Several paleoceanographic and climate modeling studies have shown both oceanic and atmospheric teleconnections between climate in the tropics and the high latitudes on timescales ranging from decadal to multi-centennial. The last glacial period is characterized by millennial-scale abrupt warmings (interstadials) followed by rather gradual coolings to colder (stadials) Dansgaard-Oeschger (DO) events. The more pronounced of these stadial phases coincide with occurrences of ice-rafted debris in sediments from the mid-latitude Atlantic Ocean, referred to as Heinrich events (HE). Climate oscillations associated with DOs and HEs are also recorded in tropical climate archives around the Indian Ocean and on the Asian continent. However, forcing and response mechanisms of the Indo-Asian monsoon system and ocean-atmosphere exchange processes in conjunction with these millennial-scale oscillations are still poorly understood. Here, we present high-resolution geochemical and micropaleontological data from a sediment core located at 571 m water depth offshore Pakistan, representing the past 80,000 years at millennial-scale resolution.

Alkenone unsaturation-derived sea surface temperature (SST) estimates show overall variations between 23 and 28°C. Millennial scale SST changes of 2°C are modulated by longer-term SST fluctuations. Interstadial intervals are characterized by higher organic carbon (TOC) concentrations, whereas sediments with low TOC contents mark stadials. Productivity-related and anoxia-indicating proxies show abrupt shifts with a 50-60 year duration at climate transitions, such as interstadial inceptions. Inorganic data consistently indicate that enhanced fluxes of terrestrial-derived sediments are paralleled by productivity maxima, and are characterized by an increased fluvial contribution from the Indus River during interstadials. The hydrogen isotopic composition of terrigenous plant waxes indicates that stadials are dry phases whereas humid conditions seem to have prevailed during interstadials. Stadials are characterized by an increased contribution of aeolian dust. HEs are especially dry, indicating a dramatically weakened Indian summer monsoon and increased continental aridity.  

The stable oxygen isotope (δ¹⁸O) records of the surface-dwelling foraminifera G. ruber and of the thermocline-dwelling P. obliquiloculata both show a strong correspondence to Greenland ice core δ¹⁸O, whereas the δ¹⁸O signal of benthic foraminifera (U. peregrina and G. affinis) reflects patterns similar to those observed in Antarctic ice core records. Distinct shifts in benthic δ¹⁸O during stadials indicate frequent injections of oxygen-rich intermediate water masses of Southern Ocean origin into the Arabian Sea. The most pronounced oceanographic changes occur during the transition and the termination of HE 4, respectively. Mg/Ca ratios of G. affinis show a rapid increase (decrease) of bottom water temperatures during the onset (termination) of HE 4, which is in good agreement to modelling studies. The hydrogen isotopic composition of terrigenous plant waxes indicates that HE 4 is much drier than the surrounding DOs.

Overall, our results strengthen the notion that North Atlantic temperature changes and shifts in the hydrological cycle of the Indian monsoon system are closely coupled, with significant impacts on regional environmental conditions such as river discharge and ocean margin anoxia. These shifts were modulated by changes in the supply of water masses from the Southern Hemisphere.

How to cite: Lückge, A., Hollstein, M., Kienast, M., Groeneveld, J., Schefuß, E., Mohtadi, M., and Steinke, S.: Southern Hemisphere water mass transport to the Arabian Sea linked to Greenland climate variability during Heinrich Event 4 , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3030, https://doi.org/10.5194/egusphere-egu25-3030, 2025.

The Indian Ocean is affected by seasonal and interannual climate variability, with large consequences on the water cycle over adjacent landmasses. The northern Indian Ocean influences the African and Asian monsoon systems, and its seasonal and interannual variability interacts with El Niño-Southern Oscillation (ENSO) in the Pacific Ocean, potentially triggering Indian Ocean Dipole (IOD) development. Future projections suggest that Indian Ocean modes of climate variability could be enhanced under warming scenarios, but a lack of past records precludes a deeper understanding of how the Indian Ocean modes of variability could change over evolving boundary conditions.

