This session invites Indian Ocean contributions based on observations, modelling, theory, and palaeo proxy evidence across a range of timescales from synoptic, interannual, decadal to centennial and beyond. Topics of interest include past, current, and projected changes in Indian Ocean physical and biogeochemical properties and their impacts on ecological processes, diversity in Indian Ocean modes of variability, extent of the Indo-Pacific Warm Pool, interactions and exchanges between the Indian Ocean and other ocean basins via both oceanic and atmospheric pathways, as well as impacts on regional hydroclimate and adjacent monsoon systems.
Submissions are sought on assessing and predicting weather and climate extremes of societal relevance in the Indian Ocean and surrounding regions. We especially welcome submissions addressing compound extreme events that span across the ocean, atmosphere, and/or biogeochemical and ecological realms. Furthermore, studies evaluating climate risks, vulnerability, resilience, adaptation and mitigation strategies in coastal regions affected, for example, by tropical cyclones and extremes in sea level are encouraged.
We also welcome contributions that address research on the Indian Ocean, using advanced techniques such as artificial intelligence/machine learning, and new technological advances in observing systems, such as deep and biogeochemical Argo.
Tropical cyclones are among the most devastating natural hazards to occur in the South-West Indian Ocean basin (SWIO), posing considerable risk to vulnerable countries such as Madagascar and Mozambique. This study examines changes in tropical cyclone risk across the SWIO, the Main Tropical Cyclone Region, and the Madagascar Region over the last 45 cyclone seasons (1981–2025). Seasonal and monthly time-series of key tropical cyclone metrics, namely frequency, maximum sustained wind (MSW), and accumulated cyclone energy (ACE) were computed and analysed for trends. The relationship these metrics have with major oceanic and atmospheric drivers, such as ocean temperature, the El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Madden–Julian Oscillation (MJO), were examined.
Results indicate a significant decrease in tropical cyclone frequency in the SWIO and the Main Tropical Cyclone Region, while frequency remained relatively stable in the Madagascar Region. In contrast, MSW increased significantly across all regions (+3.91 Knots per decade), with the strongest intensification occurring within the Madagascar Region (+4.79 Knots per decade). This suggests risk has increased over time in the SWIO despite the occurrence of fewer storms.
Ocean temperatures exhibited significant warming at both surface and subsurface levels, with depths of 35 m and 45 m indicating the greatest warming trends and the strongest relationship with increased cyclone MSW. Cyclone frequency on the other hand was negatively correlated with ocean warming, suggesting warmer waters in the SWIO may create conditions less conducive to the formation of tropical cyclones.
ENSO was found to be a considerable driver of regional cyclone variability, with La Niña conditions associated with higher frequency and stronger cyclones. The MJO was also identified as a key modulator of cyclonic activity, particularly in the Madagascar Region, where active phases 3, 4, and 5 coincided with increased cyclone frequency and MSW. The IOD on the other hand showed little to no influence on cyclone metrics in the SWIO. The incorporation of this research into forecasting and intensity models has the potential to enhance early warning systems in the SWIO, thereby providing a valuable tool for the highly vulnerable region.
(Swan and Hallam, in prep, 2026)
How to cite: Swan, L. and Hallam, S.: The Changing Tropical Cyclone Risk in the South-West Indian Ocean over the last 45 tropical cyclone seasons (1981-2025), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5435, https://doi.org/10.5194/egusphere-egu26-5435, 2026.
In this presentation, we review the recent research progress based on moored observations of the Leeuwin Current, an eastern boundary current of the south Indian Ocean, off the west coast of Australia, over the past 15 years. The focus will be on the seasonal cycle of the Leeuwin Current, as well as the interannual temperature variability and its drivers, with a focus on the Ningaloo Niño – an austral summer marine heatwave event. In the future climate projections, the Leeuwin Current (along with the Indonesian Throughflow) will become weaker, as shown in both climate model projections and downscaling models. However, the Ningaloo Niño is expected to strengthen in the future climate, with its peak month shifting from February to March in the austral summer. Climate model projections suggest that both enhanced local air-sea coupling and remote forcing from the Pacific may induce such a strengthening of the warming events.
How to cite: Feng, M.: Observations of the Leeuwin Current and variability, and future projections of Ningaloo Niño marine heatwaves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16944, https://doi.org/10.5194/egusphere-egu26-16944, 2026.
Mesoscale eddies in the southeastern tropical Indian Ocean (SETIO) are crucial for regional circulation, heat transport, and ecosystem dynamics. Their interannual variability is closely associated with ENSO and IOD. Eddy activity is enhanced during pure La Niña and positive IOD years, but suppressed during pure El Niño and negative IOD years. When ENSO and IOD co-occur, their influences tend to counteract each other: the IOD dominates during the ENSO developing phase, whereas ENSO exerts a stronger influence during the decay phase. This variability is linked to changes in the Indonesian Throughflow and wind-driven upwelling associated with ENSO and IOD events. Numerical experiments further indicate that the interannual variability of SETIO eddies is primarily wind-driven, with winds over the equatorial Pacific, equatorial Indian Ocean, and SETIO all contributing significantly. Oceanic channel effects induced by equatorial Indo-Pacific winds are stronger than those arising from purely atmospheric processes.
How to cite: Zhou, Y., Cheng, X., Duan, W., Yang, C., and Chen, J.: Impacts of ENSO and IOD on Mesoscale Eddy Activity in the Southeastern Tropical Indian Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3044, https://doi.org/10.5194/egusphere-egu26-3044, 2026.
