Antarctic Intermediate Water (AAIW) plays a crucial role in the global thermohaline circulation and is a vital component of the Atlantic Meridional Overturning Circulation (AMOC). It significantly contributes to the redistribution of heat, oxygen, and nutrients across the global ocean. Understanding the dynamics of intermediate water circulation over millennial timescales is essential for evaluating how changes in the AMOC affect ocean heat transport during abrupt climatic events. Despite its importance, the relationship between global intermediate water circulation and abrupt high-latitude climatic events such as the Younger Dryas (YD) and Heinrich Stadials (HS) remains partly understood, particularly in the Indian Ocean. To address this gap, we present a high-resolution ~29 kyr record of Neodymium isotopes (ƐNd) from the authigenic phases of a sediment core (SK-17) collected at 840 m depth in the eastern Arabian Sea, off Goa. Our ƐNd data shows significant temporal variations from -9.5 to -6.1 throughout the core. Climatic periods (such as YD and HS) with enhanced radiogenic Nd signatures indicate increased northward penetration of AAIW into the northern Indian Ocean during these intervals. These episodes correspond to colder periods in the Northern Hemisphere, suggesting a direct linkage between Northern Hemisphere climate dynamics and the formation of AAIW in the Southern Ocean. Specifically, the enhanced formation of AAIW during these times may have been driven by warming-induced deceleration of the AMOC, which likely triggered increased AAIW production in the Southern Ocean. This connection highlights the interplay between the North Atlantic Deep-Water formation and Southern Ocean climate processes governed by the "bipolar seesaw" mechanism. By linking AAIW variability in the Arabian Sea to global climatic events, our study underscores the importance of intermediate water masses in understanding the mechanisms driving past and potential future changes in the AMOC.
How to cite: Shukla, A., Mishra, T. K., Singh, S. K., and Singh, A. D.: Extensive Intrusion of Antarctic Intermediate Water into the Arabian Sea during Younger Dryas , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-270, https://doi.org/10.5194/egusphere-egu25-270, 2025.
The Arabian Sea (AS) hosts the world’s thickest and most intense oxygen minimum zone (OMZ), and previous studies have documented a dramatic decline of dissolved oxygen (DO) in the northeastern AS in recent decades. In this study, using the recently released data from Biogeochemical-Argo floats, we found a surprising trend of recovery in deoxygenation within the core region of the OMZ in the AS (ASOMZ) since 2013. The average DO concentration increased by approximately threefold, from ~0.63 μM in 2013 to ~1.68 μM in 2022, and the thickness of the ASOMZ decreased by 13%. We find that the weakening of Oman upwelling resulting from the weakening of the summer monsoon is the main driver of oxygenation in the ASOMZ. In addition, the reduction of primary production linked to warming-driven stratification reinforces deoxygenation recovery at depth.
How to cite: Liu, T., Qiu, Y., and Lin, X.: Dissolved oxygen recovery in the oxygen minimum zone of the Arabian Sea in recent decade as observed by BGC-Argo floats, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2038, https://doi.org/10.5194/egusphere-egu25-2038, 2025.
The Maldives archipelago has become one of the most famous touristic locations in the world and is under noticeable pressure from various human activities such as extensive fisheries and tourism. Because of this, the Pristine Seas Project (National Geographic) included the area in its efforts to fill existing knowledge gaps such as its species diversity and distribution as part of a larger aim to establish science-based protected marine reserve zones. Foraminifera, unicellular marine marvels, are renowned for their use as an environmental bioindicator of broader conditions. The present study investigates the diversity and biogeographic patterns of recent shallow water foraminifera that inhabit three distinctive atolls in the Maldives archipelago. Sediment samples were collected from reef and lagoon environments of the 26 distinct localities across three southern Maldives atolls - Addu, Fuvahmulah, and Huvadhoo. The most abundant taxa are Amphistegina followed by Calcarina, Heterostegina and Sorites. A species richness and diversity varied among sampling sites, with higher richness observed in MV18 station (Addu Atoll). Cluster analysis revealed distinct foraminiferal assemblages associated with different reef zones and sediment types. Here we discuss environmental parameters such as depth, substrate characteristics, ocean current influence, foraminiferal distribution patterns within and between different atolls of Maldives and also in comparison to the greater Indian Ocean datasets. The empirical data generated in this study offers a better understanding of ecosystem biodiversity in this remote location which may act as a baseline for future experimental and ecological studies, assessing possible anthropogenic influences and provides valuable insights into the regional vulnerability to climate change. Present study highlights the importance of habitat, microhabitat conservation and contributes to our knowledge of Indian Ocean marine biodiversity and biogeography.
