How El Niño Reshapes the Salty Map of the Bay of Bengal

The Bay of Bengal has long been known as one of the strangest corners of the world ocean. Fed by some of the mightiest rivers on Earth, including the Ganges and the Brahmaputra, its northern waters are so fresh that they form a near-floating lid atop saltier water below. That lid, oceanographers have learned, can suppress mixing, trap heat, and feed back into the monsoon itself. Yet a fundamental question has remained stubbornly open: how exactly does the planet’s most powerful climate oscillation, the El Niño–Southern Oscillation, reach into this freshwater-dominated basin and rearrange its surface salinity from one year to the next?

A new study published in the journal Climate Dynamics by Hui Teng, Xinyu Lin, and Yun Qiu of the Third Institute of Oceanography in Xiamen, China, offers the most detailed answer yet. Drawing on more than three decades of ocean reanalysis and observational data spanning 1980 to 2015, the team shows that the interannual variability of sea surface salinity in the Bay of Bengal is tightly coupled to ENSO events, and that this coupling unfolds in two distinct, seasonally locked stages with strikingly different spatial fingerprints.

The first stage arrives with the northeast monsoon, the dry winter season, and peaks between December of the developing El Niño year and the following February. During this window, the researchers found that sea surface salinity drops significantly across the southern Bay of Bengal, the Andaman Sea, and the eastern equatorial Indian Ocean, while at the same time salinity rises markedly in the northern bay. It is a seesaw pattern: as one end of the basin freshens, the other grows saltier, and the two changes are physically connected rather than coincidental.

The mechanism behind the southern freshening, according to the study, lies in the winds. El Niño’s influence on the tropical atmosphere generates an anomalous anticyclonic circulation over the region, a giant swirl of air that spins the surface ocean along with it. This circulation drives a southward advection of low-salinity water, pushing the bay’s famously fresh surface waters down toward the southern bay, the Andaman Sea, and the equatorial Indian Ocean. In effect, El Niño spreads the freshwater signature of the northern bay across a much wider swath of the tropical Indian Ocean than it would normally occupy.

The northern bay, meanwhile, moves in the opposite direction for a different reason. The western boundary current of the bay, the current that flows along India’s eastern coast, intensifies during this stage of an El Niño event. That intensification enhances the northward transport of salty water drawn from the south, delivering a saltier payload to the northern reaches of the basin. The result is a simultaneous freshening in the south and salinification in the north, a dipole of sorts that had not previously been resolved with this level of seasonal precision.

Then, as the seasons turn, the pattern itself turns. During the southwest monsoon stage of El Niño events, particularly in August and October of the year following the event’s onset, the salinity map flips into a new configuration. Salinity decreases sharply in the central and southern bay and in the eastern equatorial Indian Ocean, but now it increases in both the northern bay and the Andaman Sea. The two stages are not mirror images of each other; they are governed by different combinations of forces.

In the summer stage, the study identifies two processes working together to freshen the southern bay. El Niño enhances precipitation over the region, dumping additional freshwater directly onto the sea surface, while an anomalous cyclonic circulation, opposite in sense to the winter anticyclone, advects even more low-salinity water southward. The salinity increase in the northern bay during this stage arises from a strengthened northward advection of high-salinity water combined with a reduction in freshwater input, as the weakened summer monsoon delivers less river runoff and less direct rainfall to the basin’s head.

Crucially, the researchers found that La Niña events, the cool phase of the oscillation, run the same machinery in reverse. Correlation and regression analyses applied to the 36-year record show that the cold phase produces salinity anomalies of opposite sign through the same set of oceanic and atmospheric pathways. This symmetry is a hallmark of a linear response, suggesting that the bay’s salinity system can be modeled and predicted with relatively straightforward tools once the ENSO state is known, at least to first order.

The technical foundation of the work is a careful synthesis of independent datasets. The team used version 3.3.1 of the Simple Ocean Data Assimilation reanalysis, the World Ocean Atlas 2018 climatology, satellite-derived sea surface salinity from the Soil Moisture and Ocean Salinity mission, precipitation from the Global Precipitation Climatology Project, evaporation and heat fluxes from the Objectively Analyzed air–sea Fluxes product, and atmospheric fields from the ERA5 reanalysis. ENSO state was tracked with the standard Niño-3.4 index, and the Indian Ocean Dipole, the basin’s other dominant climate mode, was monitored with the Dipole Mode Index to disentangle the two influences.

Why does this matter beyond the tidy world of salinity budgets? The Bay of Bengal’s freshwater stratification is a linchpin of the regional climate system. A barrier layer of low-salinity water sitting atop denser, saltier water prevents wind-driven mixing from bringing cool water to the surface, allowing sea surface temperatures to stay high and sustaining atmospheric convection that feeds the monsoon. By showing that ENSO systematically redistributes the bay’s salinity in two predictable seasonal stages, the study provides a physical basis for anticipating how the barrier layer, and by extension monsoon convection, may respond when an El Niño or La Niña is brewing in the Pacific.

The findings also sharpen the picture of the tropical Indian Ocean as an active participant in global climate variability rather than a passive responder. Prior work had established links between the Indian Ocean Dipole and salinity anomalies in the equatorial Indian Ocean, and earlier modeling studies had identified processes controlling bay salinity variability in general. What the new analysis adds is a coherent, seasonally resolved framework that ties the bay’s salinity seesaw directly to the evolution of ENSO events, complete with the specific current systems and wind-driven circulations responsible at each stage.

For forecasters and climate scientists, the practical implications are considerable. Because the winter-stage salinity dipole emerges during the peak of an El Niño event, and the summer-stage pattern follows with a predictable lag, sea surface salinity in the bay could serve as an observable fingerprint of ENSO’s downstream reach, one that satellites now monitor routinely. As climate change continues to alter both the strength of ENSO events and the freshwater delivery from Himalayan-fed rivers, understanding the baseline mechanics of this coupling becomes essential for projecting the future of the South Asian monsoon, one of the most consequential climate systems on the planet.

Subject of Research: Interannual variability of sea surface salinity in the Bay of Bengal and its relationship with ENSO

Article Title: Interannual variabilities of sea surface salinity in the Bay of Bengal and its relationship with ENSO

Article References: Teng, H., Lin, X., & Qiu, Y. (2026). Interannual variabilities of sea surface salinity in the Bay of Bengal and its relationship with ENSO. Climate Dynamics, 64(10), Article 425. https://doi.org/10.1007/s00382-026-08383-x

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08383-x

Keywords: Bay of Bengal, sea surface salinity, ENSO, El Niño, La Niña, Indian Ocean, monsoon, ocean circulation, Climate Dynamics, barrier layer, Indian Ocean Dipole, climate variability