Six major summer monsoon floods occurred in the Yangtze Basin between 1992–2024 affecting millions of people, compared to only one during 1960–1991. This significant rise in hydroclimatic extremes is closely associated with an approximately 50% increase in variability at the quasi-biennial timescale. In this study, using sea surface height and thermocline depth from the ORAS5 reanalysis and EN4 observational analysis, we demonstrate that the increased quasi-biennial variability in East Asian summer monsoon rainfall over the Yangtze River Basin is strongly coupled with intensified quasi-biennial scale wave dynamics in the Indian Ocean. We provide evidence of fundamental changes in the characteristics of baroclinic waves in the tropical Indian Ocean over recent decades. We find that the mean phase speed of westward-propagating tropical Rossby waves has increased by 70%, along with their overall variance. These shifts are likely associated with changes in large-scale atmospheric forcing. Our findings highlight that evolving Indian Ocean wave characteristics are a key driver of changes in East Asian summer monsoon variability at quasi-biennial timescales and the associated hydrological extremes over East Asia, with important implications for the predictability of East Asian summer monsoon rainfall at these timescales.
How to cite: Dasgupta, P., Nam, S., McPhaden, M. J., Kang, D., Mathew Koll, R., and Jayanthi Sasikumar, S.: Intensified Indian Ocean Rossby Wave Dynamics as a Driver of Increased Quasi-Biennial Summer Monsoon Floods in the Yangtze River Basin , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15991, https://doi.org/10.5194/egusphere-egu26-15991, 2026.
Uncertainty in Earth System Model (ESM) projections of Indian Ocean biogeochemistry is often attributed primarily to differences in biogeochemical process representations. Here, we demonstrate that large present-day physical biases and divergent future physical climates also play a substantial role. Using a bias-corrected ocean-only model forced by air–sea flux anomalies from multiple CMIP6 models, we show that correcting present-day physical biases strongly amplifies projected summer surface chlorophyll (SChl) changes and substantially improves inter-model consistency.
Across key Indian Ocean upwelling regions, increased upper-ocean stratification driven by heat-flux anomalies consistently reduces SChl, highlighting the role of ocean warming in shaping future biogeochemical change. In contrast, wind-driven changes dominate the SChl response in several regions, particularly off southern India and off Sumatra, emphasizing strong regional differences in physical controls. These results underscore the central importance of monsoonal wind variability and its future evolution for Indian Ocean biogeochemistry, with implications for ecosystem functioning and the predictability of regional climate impacts.
How to cite: Kwatra, S., Lengaigne, M., Iyyappan, S., Dutheil, C., and Vialard, J.: Physically Driven Uncertainty in Future Indian Ocean Chlorophyll: Roles of Stratification, Winds, and Bias Correction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20467, https://doi.org/10.5194/egusphere-egu26-20467, 2026.
The Bay of Bengal (BoB) is characterized by intense stratification driven by monsoonal freshwater flux coupled with high irradiance levels, which limits nutrient supply to the euphotic zone and restricts oxygen ventilation. While the northern part of the Bay remains hypoxic, ubiquitous mesoscale eddies provide a mechanism to break this stratification, pumping nutrients into surface layers and transporting shelf-associated microbial communities to the central bay. The response of these communities to the dynamics of this region remains however poorly understood.
The objective of the present study was to analyse the response of two distinct microbial communities to the environmental dynamics of this region. We conducted a series of onboard incubations using natural microbial communities collected from surface and deep chlorophyll maximum (DCM) waters during a cruise aboard the R/V Thomas G. Thompson in the summer months of 2025. Two distinct treatments were established: a control representing standard stratified conditions (characterized by an abrupt oxygen gradient and low irradiance at depth), and an experimental treatment designed to simulate the nutrient injection and mixing typically induced by cyclonic eddies, under well-oxygenated conditions and the local photoperiod. We monitored physiological parameters as chlorophyll-a concentration, the maximum photosynthetical quantum yield (Fv/Fm) and intracellular nitrate pools.
Our findings indicate that both communities (surface and DCM) exhibited similar response patterns under stratified conditions, with no significant growth and intracellular nitrate levels remaining lower (≈ 0.1 µM) than the freely dissolved pool (0.9-1.2 µM). In contrast, nutrient enrichment from bottom waters resulted in a rapid community response. The surface community exhibited a rapid uptake of nitrate within the first hours of incubation, resulting in an increase in the intracellular pool, which was followed by a gradual consumption over the following days. These results demonstrate the physiological plasticity of the community in response to a highly dynamic environment, with the capacity to utilize episodic nutrient enrichment within this highly variable system. Such plasticity may have significant implications for the nitrogen biogeochemical cycle as well as for the overall microbial community composition in the highly dynamic and increasingly deoxygenated North Indian Ocean.
How to cite: Rodríguez-Márquez, J., Bartual, A., Bourbonnais, A., Pachiadaki, M., and Garcia-Robledo, E.: Surface and deep chlorophyll microbial community response to mixing events in the stratified Bay of Bengal (NE Indian Ocean), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17507, https://doi.org/10.5194/egusphere-egu26-17507, 2026.