Keywords: Indian Ocean, assemblage, habitat, distribution, large benthic foraminifera
How to cite: Baroliya, H., Anagnostoudi, T., Oron, S., Sala, E., Freidlander, A., and Goodman- Tchernov, B.: A Baseline for Recognizing Change: Diversity and Biogeography of the Maldives Atolls' Shallow Water Foraminifera, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8934, https://doi.org/10.5194/egusphere-egu25-8934, 2025.
Mesoscale eddies have a significant impact on the biological productivity in the Bay of Bengal (Eastern Indian Ocean). However, we do not currently have a complete understanding of them. The Bay of Bengal is a region that is highly dependent on fishing for its population’s survival, with surrounding countries such as Bangladesh depending on fish from the bay for approximately 60% of their consumed animal protein, and hence understanding any changes in this biological productivity is important. In this work, using 25-42 years of satellite data and the py-eddy-tracker eddy detection algorithm (Mason et al., 2014), we observe a yearly recurring mesoscale eddy (called hereafter the Odisha Eddy) on the western coast of the Bay of Bengal, which was noted before but investigated here, for the first time, in terms of physics and biological response. We also examine the impact of a confluence zone that forms between the EICC as it reverses poleward and an equatorward flowing boundary during May to September on the Odisha Eddy variability, using a new algorithm that automatically identifies this confluence zone based on satellite altimetry currents data. We further investigated the impact that the EICC confluence zone has on the biological productivity of the Odisha Eddy. We found that the eddy has an increase in the concentration of chlorophyll-a by 42% as compared to the surrounding waters, and that when the eddy occurs alongside the confluence zone, the chlorophyll-a content within the Odisha Eddy is 36% higher than when the eddy occurs alone. The Odisha Eddy also presents a larger radius (31% increase), amplitude (47% increase) and faster rotational velocity (15% increase) when it occurs alongside the confluence zone. We conclude that the EICC confluence zone amplifies the positive effect that the Odisha Eddy has on the biological productivity of this region of the Bay of Bengal. We hypothesise that this is due to the confluence zone enhancing advection of nutrients within and around the eddy. More research is needed to fully examine this mechanism and other controls (e.g., impact of ocean planetary waves, wind field) influencing the Odisha Eddy productivity in the Bay.
How to cite: Luty, W., Jebri, F., Srokosz, M., Ross, A., Griffiths, S., and Jacobs, Z.: Impact of a newly observed recurring eddy on the western Bay of Bengal productivity , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9261, https://doi.org/10.5194/egusphere-egu25-9261, 2025.
Salinity plays a major role in the global hydrological cycle and climate by modulating upper ocean stratification and sea surface temperature. Past studies have revealed the salinity variation in different major ocean basins from interannual to decadal time scales. However, understanding of upper ocean salinity variation in the Indian Ocean is limited, especially on decadal time scale and thus its global impacts. Our study reveals the presence of a horseshoe pattern in the upper ocean salinity, which varies on a decadal time scale. This pattern is generated from air-sea interaction mechanisms and is linked with Ningaloo Niño. The sea surface temperature anomalies in the southeastern tropical Indian Ocean in relation to Ningaloo Niño triggers the circulation anomaly causing the variation in the precipitation pattern. As a result, the upper ocean freshens or gets more saline and forms as a horseshoe pattern in upper ocean salinity on a decadal time scale. This variation could be useful for better presentation of salinity variation in the Indian Ocean and its associated impacts.
How to cite: Sahu, L., Senapati, B., and Dash, M. K.: A horseshoe salinity pattern in the Indian Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12271, https://doi.org/10.5194/egusphere-egu25-12271, 2025.
