A new study from researchers at the Woods Hole Oceanographic Institution (WHOI), published in Nature Communications on August 26, 2026, finds that large volcanic eruptions and human-caused climate warming can disrupt the long-standing coupling between climate variability in the tropical Indian and Pacific Oceans.

Climate conditions in the Indian Ocean (including the Indian Ocean Basin Mode, or IOBM, and the Indian Ocean Walker Circulation, or IWC) typically follow or co-vary with those in the Pacific (via the Pacific Walker Circulation, or PWC, linked to ENSO-like patterns) on interannual to multidecadal timescales. This inter-basin interaction helps drive rainfall and other climate patterns affecting surrounding continents.

Instrumental records (roughly the past century) show a breakdown in this coupling since the mid-20th century/1980s, particularly for the IOBM–PWC relationship, attributed to greenhouse gas-driven warming of the Indian Ocean that overwhelms Pacific influences. The short observational record left uncertainty about whether this was unprecedented.

To provide longer context, the team (lead author Shawn Wang, with co-authors Delia Oppo and Caroline Ummenhofer) reconstructed the relevant modes back to the early 1600s (or past four centuries) using paleoclimate archives—primarily marine records such as corals (δ¹⁸O and Sr/Ca), plus some tree rings and a stalagmite. These show that the basins were largely coupled throughout most of the pre-industrial period.

An anomalous decoupling occurred in the early 19th century (~1810–1850). This coincided with a cluster of large tropical volcanic eruptions (stratospheric sulfur injections exceeding that of the 1991 Pinatubo event), including the major 1815 Tambora eruption. Climate model simulations (e.g., CESM Last Millennium Ensemble) support that these eruptions disrupted inter-basin teleconnections; the effect depended on eruption strength and background climate conditions. Volcanism causes temporary weakening, but the recent (post-mid-20th century) PWC–IOBM decoupling appears exceptional even compared to past volcanic periods.

The Indian Ocean can act more independently as a major heat reservoir. Stable basin interactions normally aid near-term climate predictability (e.g., for rainfall). The findings highlight how external forcings—natural (volcanism) or anthropogenic—can alter these links, with relevance for understanding climate risks and improving models of future Indo-Pacific trends.

The study was supported by the U.S. National Science Foundation and WHOI programs. The full paper is titled “Coupling of Pacific and Indian Ocean variability disrupted by 19th century volcanism” (Wang, Oppo & Ummenhofer, Nat Commun 17, 8572, 2026).

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The Indian Ocean Basin Mode (IOBM), also called the Indian Ocean Basin-Wide mode or IOB mode, is the leading (first empirical orthogonal function) pattern of interannual sea surface temperature (SST) variability in the tropical Indian Ocean.

It features nearly uniform (basin-wide) warming or cooling across much of the tropical Indian Ocean, in contrast to the more zonal dipole structure of the Indian Ocean Dipole (IOD).

Seasonality and timing: It typically develops during the mature phase of El Niño (boreal winter, December–February), peaks in the following spring (March–May), and can persist into summer (through about August in some cases). This lagged response relative to ENSO is a hallmark.

Positive phase: Basin-wide warming (often following El Niño).

Negative phase: Basin-wide cooling (often following La Niña).

Index: Commonly defined as the area-averaged SST anomaly over a tropical Indian Ocean box (e.g., roughly the region shown in paleoclimate reconstructions as a broad tropical domain). It is distinct from the IOD’s Dipole Mode Index (western minus eastern SST anomalies).

The IOBM explains a large fraction of tropical Indian Ocean SST variance on interannual timescales and also has counterparts or expressions on decadal/interdecadal timescales (sometimes called ID-IOBM), often linked to the Interdecadal Pacific Oscillation (IPO).

The IOBM is primarily forced by remote ENSO influences via the “atmospheric bridge,” though local ocean–atmosphere processes and ocean dynamics play important reinforcing roles:

1. Atmospheric bridge (surface heat fluxes): During El Niño, the Pacific Walker Circulation weakens. This leads to anomalous descent (subsidence) over the Maritime Continent and tropical Indian Ocean, reducing cloud cover (increasing shortwave radiation into the ocean) and weakening surface winds (reducing evaporative cooling/latent heat loss). Net result: surface heat flux anomalies that warm the Indian Ocean.
2. Oceanic processes: Downwelling Rossby waves (generated by anticyclonic wind anomalies in the southeastern tropical Indian Ocean) propagate westward, deepening the thermocline and contributing to warming, especially in the southwestern Indian Ocean. This can further enhance basin-wide SST anomalies. Local air–sea feedback (e.g., wind–evaporation–SST feedback) help sustain the anomalies into summer.