The Indian Ocean climate response to natural external forcing, such as volcanic eruptions, is still uncertain due to potential model biases and lack of validation from observations and subannual-resolution marine palaeorecords. Here we present monthly temperature and hydrology reconstructions derived from coral Sr/Ca and oxygen isotopes in the northeastern Indian Ocean back to 1774. Our reconstructions reveal anomalous and prolonged cooling and freshening during the early 19th century (~1809-1824), which we attribute to a cluster of tropical volcanic eruptions that includes the unidentified 1809 and Tambora 1815 eruptions. The regional cooling and freshening were unusually strong compared to the wider Indian Ocean. The eruptions forced negative Indian Ocean Dipole (IOD)-like conditions in our reconstructions, followed by positive IOD-like conditions in subsequent years, regardless of eruption magnitude. Our results and other palaeorecords suggest positive IOD-like mean conditions during the early 19th century, accompanied by stronger summer rainfall over areas of India and a negative Interdecadal Pacific Oscillation state, were associated with the regional cooling and freshening. Our findings highlight the sensitivity of the northeastern Indian Ocean to external forcing and that available observations, proxy records, and climate model simulations do not capture the full range of regional climate variability, complicating climate change projections for this highly populated region vulnerable to future climate extremes.
How to cite: Camelia, H., Felis, T., Kölling, M., Scheffers, S., and Chavanich, S.: Pronounced volcanic cooling and freshening in the northeastern Indian Ocean during the early 19th century, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20293, https://doi.org/10.5194/egusphere-egu26-20293, 2026.
As a link between the surface waters and the deep ocean, intermediate water masses play a key role in transmitting atmospheric variations into the deep sea. The paleo-oceanographic reconstructions of intermediate water mass variability are therefore essential to comprehend the pace and extent with which shallow marine changes are transferred into ocean basins. Cold water corals (CWC) thriving in intermediate water depths have been identified as an adequate geochemical proxy archive to reconstructed temporal changes in water mass properties, due to the sustained growth of their carbonate skeleton and their long lifespan (several hundred years).
Two living CWC colonies of Enallopsammia rostrata (Pourtalès, 1878) have been collected in the northern part of the Mozambique Channel around the island of Mayotte, during the research cruise SO306 with the RV Sonne in August 2024. The corals were collected with a ROV from water depths between 600 to 900 meters within the transition zone of South Indian Central Water (SICW) and the underlying Red Sea Water (RSW). The chemical composition (Ca, Li, Mg) of different branches from each colony was analysed using line scan laser ablation inductively coupled mass spectrometry (LA-ICP-MS). U/Th dating enables the determination of ages and calculation of growth rates for the individual colonial parts.
The sclerochronological aligning of the geochemical data along the U/Th-based growth rates enabled a reconstruction of Li/Mgcoral variations until the end of the penultimate century. Pronounced cyclic variabilities in ranges of duration from years to decades could be identified within the Li/Mg-records, due to the spatially high-resolution LA-ICP-MS measurements. However, a significant trend in Li/Mgcoral, that would indicate a continuous change of water temperatures, could not be identified within the two colony records over the reconstructed time period. Water temperatures derived from mean Li/Mgcoral by employing Li/Mg-temperature calibration (Montagna et al. 2014) match well with observed water temperature values between of 6.6°C and 9.4°C, respectively. The reconstructed temperature variability for both colonies show variations on an average range of 3°C (2SD) over multi-year intervals, which can be attribute to a changing extent of influence of the differently temperate water masses around Mayotte.
Our reconstruction shows no long-term temperature increase in the intermediate water masses of the West Indian Ocean during the last century contrary to the anthropogenic warming of the atmosphere and surface ocean. The found temperature variability, however, points to a dynamic and periodic shifting of the different water masses, which suggests a more lateral exchange within intermediate water depths in the northern entry area of the Mozambique Channel.
- Montagna, M. McCulloch, E. Douville, M. L. Correa, J. Trotter, R. Rodolfo-Metalpa, D. Dissard, C. Ferrier-Pagès, N. Frank, A. Freiwald, S. Goldstein, C. Mazzoli, S. Reynaud, A. Rüggeberg, S. Russo, M. Taviani (2014): Li/Mg systematics in scleractinian corals: Calibration of the thermometer. Geochim. Cosmochim. Acta 132, 288–310
How to cite: Kniest, J. F., Raddatz, J., Fietzke, J., Frank, N., Kaiser, T., Freiwald, A., and Flögel, S.: One century of intermediate water masses temperature variability of the West Indian Ocean reconstructed by Li/Mg-thermometer in scleractinian cold-water corals, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9018, https://doi.org/10.5194/egusphere-egu26-9018, 2026.
South Asian summer monsoon (SASM) delivers substantial rains to Indian subcontinent and drives upwelling in the Arabian Sea that sustains one of the world's most productive fisheries there. Both marine upwelling records and terrestrial rainfall records have been established as fundamental archives for reconstructing past SASM variability. However, the upwelling records vary in opposite direction to the terrestrial rainfall records on orbital timescale, leading to a long-standing paradox in the past monsoon variability. To understand this paradox, here we combine paleoclimate records with novel transient climate simulations that explicitly separate the effects of the Northern and Southern Hemisphere insolation forcing. Our results show that the SASM rainfall is governed by Northern Hemisphere (NH) insolation, whereas the Arabian Sea upwelling is dominated by Southern Hemisphere (SH) insolation. When boreal summer occurs at perihelion, insolation is strongly enhanced not only in the NH but also in the tropical-subtropical SH. The former enhances the SASM rainfall through Eurasian warming, while the latter weakens the Arabian Sea upwelling by inducing South African warming and subsequent atmospheric teleconnections. We reconcile the long-standing paradox, and more broadly, reveal that warming in South Africa could exert a significant and previously overlooked remote forcing on the SASM system.