The rapid warming of the oceans is increasingly recognized for its significant impacts on the climate system and is a central issue in the Climate Variability and Predictability (CLIVAR) research program. However, the effect of future ocean warming—particularly regional ocean warming—on the Hadley Circulation (HC) remains poorly understood. This study addresses the regional contributions of tropical ocean warming to future HC changes, focusing on the 1.5°C, 2°C, and 3°C warming scenarios outlined in the Paris Agreement. Through large ensemble numerical simulations, we demonstrate for the first time that the tropical Indian Ocean dominates future HC changes, while the tropical Pacific Ocean is the main source of uncertainty in HC projections. These results provide critical insights for improving Earth system models and enhancing the projection of tropical atmospheric circulation. Furthermore, they offer a scientific foundation for monitoring and forecasting climate risks associated with future HC shifts, supporting the development of key policy decisions.
Sun, Y., Ramstein, G., Fedorov, A.V., Ding, L., & Liu, B. (2025). Tropical Indian Ocean drives Hadley circulation change in a warming climate. National Science Review, 12(1), nwae375, https://doi.org/10.1093/nsr/nwae375
How to cite: Sun, Y., Ramstein, G., Fedorov, A., and Ding, L.: Tropical Indian Ocean Warming: A Key Driver of Future Hadley Circulation Changes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8405, https://doi.org/10.5194/egusphere-egu25-8405, 2025.
Accurate representation and predictability of the Indian Ocean Dipole (IOD) in seasonal forecasts are crucial given its pronounced socioeconomic impacts on countries surrounding the Indian Ocean. Using hindcasts from the coupled Met Office Global Seasonal Forecasting System (GloSea6), coupled mean state biases in the western and eastern equatorial Indian Ocean (WEIO and EEIO) and their impacts on IOD prediction are examined.
Results show that GloSea6 exhibits a pronounced cold bias in the EEIO that rapidly develops after the onset of the monsoon in boreal summer (JJA, July-August) and persists through the autumn season (SON, September-November). This cold bias, along with a dry bias, is linked to erroneous easterlies and a shallow thermocline, likely associated with the summer monsoon circulation. The seasonal evolution and relative timing of the precipitation biases between the western and eastern IOD poles, such that the biases develop through JJA in the EEIO but follow in the WEIO in SON, suggests that the EEIO plays the leading role in the development of coupled feedbacks that result in an overall large dipole pattern of atmospheric and subsurface oceanic biases in SON.
Analysis of skill metrics for the IOD shows that GloSea6 achieves a high anomaly correlation coefficient at short lead times, though it tends to overestimate IOD ampltiude, indicating higher IOD variability compared to observations. This overestimation is larger in the eastern IOD pole than in the western pole and is likely linked to the poor representation of the evolution of the SST anomalies in the EEIO during positive and negative IOD events in SON. This aligns with the skill metrics of the individual poles, which show a lower anomaly correlation coefficient and higher prediction errors observed in the eastern pole compared to the west.
Results in this study highlight the crucial role of regional biases, particularly in the EEIO, in shaping IOD variability and suggest that addressing these regional biases in GloSea6 could improve IOD prediction skill, enhancing forecasts of climate impacts for countries surrounding the Indian Ocean.
How to cite: Turner, A., Gler, M., Hirons, L., Marzin, C., and Wainwright, C.: Systematic biases over the equatorial Indian Ocean and their influence on seasonal forecasts of the IOD, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19800, https://doi.org/10.5194/egusphere-egu25-19800, 2025.
The Indian Ocean Dipole (IOD) and Tripole (IOT) represent primary modes of interannual variability in the Indian Ocean, impacting both regional and global climate. Unlike the IOD, which is closely related to the El Niño-Southern Oscillation (ENSO), our findings unveil a substantial influence of the Australian Monsoon on the IOT. An anomalously strong Monsoon induces local sea surface temperature (SST) variations via the wind-evaporation-SST mechanism, triggering atmospheric circulation anomalies in the eastern Indian Ocean. These circulation changes lead to changes in oceanic heat transport, facilitating the formation of the IOT. Our analysis reveals a strengthening connection between the Australian Monsoon and the IOT in recent decades, with a projected further strengthening under global warming. This contrasts with the diminished coupling between ENSO and IOD in recent decades from observations and model projections, illustrating evolving Indian Ocean dynamics under the warming climate.
How to cite: Chen, M.: Emerging influence of the Australian Monsoon on Indian Ocean interannual variability in a warming climate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10702, https://doi.org/10.5194/egusphere-egu25-10702, 2025.