The mode is not purely passive: once established, it can influence the atmosphere and even feedback onto the Pacific (e.g., helping terminate El Niño via enhanced convection over the Indian Ocean that strengthens easterlies over the western Pacific).

There is diversity in IOBM evolution—some events persist strongly into summer, while others transition into IOD-like patterns depending on ENSO phase transitions, equatorial symmetry of SST anomalies, and other factors.

Comparison with the Indian Ocean Dipole (IOD)

Feature	Indian Ocean Basin Mode (IOBM)	Indian Ocean Dipole (IOD)
Pattern	Basin-wide uniform warming/cooling	Zonal dipole (west vs. east/southeastern IO)
Peak season	Boreal spring (after ENSO peak)	Boreal autumn (SON)
Primary driver	Remote ENSO forcing (atmospheric bridge)	Local Bjerknes feedback; can be triggered by ENSO or independent
Ocean dynamics	Heat fluxes + Rossby waves	Strong equatorial wind–thermocline–SST coupling

Both modes interact with ENSO, but the IOBM is more of a lagged, basin-scale response, while the IOD involves stronger local coupling and can occur more independently.

The IOBM acts as a “capacitor” that stores and prolongs ENSO’s influence into the following seasons after the Pacific SST anomalies have decayed. Key impacts include:

• Enhanced (warm IOBM) or reduced convection and rainfall over the Indian Ocean and surrounding regions (e.g., East Africa, Maritime Continent).
• Influence on the Asian summer monsoon, East Asian rainfall (including mei-yu/baiu), the South Asian High, and Northwest Pacific anticyclone.
• Suppression of Northwest Pacific tropical cyclones in the summer after El Niño (for warm IOBM).
• Broader Northern Hemisphere midlatitude teleconnections (e.g., circumglobal teleconnection patterns).

These effects make the IOBM important for seasonal climate prediction around the Indian Ocean rim and beyond.

In the context of the recent WHOI, Nature Communications study (Wang, Oppo & Ummenhofer, 2026), the IOBM is one of the key Indian Ocean modes whose historical coupling to the Pacific Walker Circulation (PWC) was reconstructed using paleoclimate proxies (corals, etc.).

The study found that this coupling was generally stable over the past few centuries but was disrupted by early-19th- century volcanic eruptions, and that the modern (mid- 20th century onward) decoupling, driven by anthropogenic greenhouse- gas warming of the Indian Ocean, is exceptional.

Under global warming, the Indian Ocean is warming rapidly (often faster than other basins in places), which can amplify or alter IOBM behavior, reduce inter-basin coupling, and affect predictability. The mode’s persistence and strength also show decadal modulation linked to changes in ENSO characteristics.

In short, the IOBM is a fundamental mode linking Pacific ENSO variability to Indian Ocean climate and regional weather extremes. It highlights the Indian Ocean’s role not just as a passive “slave” to the Pacific but as an active player with its own dynamics and global reach.

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The Indian Ocean Dipole (IOD), also known as the Indian Ocean Zonal Mode or Indian Niño, is a coupled ocean–atmosphere mode of interannual climate variability in the tropical Indian Ocean.

It is characterized by an east–west contrast (dipole) in sea surface temperature (SST) anomalies and associated changes in the overlying atmospheric circulation (the Indian Ocean Walker Circulation).

Discovered in the late 1990s (Saji et al., 1999; Webster et al., 1999), it is the second leading mode of tropical Indian Ocean SST variability after the Indian Ocean Basin Mode (IOBM).

Positive IOD (pIOD): Warmer-than-normal SST in the western tropical Indian Ocean (roughly 50-70°E, 10°S–10°N) and cooler-than-normal SST in the southeastern tropical Indian Ocean (off Sumatra–Java, roughly 90–110°E, 10°S–0°). Associated with anomalous easterly winds along the equator.

Negative IOD (nIOD): The reverse pattern (cooler west, warmer east) with anomalous westerly winds.

Neutral: Near-normal conditions with little zonal gradient.

The standard index is the Dipole Mode Index (DMI): western box SST anomaly minus eastern box SST anomaly. Events typically develop in boreal summer (June-August), peak in autumn (September–November), and decay by winter. Positive events tend to be stronger and more frequent than negative ones due to asymmetries in feedback.