Due to low sediment accumulation rates and bioturbation in marine cores, bulk analyses cannot usually record past climate variability at seasonal or interannual timescales. In contrast, individual foraminifera analyses (IFA) can theoretically estimate the total variance in a population of foraminifera that experienced upper ocean variability at sub-centennial timescales. Geochemical records (δ¹⁸O, Mg/Ca) based on IFA have been used to reconstruct past climate variance including seasonality, ENSO in the Pacific Ocean, and IOD in the eastern Indian Ocean. However, using variance in a foraminifera population to pinpoint which mode of variability is the ultimate driver remains a challenge.

Here we model the range of variability of different key sites in the Indian Ocean to seasonal vs interannual climate variability using ORAS-5 re-analysis temperature and salinity data spanning the period from 1958 to 2018, and to assess the impact of changing the amplitude of these modes of variability on total δ¹⁸O variance of model foraminifera populations. In light of these results, we then analyzed IFA (δ¹⁸O and morphology) in three core-top samples, targeting two planktonic foraminifera species with different seasonalities and depth habitats: Ocean Drilling Program Site 722 in the Arabian Sea upwelling region, monsoon-influenced Site MD77-191 near the southern tip of India, and Site MD96-2060 core offshore Tanzania in the western Indian Ocean. Results will allow us to better evaluate how to interpret IFA in different oceanographic biomes of the Indian Ocean, ultimately leading to a better understanding of past climate extremes embedded in paleoclimate record.

How to cite: Lichterfeld, Y., Leduc, G., Thirumalai, K., Vidal, L., De Garidel-Thoron, T., Sonzogni, C., and Bolton, C.: Calibrating Individual Foraminifera Analysis for Climate reconstruction in the Western Indian Ocean: Assessing Seasonal and Interannual Variability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8783, https://doi.org/10.5194/egusphere-egu25-8783, 2025.

Climate variability in the Indian and Pacific Oceans exhibits strong coupling on interannual to decadal timescales. Since the early 2000s, however, synchronization of decadal climate modes between the two basins has decreased due to enhanced greenhouse gas forcing and anthropogenically driven warming of the Indian Ocean. Understanding mechanisms of decoupling is crucial for properly characterizing and predicting low-frequency (decadal-multidecadal) climate variations which have a large impact on regional water resources around the Indian Ocean rim and marine ecosystems.

Here we contextualize the recent inter-basin decoupling by reconstructing Indo-Pacific basin interactions over the past four centuries (1630-2000 CE) through leveraging a compilation of tropical paleoclimate archives and two reconstruction methods. Specifically, we employ a network of coral proxy records from the Indian Ocean, Maritime Continent, and Pacific Ocean, alongside select hydroclimatically-sensitive stalagmite and tree-ring records from the Indian Ocean rim to reconstruct the Indian and Pacific Walker circulations, as well as the Indian Ocean Basin Mode over the past four centuries.

Our results confirm that Indo-Pacific coupling was present throughout the preindustrial era, and was disrupted only by a series of strong tropical volcanic eruptions during the early 19th century. We find, based on last millennium climate model simulations and hemispheric temperature reconstructions, that the interhemispheric asymmetry of cooling in response to volcanic forcing as well as the Indian Ocean’s strong sensitivity to external forcings caused this anomalous decoupling of Indo-Pacific climate. Additionally, the mechanisms of past decoupling associated with volcanism provide insights into the source of inter-model spread on the magnitude of future Indo-Pacific trends.

How to cite: Wang, S., Ummenhofer, C., and Oppo, D.: Low-frequency coupling of the Indian and Pacific Walker circulation modulated by volcanic forcing, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21348, https://doi.org/10.5194/egusphere-egu25-21348, 2025.

Oxygen minimum zones (OMZ) contribute to 20 to 40 % of global fixed nitrogen loss despite occupying only about 1 % of the ocean. The Bay of Bengal (BoB) contains one of the most pronounced OMZ in intermediate waters worldwide, with oxygen levels near anoxic conditions. Understanding nitrogen cycling in OMZs is critical for comprehending and accurately modeling the global oceanic nitrogen cycle.

In this study, we examined nitrogen cycling in the East Equatorial Indian Ocean (EEIO) and the BoB using water column properties—including temperature, salinity, oxygen, nutrients, and dual stable isotopes of nitrate—collected during a cruise in April/May 2024. Potential temperature and salinity profiles revealed a clear separation between the BoB and the EEIO at 5°N, with distinct water mass distributions and limited mixing between the two regions.