How to cite: Wen, Q., Liu, Z., Wang, T., Ji, C., Liu, J., Cheng, H., Yan, M., Ning, L., Jing, Z., Liu, H., Lei, J., Lu, J., Creutzig, F., and Yin, Q.: Reconciling Contrasting Marine and Terrestrial Responses of South Asian Summer Monsoon Reveal a Remote Control from South Africa, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7310, https://doi.org/10.5194/egusphere-egu26-7310, 2026.
The Last Glacial Maximum (LGM) and the subsequent deglacial period likely featured climate-ocean dynamics and deep-ocean carbon storage states that contrasted with those of today. In this study, we focus on the Indian Ocean and place regional reconstructions in a global context by integrating published results from other ocean basins. Differences in deep-ocean carbon storage between basins across the LGM and deglaciation reflect changes in (1) deep-water mass sources and circulation structure and (2) regional regulation processes within the ocean basin. We reconstruct deep-water oxygen concentrations ([O₂]) between 26 and 10 ka BP using sediments from IODP Site 353-U1445 (Bay of Bengal) and IODP Site 361-U1479 (Cape Basin). Deep-water [O₂] is inferred from the carbon isotope gradient between epifaunal and infaunal benthic foraminifera (Δδ13Cepi-in). Changes in biological pump efficiency are assessed from the carbon isotope gradient between planktonic and benthic foraminifera (Δδ13Cp-b). Reconstructed [O₂] records are combined with outputs from the TraCE-21ka simulations and CMIP6 EC-Earth3-CC model to estimate respired carbon storage (Pg C).
During the LGM, deep-water [O₂] variations in the Cape Basin and the Bay of Bengal showed broadly synchronous trends, with major inflection points occurring at similar times. However, changes in the Cape Basin systematically preceded those in the Bay of Bengal. This temporal offset indicates a more rapid response in the Cape Basin relative to the Bay of Bengal. From the LGM to the deglaciation, increasing deep-water [O₂] and declining carbon storage in the Cape Basin are closely associated with reduced biological pump efficiency. In contrast, the Bay of Bengal exhibited stronger variability during the deglaciation, with a pronounced response during the Bølling–Allerød (B/A) interval, when deep-water [O₂] sharply decreased. During the B/A stage, the Antarctic Cold Reversal in the Southern Ocean was characterized by weakened AABW formation and reduced deep-water [O₂]. These changes slowed deep-water renewal and enhanced deep-water organic carbon remineralization, which probably resulted in increased deep-water respired carbon storage in the Indian Ocean. The larger LGM–deglacial amplitude in the Cape Basin reflects its location at the confluence of Atlantic, Southern Ocean, and Indian Ocean water masses, resulting in a more rapid and pronounced response to circulation reorganization, whereas the Bay of Bengal exhibits weaker and delayed responses as a distal deep-water reservoir. Estimated respired carbon storage efficiency in the Cape Basin is higher during the LGM by~0.03 mol m⁻³ and ~0.05 mol m⁻³ relative to Heinrich Stadial 1 (H1) and the B/A, respectively. Consistent with this difference, mean respired carbon storage decreased from ~5.51 Pg C (~2.58 ppm CO₂ equivalent) during the LGM to ~2.84 Pg C (~1.33 ppm) and 1.10 Pg C (~0.52 ppm CO₂) during H1 and the B/A, respectively. In contrast, the Bay of Bengal exhibits higher respired carbon storage during the B/A (1.24 Pg C; ~0.58 ppm CO₂ equivalent) than during the LGM (0.51 Pg C; ~0.24 ppm CO₂ equivalent). This study highlights the heterogeneous response of the Indian Ocean deep carbon reservoir during glacial-interglacial transitions.
How to cite: Shen, W. and Zhao, N.: Evolution of deep-ocean carbon storage in the Indian Ocean since the Last Glacial Maximum, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15512, https://doi.org/10.5194/egusphere-egu26-15512, 2026.
The tropical Indian Ocean (TIO) has experienced pronounced warming trends in recent decades, with dynamical processes recognized as key drivers. However, the role of thermal processes remains uncertain due to discrepancies in surface wind-induced heat flux across existing datasets. The present study introduces a random forest machine learning algorithm that synergistically integrates the advantages of in situ observations and satellite data, yielding a monthly surface wind (MLAWind) dataset and corresponding air-sea heat flux from 1950–2022 with a horizontal resolution of 1°×1°. MLAWind exhibits high accuracy and robust generalization capability based on evaluations using both satellite and buoy observations. Besides, it is capable of effectively representing spatial and temporal characteristics of surface wind. In contrast to the majority of existing reanalysis datasets, MLAWind reveals a decline in surface wind over the TIO since 1950, which is further supported by the west-to-east asymmetrical variations in sea surface height and thermocline depth. The attenuation of surface wind is more significant in the eastern TIO as compared to the western TIO, leading to a remarkable reduction in evaporative cooling within the eastern TIO. The thermal processes associated with surface wind-induced heat flux serve as the essential drivers of the warming in the eastern TIO, with a contribution accounting for approximately 45% of that of dynamical processes. The findings of our study challenge existing reanalysis results but are aligned with state-of-the-art models, highlighting that the significance of thermal processes is substantially underestimated in most existing reanalysis datasets.
How to cite: Guo, W.: Unveiling the drivers of tropical Indian Ocean warming through machine learning-assisted surface wind, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3733, https://doi.org/10.5194/egusphere-egu26-3733, 2026.