The primary growth mechanism is a Bjerknes-like positive feedback (analogous to ENSO in the Pacific), active mainly during boreal summer–autumn when climatological conditions favor it (southeasterly winds along the Sumatra–Java coast and a relatively shallow eastern thermocline):

1. Initial trigger — Anomalous easterly (westerly) winds for pIOD (nIOD). These can arise from remote ENSO forcing (via the atmospheric bridge/Walker Circulation changes), internal Indian Ocean processes, or other atmospheric variability (e.g., MJO).
2. Ocean response — Easterlies enhance upwelling and shoal the thermocline in the east (cooling SST via vertical advection of colder subsurface water). They also generate upwelling Kelvin waves that further cool the east and downwelling Rossby waves that warm the west (deepening the thermocline there).
3. SST gradient reinforcement — The resulting west-minus-east SST gradient strengthens the anomalous easterlies (atmospheric response to the zonal SST contrast).
4. Amplification loop — Stronger winds further enhance upwelling/thermocline tilt, amplifying the SST dipole.

Additional processes include:

• Wind–evaporation–SST (WES) feedback: Cooler eastern SSTs can increase local winds and evaporative cooling, reinforcing the anomaly.
• SST–cloud–radiation feedback: Often acts as negative (damping) feedback.
• Nonlinear effects: Strong events involve nonlinear zonal advection and wave dynamics that can intensify extremes (e.g., 1997, 2019 pIOD events).

In the east pole, latent heat flux and vertical temperature advection dominate SST anomaly development; in the west, meridional and vertical advection are key. Rossby waves help link the eastern wind anomalies to western subsurface (and eventually surface) warming.

The mean seasonal cycle of the Indian Ocean (including monsoon winds and the shallow eastern thermocline in summer–autumn) anchors IOD development. The feedback is strongest then because the thermocline is shallow enough for subsurface anomalies to affect the surface. After the peak, seasonal changes (e.g., monsoon reversal) and often the transition into an IOBM-like pattern help terminate the event.

ENSO is a major external influence:

• El Niño often favors pIOD (via weakened Pacific Walker Circulation producing easterly anomalies over the Indian Ocean).
• La Niña favors nIOD. Roughly half of IOD events co-occur with ENSO, but many (e.g., strong 1961 and 2019 pIOD, 2016 nIOD) develop independently through internal Indian Ocean dynamics. ENSO-forced IODs tend to develop later than independent ones. The IOD can also feedback onto ENSO (e.g., certain pIOD types can accelerate the transition to La Niña via atmospheric teleconnections and Pacific thermocline adjustments).

IOD events show significant diversity (“flavors”):

• True dipoles vs. “pseudo-dipoles” (where one pole dominates or anomalies are same-signed).
• Type-I (strong western pole, often co-occurring with ENSO and linked to subsequent ENSO phase transitions via evolution into IOBM) vs. Type-II (weaker western pole, less impact on ENSO).
• Variations in spatial extent, strength of poles, and meridional structure (ENSO-forced events often broader; internal events more equatorially confined).

Climate impacts

• Positive IOD: Enhanced rainfall and flooding risk in East Africa; drought and wildfire risk in Indonesia, Australia, and parts of Southeast Asia; often beneficial for the Indian summer monsoon.
• Negative IOD: Opposite rainfall patterns (wetter Indonesia/Australia, drier East Africa). These effects arise from shifts in the atmospheric Walker Circulation and convection centers. The IOD also influences marine ecosystems, coral bleaching risk, and can modulate broader Indo-Pacific climate.

Under global warming, models project changes in IOD frequency, intensity, and asymmetry (often more extreme pIOD events), though uncertainties remain due to mean-state changes (e.g., thermocline depth). In the context of the 2026 WHOI study on Indo-Pacific coupling, the IOD (via the Indian Ocean Walker Circulation or IWC) is one of the modes whose historical relationship to the Pacific was reconstructed; volcanic eruptions and anthropogenic warming can disrupt typical inter-basin links.

In summary, IOD dynamics center on a seasonally modulated Bjerknes feedback that allows self-sustaining growth of zonal SST and wind anomalies, strongly influenced by (but not solely dependent on) ENSO, with important regional climate consequences and notable event-to-event diversity.

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The 1815 eruption of Mount Tambora (Sumbawa, Indonesia) was the largest volcanic eruption in recorded history (Volcanic Explosivity Index VEI-7).

It ejected an estimated 100–150 km³ of material and injected roughly 60 Tg of sulfur dioxide into the stratosphere. The resulting sulfate aerosols formed a global veil that reflected incoming solar radiation, producing multi-year climate perturbations often described as a “volcanic winter.”