This study investigated the influence of interannual variations in tropical Indian Ocean tripole (IOT) on the surface air temperature (SAT) over the western Tibetan Plateau (TP) during boreal summer. During the positive phase of the IOT, two northward cross-equatorial airflows are induced over the tropical eastern and western Indian Ocean. These airflows reinforce the ascending motion over southern tropical Asia (80°–125°E, 15°–25°N), increasing local precipitation, as confirmed by observations and simulations by the Community Atmosphere Model. The upper-level Asian Continental Meridional Teleconnection (ACMT) pattern is excited by the latent heat released from precipitation and transmits signals from southern tropical Asia to the western TP, leading to the positive geopotential height anomalies and anomalous anticyclones over there. Upper-level circulation anomalies over the western TP enhance atmospheric thickness through adiabatic processes, consequently elevating local SAT. The ACMT associated with precipitation anomalies thus serves as an atmospheric bridge connecting the IOT and the SAT variations over the western TP.
How to cite: Zhang, Y., Zhu, M., and Li, J.: Physical connection between the tropical Indian Ocean tripole and western Tibetan Plateau surface air temperature during boreal summer , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6573, https://doi.org/10.5194/egusphere-egu25-6573, 2025.
Sea Surface Temperature (SST) extreme, known as the Marine Heat Extremes (MHEs), has become more frequent and intense over the years in the Northern Indian Ocean (NIO), leading to increased uncertainty in monsoon and cyclones. In this study, we characterised the evolution of MHEs utilising the monthly Hadley Centre Sea Ice and SST (HadISST) for 1900–2020 over the NIO. For a comparative analysis of evolution of MHEs, the region was further divided into Eastern Equatorial IO (EEIO), Western Equatorial IO (WEIO), Arabian Sea (AS), and Bay of Bengal (BoB). A MHE event is defined when the SST crosses the monthly varying 98th percentile threshold corresponding to the fixed climatological baseline of 1901–1950. Two normalized indices, i.e., Normalized Extreme Frequency Index and Heat Index, have been utilized to understand the spatio-temporal characteristics of intensity and frequency, respectively. Both the indices show a non-linear exponential increment. Moreover, the area fraction experiencing MHEs was also found to increase swiftly, following a sigmoidal curve. Frequent mean regime-shifts in these quantities have been observed, increasing the unpredictability of the climate system. Moreover, statistical tests revealed that the MHE attributes are increasing because of the increasing mean SST rather than its variance. A mixed layer heat budget analysis shows that the MHE attributes have been increasing more rapidly over the WEIO, followed by EEIO, AS, and lastly the BoB, majorly due to the net heat flux followed by the horizontal advection. These findings underscore the non-linear escalation of thermal stress on marine ecosystems and the broader climate, emphasizing the need to develop mitigation strategies.
How to cite: Gupta, H., Deogharia, R., and Sil, S.: Regime Shifts in Marine Heat Extremes in the Northern Indian Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15053, https://doi.org/10.5194/egusphere-egu25-15053, 2025.
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 Equatorial Intermediate Current (EIC) impacts the distribution and the transport of biogeochemical tracers such as oxygen. The EIC in Indian Ocean, covering the range from 200 to 1000 m between 2°S and 2°N , has higher velocity and a lower-frequency variability in the central basin than in the east. The EIC variability is forced by the wind stress forming equatorial beams and is also strengthened by basin resonance. We use zonal current velocity timeseries of 2015-2023 obtained from different equatorial moorings and a continuous timeseries of 2000-2022 years derived from a global NEMO ocean model configuration at 0.25° horizontal resolution with 46 z-levels (ORCA025.L46) and apply the method of vertical mode decomposition aiming to characterize equatorial zonal velocity variability of the Indian Ocean by equatorial beams, baroclinic modes, and equatorial basin resonance.