The acoustic travel time (τ) measured by an inverted echo sounder (IES) can be converted into vertical temperature profiles using the gravest empirical mode (GEM) technique based on the relationship between τ and temperature. In the Seychelles–Chagos Thermocline Ridge (SCTR) in the southwestern tropical Indian Ocean, where persistent subsurface upwelling occurs, continuous vertical temperature profiles are crucial for monitoring the upwelling variability. However, the conventional GEM method has large uncertainties at the SCTR due to temporal variability in upwelling strength. This study introduces a new approach, termed hybrid GEMs, to improve IES data analysis by reflecting upwelling strength based on the depth of the 20°C isotherm (D20). The hybrid GEMs consist of one moderate GEM and two combined GEMs derived from three groups of historical hydrographic profiles in the SCTR, categorized by D20 ranges. When applied to the in situ τ measured by a pressure-recording IES at Station K (61°E, 8°S) in the SCTR from May 2019 to December 2021, the absolute dynamic topography from satellite altimetry is used as an index to select the appropriate hybrid GEM based on the consistency between the absolute dynamic topography and D20 variability. The vertical temperature profiles based on hybrid GEMs show significant improvements in both the mean and maximum root mean square errors of the upper 300 m temperature, which are reduced by approximately 29% and 20%, respectively. The hybrid GEM–derived temperature profiles reliably capture temperature variability in the upper 300 m, demonstrating the strong potential of acoustic travel time as an essential observational variable in data-sparse tropical upwelling regions of the Indian Ocean.
How to cite: Lee, E., Na, H., and Nam, S.: A Hybrid Gravest Empirical Mode Method for Reconstructing Temperature Profiles in the Seychelles–Chagos Thermocline Ridge , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6757, https://doi.org/10.5194/egusphere-egu26-6757, 2026.
This study investigates decadal changes in boreal summer Indian Ocean Basin Mode (IOBM) predictability (1948–2022) using the Model-based Analog Forecast (MAF) method, based on a library of 20 CMIP6 models. A pronounced decadal shift is identified, with forecast skill markedly increasing after 1980. This shift is primarily attributable to the decadal modulation of the ENSO–IOBM teleconnection. During the high-skill period, prolonged El Niño events induce significant southwestern Indian Ocean (SWIO) warming. This, in turn, activates a robust wind-evaporation-SST (WES) feedback, which maintains the basin-wide warming into summer, thereby providing an enhanced signal component for IOBM predictions. In contrast, during the low-skill period, weaker ENSO events fail to sustain this feedback, leading to premature termination of IOBM events and consequently lower forecast skill. These findings demonstrate that boreal summer IOBM predictability is nonstationary and reveal that accurately representing the ENSO–IOBM teleconnection is essential for advancing forecast skill.
How to cite: Xu, C. and Wu, Y.: Decadal change in seasonal prediction skills of the Indian Ocean Basin Mode during boreal summer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6100, https://doi.org/10.5194/egusphere-egu26-6100, 2026.
The tropical southeastern Indian Ocean regarded as a pivotal region for Indo-Pacific climate teleconnections, including phenomena such as the El Niño-Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Interdecadal Pacific Oscillation (IPO). However, long-term instrumental climate data are often lacking for tropical oceans. The geochemistry of massive stony corals provides a valuable record of past hydroclimatic conditions that compensates for this lack and surpasses existing data.
Using sub-seasonally resolved coral Sr/Ca and δ18O records from Browse Island, Australia spanning 1753–2011, this work provides new insights into sea surface temperature (SST) and salinity variability over interannual to multidecadal timescales. The Sr/Ca record reveals robust correlations with instrumental SST, capturing the long-term industrial era warming starting at the end of the Little Ice Age (LIA) and accelerating trends since the early 20th century, indicative of anthropogenic forcing. The δ18Oseawater record, reconstructed from paired Sr/Ca and δ18O data, highlights hydrological variability driven by precipitation-evaporation dynamics, closely tied to the Australian monsoon, and ITF transport. While the imprint of the IOD seems to be reflected more in SST anomalies in the region, the influence of ENSO is recorded in hydrological anomalies due to changes in ocean advection. Long-term trends in δ18Osw indicate centennial variability, reflecting complex interactions between monsoon-driven freshwater fluxes and ITF circulation. Freshening since the 1950s is likely caused by the intensified hydrological cycle due to anthropogenic warming. The SST reconstruction tracks the cooling and warming periods indicated by the IPO.
The findings underscore the influence of interannual and decadal variability, particularly the IOD, ENSO, and the Interdecadal Pacific Oscillation (IPO), on SST and salinity, mediated by the combined effects of monsoon dynamics and ITF transport. Discrepancies between δ18Osw and Sr/Ca-SST trends emphasize the need for further investigation into the driving mechanisms of long-term climate variability by pantropical teleconnections.
How to cite: Zinke, J., Krawczyk, H. A., Behera, P., Boom, A., Hambach, B., Pfeiffer, M., Cantin, N., Lough, J. M., and Wilson, P.: A 250-year SST and salinity coral record reflecting Indo-Pacific teleconnections by the Indonesian Throughflow , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21158, https://doi.org/10.5194/egusphere-egu26-21158, 2026.