Global temperature and precipitation effects

• Global mean surface cooling of approximately 0.4–1.0 °C (estimates vary by reconstruction and baseline; Northern Hemisphere land cooling often larger, around 0.5–1.9 °C in peak years).
• Peak impacts in 1816, with effects lasting 2–3 years (and detectable cooling for up to 6–10 years in some records). The 1810s overall were among the coldest decades of the past several centuries, partly due to Tambora plus an earlier unidentified tropical eruption (~1808/1809).
• Global precipitation decreased by ~3–4% in model simulations for 1816, with a general slowdown of the hydrological cycle.
• Stronger cooling over land than ocean; Northern Hemisphere extratropics were particularly affected. Southern Hemisphere signals are weaker or less consistent in proxies.

The “Year Without a Summer” (1816)

This is the best-known regional manifestation, especially in Europe and northeastern North America:

• Unusually cold, wet conditions: frosts and snow in June–August in New England and parts of Canada; mean July temperatures in parts of Europe among the lowest in centuries.
• Crop failures, livestock deaths, and famine across Europe (e.g., Switzerland, Ireland, Germany) and North America. Grain prices soared; malnutrition and disease (including typhus) followed. Tens of thousands emigrated.
• Cultural notes: Mary Shelley wrote Frankenstein during the cold, rainy Swiss summer of 1816; spectacular sunsets and atmospheric optical effects were widely observed.

Atmospheric and oceanic responses

Climate models forced with Tambora-like aerosols typically show:

• Strengthened wintertime stratospheric polar vortex.
• Weakened summer monsoon circulations.
• Changes in atmospheric circulation and jet streams.
• Temporary strengthening of the Atlantic Meridional Overturning Circulation (AMOC).
• Reduced atmospheric CO₂ (models estimate a drop of several ppm due to altered carbon-cycle fluxes).
• Ocean heat-content release that partially dampens surface cooling; increased snow/sea-ice cover that amplifies cooling via albedo feedback.

Indo-Pacific and tropical impacts

As a major tropical eruption, Tambora contributed to the cluster of early-19th-century events that disrupted normal coupling between Pacific and Indian Ocean climate modes (IOBM and IWC with the Pacific Walker Circulation).

Model analyses indicate widespread tropical cooling, a negative IOBM-like response, and alterations to inter-basin teleconnections. Suppression of summer rainfall occurred in parts of Southeast Asia, and the eruption helped drive the broader multi-year climate anomaly of the 1810s.

Direct and indirect deaths are estimated in the tens to ~100,000 range (mostly from local pyroclastic flows, ash, and subsequent famine/disease).

The event is a key test case for understanding high-impact tropical eruptions and their Earth-system effects. It occurred against the backdrop of the Dalton Minimum (low solar activity) and the broader Little Ice Age, which amplified the relative impact.

Tambora produced clear, multi-year global cooling, hydrological changes, monsoon disruption, and severe regional weather extremes that caused widespread agricultural failure and social hardship—most famously the 1816 “Year Without a Summer.”

Its scale makes it a benchmark for studying volcanic climate forcing, including interactions with ocean basins and potential future analogous events.

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Coupling of Pacific and Indian Ocean variability disrupted by 19th century volcanism

“Coupling of Pacific and Indian Ocean variability disrupted by 19th century volcanism” is a 2026 open-access paper in Nature Communications (Wang, Oppo & Ummenhofer; volume 17, article 8572) from researchers affiliated with the Woods Hole Oceanographic Institution (WHOI).

Tropical basin interactions normally cause the Indian Ocean Walker Circulation (IWC) and Indian Ocean Basin Mode (IOBM) to co-vary with the Pacific Walker Circulation (PWC) on interannual-to-decadal timescales.

Recent decades show degraded IOBM–PWC coupling due to greenhouse-gas forcing, but the short instrumental record cannot confirm whether such decoupling can also arise naturally.

The authors reconstruct the three modes (IOBM, IWC, PWC) over the past four centuries with paleoclimate records.

Both Indian Ocean modes were largely coupled to the Pacific in the pre-industrial era, yet a series of early-19th-century volcanic eruptions disrupted this coupling.

Even so, the mid-20th-century-onward PWC–IOBM relationship is exceptional compared with equivalent intervals of the past millennium (including strong-volcanism periods).

Stable basin interactions are a source of near-term predictability, so the findings have implications for climate-risk management.