From west to east, the Indian Ocean is divided by the topography into three subbasins. The west basin is from the western boundary to the Maldives Islands at 73°E; the central basin is from 73°E to the 90°E ridge; the east basin is from 90°E to the eastern boundary. The frequency – baroclinic mode decomposition of the velocity field shows that semiannual and annual signals are the most significant components. For semiannual signals, the second to fourth baroclinic modes contribute at the mooring locations at 80°E and 85°E, while the fifth to eighth modes dominate at 93°E, indicating the essential role of higher baroclinic modes in the eastern basin. For annual signals, lower baroclinic modes are more significant in the east than in the central subbasin. The model output agrees with the observed distribution of contributing baroclinic modes. Observations further reveal several strong EIC events occurring in 2015-2016 and 2020-2021. Atmospheric data showed corresponding strong anomalies in zonal wind stress and outgoing long-wave radiation. Sea surface temperature anomalies happened along with them. With the distribution of the contributing baroclinic mode, the equatorial beams could explain the strong current events at intermediate depths. The energy input from atmospheric forcing propagates along beams, which are predominantly formed by the second baroclinic mode in the central basin and by the superposition of several higher baroclinic modes in the eastern basin. Future research would focus on the role of equatorial beams in the deeper current variability with the knowledge of contributed baroclinic modes in the Indian Ocean.
How to cite: Zhong, Q., Brandt, P., Czeschel, R., and Schwarzkopf, F. U.: Intermediate circulation variability in the equatorial Indian Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3617, https://doi.org/10.5194/egusphere-egu25-3617, 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.
Posters virtual: Wed, 30 Apr, 14:00–15:45 | vPoster spot 4
EGU25-12429 | ECS | Posters virtual | VPS18
Unraveling the Arabian Sea’s Thermal Pulse: Seasonal and Interannual SST Variability Amidst Climate DynamicsWed, 30 Apr, 14:00–15:45 (CEST) | vP4.2
This study investigates the spatio-temporal variability and long-term warming trends in sea surface temperature (SST) across the Arabian Sea from 2000 to 2019, using daily AVHRR satellite observations with a 1°x1° spatial resolution. Seasonal and interannual SST dynamics reveal patterns shaped by monsoonal processes and global climate phenomena, such as El Niño and La Niña. Wavelet spectrum analysis highlights periodic fluctuations and dominant frequencies associated with interannual climate variability, further emphasizing the influence of seasonal processes. Spring (MAM) exhibits the most pronounced interannual warming, particularly in the central and northern regions, while autumn (SON) demonstrates significant warming trends, especially in the southern basin. Monsoonal processes influence seasonal variability, with winter (DJF) cooling in the northern Arabian Sea and summer (JJA) upwelling along Oman and Somalia, resulting in localized cooling amidst broader warming trends in central and southern regions. Wavelet power spectra from critical regions, including the Gulf of Oman, Balochistan Coast, and Mumbai, indicate dominant periodicities of interannual warming, with variations corresponding to regional oceanographic processes. For instance, the Balochistan Coast displays the highest warming rate (0.0519°C/year), underscored by strong wavelet power at periodicities tied to El Niño–Southern Oscillation (ENSO) cycles. Similarly, the Gulf of Oman and Mumbai exhibit distinct spectral peaks, reflecting localized climate dynamics and variability. Regionally, the warming trend varies significantly. The Gulf of Aden (0.0181°C/year), Gulf of Oman (0.0164°C/year), and Gulf of Kutch (0.0269°C/year) exhibit moderate warming rates, while areas like the Balochistan Coast and South of Salalah (0.023°C/year) highlight significant localized warming. Southwestern Arabian Sea regions west of Kochi (0.0209°C/year) and Mangalore (0.0323°C/year) also demonstrate notable trends. In contrast, regions like Minicoy (0.0162°C/year) and the Male-Maldives area (0.0073°C/year) show relatively weaker warming. These findings underscore the critical role of spatial and seasonal variability in shaping SST changes and their implications for regional climate patterns, monsoonal behavior, marine ecosystems, and fisheries. The pronounced warming in key regions, coupled with insights from wavelet spectrum analysis, highlights the influence of localized oceanographic processes, such as upwelling, heat transport, and climate-induced variability. These results necessitate further study to assess future impacts and develop mitigation strategies for sensitive marine biodiversity and economic resources in the Arabian Sea .
How to cite: Saha, S. and Mukherjee, A.: Unraveling the Arabian Sea’s Thermal Pulse: Seasonal and Interannual SST Variability Amidst Climate Dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12429, https://doi.org/10.5194/egusphere-egu25-12429, 2025.