It has been recognized that the low-tropospheric circulations associated with Northeast Asian and the western North Pacific monsoons are closely related to each other via atmospheric teleconnection in boreal summer. This teleconnection can be understood by the responses to the stationary Rossby wave. In addition, the climate mode including the atmospheric teleconnection has a large variability on inter-decadal as well as interannual time scales associated with adjacent climate variability. This study focuses subseasonal climate modes, which were decomposed by the self-organizing map (SOM) analysis. This study suggests that those modes are significantly increased in the last few decades, and the changes in the climate mode are related to upper ocean warming of Northern Indian Ocean due to global warming and changes in the climate variability in the Indian Ocean. This study also shows the possible reasons for the analysis results.
How to cite: Lee, H. and Kwon, M.: Inter-decadal changes in a subseasonal climate variability in the western North Pacific region with Northern Indian Ocean in boreal summer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8740, https://doi.org/10.5194/egusphere-egu26-8740, 2026.
Climate conditions over East Asia are significantly affected by coupled ocean–atmosphere interaction in the tropical Indo-Pacific Ocean. In April 2024, South China suffered two rounds of extreme rainfall, occurring from 30 March to 6 April and from 19 to 30 April, resulting in the earliest flood over the Pearl River basin since 1998. This study finds that the early Indo–western Pacific Ocean capacitor (IPOC) effect and the Madden–Julian oscillation (MJO) jointly contributed to the extreme rainfall. Co-occurrence of El Niño and positive Indian Ocean dipole events in 2023–24 led to strong sea surface temperature (SST) warming in the western tropical Indian Ocean via wind forcings and oceanic waves. Such SST warming induced persistent easterly wind anomalies and maintained the anomalous anticyclonic circulation (AAC) over the western North Pacific. The IPOC effect was hence activated in April, approximately 2 months earlier than expected, inducing stronger northward water vapor transport. Moreover, two MJO events were observed in April. With the eastward propagation into the eastern Indian Ocean (phases 1–3), the MJO events facilitated the southwest flank of the AAC and enhanced the northward water vapor transport, leading to extreme rainfall along with strong convection in South China. This study emphasizes the synergistic contributions of climate modes on different time scales to extreme weather.
How to cite: Du, Y., Zhang, L., Zhang, Y., and Chen, Z.: Extreme Rainfall over South China in April 2024 Associated with Early IPOC and MJO Events, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4649, https://doi.org/10.5194/egusphere-egu26-4649, 2026.
The Indo-Pacific region is vulnerable to changes in the Indian summer monsoon and its onset. The associated monsoon rainfall strongly affects agriculture, water resources, and human security across the densely populated regions of South Asia. The Indian Ocean summer monsoon often develops in association with the southeast Arabian Sea warm pool and a monsoon onset air pressure vortex, giving rise to a complex air-sea coupled system, though their precise interactions and impacts remain unclear. In this study, our analysis covering 1992-2017 demonstrates that the vortex triggers an earlier onset due to vortex-induced rainfall when the monsoon system has not yet developed to its climatological intensity. The weaker monsoon at the onset time means the large-scale moisture transport is expected to be lower over the Indian subcontinent, and rainfall analysis confirms a drier central India, where agriculture is mostly rain-fed, at the monsoon onset time in the vortex years. These results help to understand the transition from localized synoptic activity to the large-scale monsoonal system, highlighting the crucial role of the vortex. More importantly, when a vortex is pre-observed, rainfall available for agricultural irrigation is expected to be lower, providing guidance for agricultural irrigation.
How to cite: Zhao, Z., Sprintall, J., and Du, Y.: How the Vortex over the Arabian Sea Warm Pool Triggers an Early Indian Summer Monsoon, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3220, https://doi.org/10.5194/egusphere-egu26-3220, 2026.
Marine heatwaves (MHWs) are ocean temperature extremes that can occur at any ocean depth. Surface features and drivers of MHWs have been extensively explored based on satellite observations; however, their subsurface features and drivers remain unclear. This study investigates the characteristics and drivers of subsurface MHWs near the thermocline in the Bay of Bengal (BoB) from 1993 to 2024 using high-resolution ocean reanalysis datasets. The subsurface MHW days exhibit a dipole pattern in response to the El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). During El Niño or positive IOD, the anticyclonic mesoscale eddies in the western BoB are favorable for MHW generation in this region, related to the anomalous anticyclonic winds and currents over the BoB. Meanwhile, the equatorial easterly anomalies drive upwelling Kelvin waves to propagate eastward into the eastern BoB, inhibiting MHW formation in that area. Thus, the subsurface MHW days in the BoB increase in the west but decrease in the east during El Niño or positive IOD events. Over the past decades, a significant increasing trend in the subsurface MHW days has been observed due to the rise in mean temperature over the BoB. This study highlights the inconsistent spatial responses of subsurface MHWs to distinct ocean dynamics induced by ENSO and IOD.
How to cite: Zhang, Y., Du, Y., Lin, X., and Qiu, Y.: Dipole variability of subsurface marine heatwaves in the Bay of Bengal, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6646, https://doi.org/10.5194/egusphere-egu26-6646, 2026.