Indian Ocean SST variability strongly influences temperature and hydroclimate across Africa, Asia, and Australia. On interannual timescales the two main modes are:

• IOBM — basin-wide warming/cooling that typically peaks in March–May after the ENSO peak, driven largely by Pacific Walker Circulation changes via the atmospheric bridge (surface heat fluxes).
• IOD / IWC — zonal SST gradient changes sustained by Bjerknes feedback; positive IOD features easterly anomalies, eastern cooling (off Java–Sumatra), and western warming.

Both modes are influenced by ENSO and PWC but can also arise from intrinsic Indian Ocean dynamics. On decadal timescales similar patterns exist and are normally linked to Pacific variability. Instrumental data (~100 years) show recent IOBM warming despite a strengthening PWC (since the 1980s), attributed to anthropogenic Indian Ocean warming that overwhelms Pacific teleconnections. The short record leaves open whether this is unprecedented.

The team reconstructed annual SST anomalies (1631–1990 CE) using the Composite Plus Scale (CPS) method, relying almost exclusively (~90%) on proximal marine records:

• 35 coral records (23 δ¹⁸O, 12 Sr/Ca),
• 3 tree-ring width records,
• 1 speleothem δ¹⁸O record.

Targets:

• IOBM (March–May tropical Indian Ocean box),
• IWC (July–November eastern-minus-western Indian Ocean SST gradient; positive = negative-IOD-like),
• PWC (December–February western-minus-eastern Pacific SST gradient; stronger = more La Niña-like).

Results were validated against instrumental data (ERSSTv5) and cross-checked with Paleoclimate Data Assimilation (Last Millennium Reanalysis). Low-pass filtering (10-year window) isolated decadal-to-multidecadal variability. Mechanisms were examined with the CESM Last Millennium Ensemble (all-forcing and volcanic-only members) and superposed-epoch analysis of large tropical eruptions.

Key results

• Pre-industrial coupling — Reconstructed IOBM and IWC multi-decadal variability closely tracked the PWC for most of the past four centuries (IOBM warming and IWC weakening during PWC weakening intervals, and vice versa). This underscores the dominant role of Indo-Pacific interactions.
• Early-19th-century decoupling (~1810–1850) — An anomalous, unusually long and strong breakdown in coupling of both Indian Ocean indices from the PWC. This interval coincided with at least four large tropical volcanic eruptions whose stratospheric sulfur injections exceeded the 1991 Pinatubo event, including the 1815 Tambora eruption (the most explosive of the past 500 years).
• Model support — CESM-LME simulations show that these eruptions can disrupt inter-basin teleconnections. The magnitude of the forced departure depends on eruption strength and background Indo-Pacific internal variability (especially for the IWC). Volcanic forcing produces temporary deviations via mechanisms such as interhemispheric cooling asymmetry and the Indian Ocean’s sensitivity to external radiative forcing.
• Modern decoupling is exceptional — The post-mid-20th-century PWC–IOBM relationship stands out relative to any comparable-length interval of the past millennium, including earlier strong-volcanism periods. Anthropogenic greenhouse-gas forcing is the primary driver of the recent, sustained breakdown.

Stable Indo-Pacific interactions normally enhance near-term climate predictability (rainfall, extremes, etc.). Temporary volcanic disruption illustrates how external forcings can break these links; the ongoing anthropogenic decoupling is more persistent and exceptional.

The Indian Ocean can behave more independently as a large heat reservoir. The results help frame the uniqueness of recent changes and inform climate-risk management and model projections of future Indo-Pacific trends.

 Published:  Nature Communications (2026)

DOI: 10.1038/s41467-026-76705-y

Provided: Woods Hole Oceanographic Institution 

Authors: Shawn Wang, 
Delia W. Oppo & 
Caroline C. Ummenhofer 

Abstract

Through tropical basin interactions, the Indian Ocean Walker circulation (IWC) and basin mode (IOBM) covary with the Pacific Walker circulation (PWC) on interannual-decadal timescales. Coupling between IOBM and PWC has degraded considerably in recent decades due to greenhouse gas forcing; however, the instrumental record is too short to unequivocally determine whether such a decoupling could also have arisen naturally. Here we contextualise recent trends by reconstructing Indo-Pacific modes (IOBM, IWC, and PWC) over the past four centuries using a compilation of paleoclimate records. We demonstrate that both Indian Ocean modes were largely coupled to the Pacific throughout the pre-industrial era, but this coupling was disrupted by a series of volcanic eruptions during the early 19th century. Nonetheless, the PWC-IOBM relationship since the mid-20th century appears exceptional relative to equivalent-length intervals of the past millennium, including periods of strong volcanism. Given that stable basin interactions are a source of near-term predictability, our results have implications for climate risk management efforts.