11), 2904-2920. Wang, L., Yu, J. Y., & Paek, H. (2017). Enhanced biennial variability in the Pacific due to Atlantic capacitor effect. Nature communications,8(1), 14887. Yu, L., Zhong, S., Winkler, J. A., Zhou, M., Lenschow, D. H., Li, B., ... & Yang, Q. (2017). Possible connections of the opposite trends in Arctic and Antarctic sea-ice cover. Scientific reports,7(1), 45804. Yu, L., Zhong, S., Vihma, T., Sui, C., & Sun, B. (2022). A change in the relation between the Subtropical Indian Ocean Dipole and the South Atlantic Ocean Dipole indices in the past four decades. Atmospheric Chemistry and Physics Discussions,2022, 1-22. Yu, L., Zhong, S., Vihma, T., Sui, C., & Sun, B. (2023). A change in the relationship between ENSO and the South Atlantic Subtropical Dipole in the past four decades. EGUsphere,2023, 1-30. Yu, L., Zhong, S., Vihma, T., Sui, C., & Sun, B. (2024). Multidecadal variations in North Atlantic SSTs modulate the relationship between ENSO and the South Atlantic Subtropical Dipole since 1900. Environmental Research Letters,19(9), 094001. ZILLI, M. T.; CARVALHO, L. M. V.; LINTNER, B. R. The poleward shift of South Atlantic Convergence Zone in recent decades. Climate Dynamics, v. 52, n. 5–6, p. 2545–2563, 30 mar. 2019. References