Intermediate water masses in the tropical western Indian Ocean play a key role in subsurface thermohaline circulation by contributing to the redistribution of heat and salt, yet their variability on seasonal to interannual timescales remains poorly understood due to the historical scarcity of sustained in situ observations. We present an observational analysis of intermediate water mass variability based on a continuous subsurface mooring time series collected from 2019 to 2025 at the Seychelles-Chagos Thermocline Ridge (SCTR; 8°S, with the mooring located at 61°E during May 2019-June 2024 and relocated to 65°E thereafter). We focus on the intermediate layer spanning approximately 440-1190 m depth (corresponding to ~27.0-27.4/27.5 sigma-theta), using temperature, salinity, potential density, and spiciness. Pronounced changes in physical properties are observed between the earlier (2019-2021) and later (2022-2025) periods. Relative to the earlier years, the intermediate layer during later period exhibits freshening (34.82 to 34.79 PSU, -0.1%) and warming (7.06 to 7.19 °C, +1.8%), accompanied by a decrease in potential density (27.28 to 27.23 kgm-3, -0.2%) and a concurrent increase in spiciness (0.64 to 0.66, +1.8%), suggesting potential changes in the relative contributions of intermediate water masses. To examine this possibility, we apply an optimal multiparameter (OMP) analysis to quantify the fractional contributions of Red Sea Overflow Water (RSOW), Indonesian Intermediate Water (IIW), and Antarctic Intermediate Water (AAIW). The OMP results show that RSOW has both the largest fractional contribution and the strongest interannual-scale variability among the three intermediate water masses at the SCTR accounting on average for ~0.59±0.05 of the intermediate layer indicating its dominant role in modulating intermediate-layer variability in the region. In comparison, IIW and AAIW contribute smaller mean fractions (~0.25±0.01 and ~0.13±0.03, respectively) and display comparatively weaker temporal variability. Notably, the mean RSOW fraction decreases during 2022-2025 from 0.62±0.04 to 0.57± 0.03 (-7.3%), whereas the contributions of IIW and AAIW increase from 0.24± 0.01 to 0.25± 0.01 (+7.2%) and from 0.11±0.03 to 0.15±0.02 (+34.6%), respectively. While RSOW remains the dominant intermediate water mass at the SCTR, the increased fractions of IIW and AAIW during the later years indicate an enhanced relative contributions of these water masses during the later period, consistent with the observed freshening and increase in spiciness in intermediate layer. By leveraging a rare continuous mooring time series, this study demonstrates the value of sustained in situ observations for resolving multi-year variability in intermediate water mass composition and properties at the SCTR region.
How to cite: Song, S., Nam, S., and Menezes, V. V.: Observed variability of intermediate water masses in tropical western Indian Ocean from a 2019-2025 subsurface mooring time series, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20563, https://doi.org/10.5194/egusphere-egu26-20563, 2026.
The Bay of Bengal is considered one of the largest oceanic Oxygen Minimum Zone (OMZ), characterized by oxygen levels that remains persistently near the threshold of anoxia, possibly limiting the widespread nitrogen loss observed in other major OMZs. Planktonic microbial respiration is largely responsible for the formation and maintenance of the hypoxic and anoxic conditions found in the OMZs. Understanding the respiratory kinetics of the planktonic microbial community is therefore essential to predicting the sensitivity of this area to further deoxygenation. During a cruise aboard the R/V Thomson in the summer months of 2025, we investigated the regulation and control of microbial oxygen consumption within the upper 300 m of the water column. We combined high-resolution vertical profiling with experimental rate measurements using high-sensitivity oxygen sensors to characterize the metabolic transition from the upper oxic layer through the oxycline into the nearly anoxic core. Microbial community abundance was quantified via flow cytometry to link biomass density with metabolic activity. Respiratory kinetics were characterized by onboard water incubations with samples subjected to a wide range of oxygen levels. Our results demonstrate a clear vertical stratification in respiratory potential, with the highest rates associated with the upper oxic layer and a progressive decrease as oxygen and chlorophyll levels decreased. However, higher values were also found at intermediate depths within the hypoxic water layers. By fitting oxygen consumption rates to kinetic models, we calculated the apparent half-saturation constant (Km) for the microbial community throughout the water column. These Km values showed a complex distribution, generally reaching their minimum in the oxycline and increasing within the hypoxic zones. This suggests a counterintuitive decrease in oxygen affinity at low oxygen levels, although significant consumption rates were observed even at trace levels of oxygen. This trend may indicate a taxonomic shift in the microbial community or a change in the expression of different types of terminal oxidases, thereby demonstrating adaptation of the microbial community to the episodic oxygen supply characteristic of the interior of the Bay of Bengal.
How to cite: Garcia-Robledo, E., Rodriguez-Marquez, J., Pachiadaki, M., Bourbonnais, A., and Calderon-Caro, J.: Regulation and control of the planktonic microbial respiration in the hypoxic waters of the Bay of Bengal, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17545, https://doi.org/10.5194/egusphere-egu26-17545, 2026.
The Arabian Sea is among the most productive ocean basins globally, driven by summer coastal upwelling, winter convective mixing, and aeolian dust inputs that supply nutrients to the euphotic zone. In several aspects, this region shares characteristics with High Nutrient–Low Chlorophyll (HNLC) systems, where mineral dust deposition partially alleviates iron limitation of surface waters, that are ventilated by iron-depleted waters (e.g., the Antarctic Intermediate Waters). Paleorecords indicate that enhanced dust fluxes during the Last Glacial Maximum (LGM) coincided with increased primary productivity in the northwestern Arabian Sea, suggesting a potential role for iron fertilization, although the underlying mechanisms remain poorly constrained.
Here, we reconstruct millennial-scale variations in coccolithophore growth rates in the northwestern Arabian Sea since the LGM, based on the coccolith carbon isotope vital effect (δ13CVE) recorded in sediment core MD00-2354 (61.48°E, 21.04°N). Combined with coccolithophore cell-size estimates at the studied site, and reconstructed iron fluxes in the area, these data allow us to investigate the links between iron availability and phytoplankton growth from 22 to 4 ka.