Depth profiles of nitrate stable isotopes displayed notable variations. In waters below 300 m, isotopic signatures were influenced solely by water mass distribution. In contrast, isotope variations in the upper 200 m reflected active on-site fractionation. Surface waters (<100 m) exhibited significant nitrate isotope enrichment and a nitrate deficit, driven by phytoplankton uptake. Below this layer, nitrification was observed, primarily through regenerative production using previously assimilated biomass rather than newly fixed nitrogen from N₂ fixation. A regional decoupling of nitrate dual isotopes, with more enriched δ¹⁸O-NO₃^- in more northern samples of the central BoB, suggested increased nitrite reduction followed by re-oxidation without full assimilation into organic matter in the BoB.

Within the OMZ of the BoB, we identified a persistent nitrate deficit and slightly enriched nitrate isotopes, indicative of nitrogen loss. Given that oxygen concentrations remained slightly above the threshold for significant denitrification in most samples, anammox likely represents the dominant nitrogen loss pathway in the BoB's OMZ.

How to cite: Schulz, G., Dähnke, K., Sanders, T., Penopp, J., Bange, H. W., Czeschel, R., and Gaye, B.: Nitrogen Cycling in the East Equatorial Indian Ocean and Bay of Bengal: Insights from Nitrate Isotopes and Water Masses, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8266, https://doi.org/10.5194/egusphere-egu25-8266, 2025.

Chlorophyll concentrations in the ocean exhibit significant spatial and temporal variability, driven by a complex interplay of physical, chemical, and biological factors. Seasonal changes are primarily influenced by variations in light availability, temperature, and nutrient supply, while interannual fluctuations are often linked to large-scale climate phenomena such as the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). These variations have profound implications for oceanic primary productivity, marine ecosystems, and global biogeochemical cycles. The Arabian Sea, one of the most productive regions of the global ocean, experiences high biological productivity due to seasonal upwelling driven by the Southwest Monsoon and winter convective mixing. Upwelling brings nutrient-rich deep waters to the surface, stimulating phytoplankton growth and increasing chlorophyll concentrations, which in turn support a diverse marine ecosystem and economically significant fisheries. In our study, we analysed monthly and seasonal climatologies of chlorophyll(mg/m³) in the Arabian Sea, examining anomalies over a 24-year period (1998–2021). Our results revealed significant interannual variability, with notable peaks and dips in chlorophyll anomalies. For instance, anomalies exceeded a deviation from the climatological mean by 0.2 in 2005 but dropped as low as -0.2 in 2016. By analysing the standard deviation of log-transformed chlorophyll-a anomalies, we identified five regions exhibiting the highest variability. Further investigation into these regions revealed distinct patterns in upwelling dynamics across different years and seasons, emphasizing the diverse factors influencing upwelling processes in the Arabian Sea. Focusing on the western coast of India, we observed contrasting climatic behaviours between the northern and southern regions. In the northern part, wind anomalies did not directly correspond to chlorophyll anomalies, indicating a more complex interplay of factors. Conversely, in the southern region, a strong correlation between chlorophyll and wind anomalies suggests a dominant wind-driven upwelling mechanism. These findings enhance our understanding of the regional variability in upwelling processes and highlight the intricate interactions between oceanic and atmospheric drivers in this dynamic marine system. Our study provides valuable insights into the variability of chlorophyll concentrations in the Arabian Sea, offering a better understanding of its ecological and climatic significance. These findings contribute to improved modelling and prediction of primary productivity, which is crucial for both ecosystem management and climate studies.

How to cite: Joshi, S. and Prakash Dey, S.: Interannual Variability of Chlorophyll Concentrations in the Arabian Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21671, https://doi.org/10.5194/egusphere-egu25-21671, 2025.

