Java-Sumatra Niño/Niña: two long-lost siblings of El Niño

There are many siblings, cousins, and distant relatives of El Niño spanning the Pacific, Atlantic, and Indian Oceans who share a feature in common: ocean surface temperature anomalies along eastern boundaries linked to changes in the upwelling of cooler water from below. So far, climate scientists have identified a total of 14 members of this extended family. Atlantic Niño is a little brother of El Niño in the Atlantic Ocean. We have two sisters, La Niña and Atlantic Niña. There are four more siblings in the Atlantic Ocean, Benguela Niño/Niña along the coast of Angola and Namibia, and Dakar Niño/Niña along the east coast of West Africa. There are four more in the Pacific, California Niño/Niña and Chile Niño/Niña along the coast of California and Chilean coast, respectively. In the Indian Ocean, only two have been found so far, Ningaloo Niño/Niña off the west coast of Australia. A new study, published in the Journal of Climate, found two long-lost siblings of the El Niño family and named them Java–Sumatra Niño/Niña (JSN or JS Niño/Niña). JSN is characterized by the appearance of warm/cold sea surface temperature anomalies (SSTAs) in the coastal upwelling region off Java–Sumatra in the southeastern equatorial Indian Ocean. JSN develops in July–September and sometimes as a precursor to the Indian Ocean dipole, but often without corresponding SSTAs in the western equatorial Indian Ocean. JSN is an important driver of climate variability over the tropical Indian Ocean and the surrounding continents. Notably, JS Niña events developing in July–September project onto the South and Southeast Asian summer monsoons, increasing the probability of heavy rainfall and flooding across the most heavily populated regions of the world.

Fig. 9 in Lee et al. (2022). A summary schematic of the precipitation (shading) and low-level wind anomalies (arrows) associated with (top) El Niño and (bottom) Java-Sumatra Niña in July-September. Brown and green shades represent decreased and increased precipitation, respectively. El Niño events are often linked to the associated zonal seesaw pattern of rainfall anomalies with decreased rainfall over the Maritime Continent extending to India and increased rainfall in the western tropical Pacific. In contrast, some JS Niña events produce a meridional seesaw pattern of rainfall anomalies that projects onto the South and Southeast Asian summer monsoons, driven by a Gill-type cross-hemispheric atmospheric circulation in response to the cold SSTAs along the Java–Sumatra coast.

Lee, S.-K., Lopez, H., Foltz, G. R., Lim, E., Kim, D., Larson, S. M., Pujiana, K., Volkov, D. L., Chakravorty, S., & Gomez, F. A. (2022). Java–Sumatra Niño/Niña and Its Impact on Regional Rainfall Variability, Journal of Climate, 35(13), 4291-4308. https://doi.org/10.1175/JCLI-D-21-0616.1

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