Our results show that coccolithophore growth rates and cell sizes were significantly increased during the LGM, coincident with maxima in mineral dust and iron fluxes. This pattern suggests that nutrient availability was the primary control on coccolithophore growth at that time. This interpretation is supported by a positive correlation between coccolithophore growth rates and independently reconstructed net primary productivity at the site. A likely mechanism is that increased iron supply during the LGM enhanced phytoplankton nitrogen assimilation, as further supported by ROMS–PISCES model simulations. Comparisons between simulations with and without atmospheric iron deposition indicate that, under increased iron input, the enhancement in nitrogen utilization exceeds that of phosphorus utilization, and is concomitant to elevated primary productivity.
How to cite: Zhou, X., Duchamp-Alphonse, S., Jin, X., Liu, C., Jiang, X., Bassinot, F., and Kissel, C.: Iron fertilization enhanced coccolithophore growth rate in the northwestern Arabian Sea during the Last Glacial Maximum, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7753, https://doi.org/10.5194/egusphere-egu26-7753, 2026.
Posters virtual: Tue, 5 May, 14:00–18:00 | vPoster spot 1a
EGU26-10227 | ECS | Posters virtual | VPS20
Analysis of the mechanisms underlying the low-frequency variability of the low-salinity tongue in the southeastern Indian OceanTue, 05 May, 14:24–14:27 (CEST) vPoster spot 1a
Ocean salinity serves as a key indicator of the global water cycle and exerts important controls on oceanic circulation, sea level, and stratification, thereby playing a critical role in marine thermodynamic and dynamic processes. In recent years, salinity variability in the tropical Indian Ocean, particularly its dynamic mechanisms and climatic effects, has attracted growing scientific interest. Using 31 years of satellite observations, in-situ data sets, and model reanalysis data, this study investigates the decadal variability and formation mechanisms of the low salinity tongue in the South Indian Ocean between the equator and 20°S. The results indicate that both the volume and mean salinity of the low-salinity tongue exhibit a quasi-12-year oscillation, which is primarily associated with the Interdecadal Pacific Oscillation (IPO). Further analysis reveals that on decadal timescales, variability in the volume of the upper 50 m low-salinity tongue is mainly driven by local precipitation. Through anomalous atmospheric circulation, sea surface temperature anomalies in the tropical Pacific lead to multi-year precipitation anomalies in the southeastern Indian Ocean, which subsequently alter the westward extension of the surface low-salinity tongue and ultimately govern its volume variability in the upper 50 m. However, in the subsurface layer (50 to 200 m), variability in the volume and average salinity of the low salinity tongue is dominated by freshwater transport associated with the Indonesian Throughflow (ITF). During negative IPO phases, wind anomalies over the tropical Pacific trigger oceanic wave adjustments, which enhance the ITF salinity transport. This process subsequently leads to an expansion of the low salinity tongue and a decrease in its average salinity in the southeastern Indian Ocean. Based on the three-dimensional variability of the low salinity tongue, this study reveals the relationships between the volume and average salinity of the tongue at different depths and local freshwater forcing, as well as salinity transport by the ITF, thereby contributing to an improved understanding of how regional water mass changes respond to long-term climate variability.
How to cite: yanran, P., sun, Q., zhang, Y., zhang, Y., chi, J., and du, Y.: Analysis of the mechanisms underlying the low-frequency variability of the low-salinity tongue in the southeastern Indian Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10227, https://doi.org/10.5194/egusphere-egu26-10227, 2026.
EGU26-4605 | ECS | Posters virtual | VPS20
Quantifying impacts of ENSO and internal variability on the Indian Ocean DipoleTue, 05 May, 15:21–15:24 (CEST) vPoster spot 1a
The Indian Ocean Dipole (IOD) is an intrinsic climate mode in the Indian Ocean that typically peaks during boreal fall and influences weather and climate across surrounding regions. It is influenced by both the El Niño–Southern Oscillation (ENSO) and internal variability within the Indian Ocean. However, the relative contributions of the two ENSO types—namely, Eastern Pacific (EP) and Central Pacific (CP) ENSO—and internal variability to the IOD remain poorly quantified. Here, we employ a binary combined linear regression approach to isolate and quantify the contributions of these three factors. The results show that internal variability is the dominant driver of IOD-related sea surface temperature (SST) anomalies, explaining over 60% of the variance. ENSO accounts for approximately one-third of the variance, primarily through the CP type, whereas the EP type tends to influence the IOD mainly during extreme events. Their underlying mechanisms differ. ENSO primarily modulates the Indian Ocean wind field through the Walker circulation, whose effectiveness depends on the longitudinal position of the equatorial Pacific warming—eastern for EP events and central for CP events. In contrast, internal variability generates SST anomalies through local ocean–atmosphere feedbacks that sustain the IOD. Because El Niño tends to persist longer, co-occurring positive IOD events are more likely to evolve into basin-wide Indian Ocean warming in the following spring, a transition to which El Niño contributes more than 70%. Although internal variability shows no significant statistical association with this transition, a strong positive IOD alone still has the potential to trigger the basin-wide warming in the subsequent spring. These findings enhance our understanding of climate modes and inter-basin interactions.
How to cite: Zhang, L., Du, Y., and Zhang, Y.: Quantifying impacts of ENSO and internal variability on the Indian Ocean Dipole, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4605, https://doi.org/10.5194/egusphere-egu26-4605, 2026.