The Arabian Sea, a part of the north Indian Ocean shows a trend of increasing sea surface temperature (SST) over a decadal scale. This area is particularly important due to high phytoplankton growth which is mostly governed by atmospheric forcing and also a place for carbon burial. Hence it is imperative to understand the responses of phytoplankton to this warming trend. The summer monsoon (June-August) winds develop a low-level atmospheric jet (Findlater Jet) blowing across the central Arabian Sea. The positive wind stress curl in the north of this jet leads to open ocean upwelling with consequent nutrient enrichment and phytoplankton bloom. The negative curl in the south results in down-welling and deepening of the mixed layer depth. During the winter monsoon, the wind direction reverses and speed weakens, but in the northern part the cold convective mixing occurs due to the cooling and densification of surface waters and also fuels high phytoplankton growth. However, the south remains oligotrophic, low productive, and warmer compared to the north. We present here two data sets of phytoplankton taxonomy done by both marker pigment analyses by HPLC and light microscopy collected in two field campaigns during summer monsoons 2017 (August) and 2018 (August) along the central Arabian Sea (64°E, 11 -21° N in 1 ° interval). The northern part of the Findlater Jet was mostly occupied by the cooler waters and highest nutrient levels that promoted large diatom-dominated phytoplankton biomass. The southern part was oligotrophic with deep mixed layers, warm, and dominated by (~50%) picocynaobacteria and Prochlorococcus (containing zeaxanthin and DV-Chla) followed by smaller chain-forming diatoms, and heterotrophic dinoflagellates. We have observed that the upwelling strength was stronger in 2018 with cooler waters and higher nutrient levels compared to 2017. The occurrences of warmer waters in 2017 supported higher growth of picocynaobacteria. This is also consistent with other global analyses of long-term trends observed from the Indian Ocean. We have considered a box of 64-66°E and divided it into two sectors, south (18-21°N) and north (11-15°N). The satellite-derived SST data from 2000-2024 indicates a warming trend during summer monsoon both in the north and south. However, no such trend was noticed during winter. This observation suggests that warming during summer monsoon may directly influence the phytoplankton community and may affect carbon transfer and cycling in this dynamic basin.

How to cite: Biswas, H., Chowdhury, M., and Majumder, N.: Picocyanobacteria show warm water preference in the south-central Arabian Sea (North Indian Ocean) during the summer monsoon, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8568, https://doi.org/10.5194/egusphere-egu25-8568, 2025.

With the developing observation system, our understanding of Indian Ocean circulation has advanced in recent years. We have noticed the rapid increase of the ocean content in the Indian Ocean, and its role in global climate changes. In the mean state, a relatively closed current loop is established by an eastward current along the equator and a westward current south of the equator, regarded as the Indian Ocean Tropical Gyre (IOTG). As an important component of Indian Ocean air-sea interaction, the essential impacts of IOTG have been discovered. Due to the monsoon, IOTG displays significant seasonal variations, characterized by the reversal of currents and associated heat-salt redistribution. Also, IOTG interacts with the climate modes. This paper summarizes the advances, including the multi-scale variations of IOTG, associated heat-salt transport, and its ecological impact.

How to cite: Du, Y., ZHao, Z., and Zhang, Y.: The Indian Ocean Tropical Gyre and associated heat-salt Transport and its ecological impact: A review, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15560, https://doi.org/10.5194/egusphere-egu25-15560, 2025.

The Great Whirl (GW), a prominent anticyclonic gyre in the northwest Indian Ocean, is crucial in regional circulation and energy dynamics during the summer monsoon. Using satellite observations and high-resolution ocean simulations, this study examines the mechanisms behind the growth and maintenance of Eddy Kinetic Energy (EKE) in the GW region. EKE peaks about 56 days after the summer monsoon’s peak, a delay caused by energy transfer processes. Southwest wind forcing during the monsoon initiates the EKE growth, with the barotropic energy conversions from mean flows eventually dominating the energy input. Enhanced stretching and shear effects of the Somali Currents (SC) intensify barotropic instabilities, maintaining EKE even as monsoon winds weaken. The baroclinic energy conversions act as a secondary energy input, exhibiting a positive eddy buoyancy work (potential energy to kinetic energy) at the upwelling wedge regions northwest of the GW. Our study highlights the importance of internal energy transfer processes in modulating ocean circulation and energy dynamics off the Somali Coast, emphasizing eddy-mean flow interactions and potential-to-kinetic energy transfer in the Somali upwelling system.

How to cite: Lin, J., Wang, M., Dai, L., Sasaki, H., and Du, Y.: Delayed Response of Eddy Kinetic Energy Build-up off Somali Coast During Summer Monsoon, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20979, https://doi.org/10.5194/egusphere-egu25-20979, 2025.