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.

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 (F_v/F_m) 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 19^th 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 19^th 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/Mg_coral 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/Mg_coral, 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/Mg_coral 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 (Δδ¹³C_epi-in). Changes in biological pump efficiency are assessed from the carbon isotope gradient between planktonic and benthic foraminifera (Δδ¹³C_p-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.