OS1.7 | Understanding the Indian Ocean’s past, present and future
EDI
Understanding the Indian Ocean’s past, present and future
Co-organized by BG4/CL1.2
Convener: Caroline Ummenhofer | Co-conveners: Alejandra Sanchez-Franks, Peter SheehanECSECS, Yan Du, Muhammad Adnan AbidECSECS
Orals
| Fri, 28 Apr, 08:30–10:15 (CEST)
 
Room L3
Posters on site
| Attendance Fri, 28 Apr, 14:00–15:45 (CEST)
 
Hall X5
Posters virtual
| Attendance Fri, 28 Apr, 14:00–15:45 (CEST)
 
vHall CR/OS
Orals |
Fri, 08:30
Fri, 14:00
Fri, 14:00
The Indian Ocean is unique among the other tropical ocean basins due to the seasonal reversal of monsoon winds and concurrent ocean currents, lack of steady easterlies that result in a relatively deep thermocline along the equator, low-latitude connection to the neighboring Pacific and a lack of northward heat export due to the Asian continent. These characteristics shape the Indian Ocean’s air-sea interactions, variability, as well as its impacts and predictability in tropical and extratropical regions on (intra)seasonal, interannual, decadal timescales and beyond. They also make the basin particularly vulnerable to anthropogenic climate change, as well as related extreme weather and climate events, and their impacts for surrounding regions, home to a third of the global population. Advances have recently been made in our understanding of the Indian Ocean’s circulation, interactions with adjacent ocean basins, and its role in regional and global climate. Nonetheless, significant gaps remain in understanding, observing, modeling, and predicting Indian Ocean variability and change across a range of timescales.

This session invites contributions based on observations, modelling, theory, and palaeo proxy reconstructions in the Indian Ocean that focus on recent observed and projected changes in Indian Ocean physical and biogeochemical properties and their impacts on ecological processes, diversity in Indian Ocean modes of variability (e.g., Indian Ocean Dipole, Indian Ocean Basin Mode, Madden-Julian Oscillation) and their impact on predictions, interactions and exchanges between the Indian Ocean and other ocean basins, as well as links between Indian Ocean variability and monsoon systems across a range of timescales. We encourage submissions on weather and climate extremes of societal relevance in the Indian Ocean and surrounding regions, including evaluating climate risks, vulnerability, and resilience.

We also welcome contributions that address research on the Indian Ocean grand challenges highlighted in the IndOOS Decadal Review, and as formulated by CLIVAR, the Sustained Indian Ocean Biogeochemistry and Ecosystem Research (SIBER), the International Indian Ocean Expedition 2 (IIOE-2), findings informed by the Coupled Model Intercomparison Project v6 on past, present and future variability and change in the Indian Ocean climate system, and contributions making use of novel methodologies such as machine learning.

Orals: Fri, 28 Apr | Room L3

Chairpersons: Peter Sheehan, Muhammad Adnan Abid, Yan Du
08:30–08:35
08:35–08:45
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EGU23-264
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OS1.7
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ECS
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On-site presentation
Estel Font, Bastien Y. Queste, Sebastiaan Swart, and Gerd Bruss

The decline in ocean oxygen content is one of the most alarming consequences of anthropogenic-driven climate change. A key challenge is that global climate models do not currently reproduce observed changes in deoxygenation, showing high inter-model variability and uncertainty. This uncertainty is partially due to the models’ inability to resolve features smaller than their computational grid cells resulting in large biases in ventilation. The Persian Gulf Water outflow has been pointed out by several studies as one of the sources of ventilation in the Arabian Sea Oxygen Minimum Zone (OMZ). This oxygenated water mass flows eastward along the shelf edge of the northern Omani coast at 200m depth and is fragmented by the mesoscale eddy field and rough topography, generating small “peddies”. These peddies and their relatively high oxygen concentrations have potential to ventilate the OMZ, yet this has been poorly investigated due to a lack of adequate observations. We use multi-month glider campaigns off the coast of Oman with a SeaExplorer glider equipped with an ADCP to resolve the contribution of the Persian Gulf Water outflow to oxygen supply within the Arabian Sea OMZ. We characterize its properties, seasonality, and spatial distribution and estimate mixing rates from double diffusion, salt-fingering, and shear-driven mixing to understand water mass transformations and oxygen fluxes into the OMZ.

How to cite: Font, E., Y. Queste, B., Swart, S., and Bruss, G.: Seasonality and distribution of Persian Gulf Water and its impact on ventilation: a high resolution view from gliders, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-264, https://doi.org/10.5194/egusphere-egu23-264, 2023.

08:45–08:55
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EGU23-1671
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OS1.7
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Highlight
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On-site presentation
Lei Han

The Indian Ocean Meridional Overturning Circulation (IMOC) is well known for its remarkable seasonal variation, which was attributed to Ekman flow plus its barotropic compensation (Lee and Marotzke, 1998). However, by tracking the isopycnal displacement, I defined a  sloshing MOC streamfunction, which was found highly resembling the Eulerian MOC streamfunction (see the attached figure). It was thus concluded that the IMOC is predominantly a sloshing mode, associated with the isopycnal displacement. Recognizing that these isopycnal signals were dominated by the first-baroclinic long Rossby waves, I found the IMOC strength was determined by the zonally-integrated Ekman pumping anomaly. As a result, the deep inflow into the Indian Ocean also had seasonal variation that could be attributed to this sloshing mode of overturning circulation. This could be partly verified by the cross-basin transect survey across 32oS that were fully occupied three times in history. The diffusivity dichotomy problem can be also explained by this new perspective. The importance of the Indian Ocean overturning in the global conveyor-belt was therefore challenged. This result has been published in Han (2021, JPO).

How to cite: Han, L.: New perspective on the overturning dynamics in the Indian Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1671, https://doi.org/10.5194/egusphere-egu23-1671, 2023.

08:55–09:05
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EGU23-6879
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OS1.7
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ECS
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On-site presentation
Magdalena Fritz, Leopold Haimberger, and Michael Mayer

The Indonesian Seas are characterized by numerous narrow channels connecting basins and seas of varying sizes and depths that serve as a transition between the Pacific and the Indian Ocean, known as the Indonesian Throughflow (ITF). The interaction between the ITF and important climate anomalies such as the El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), or the Australian-Indonesian monsoon indicates the high relevance for monitoring the ITF region. In situ observations of ITF transports are highly valuable but are temporally and spatially limited. Hence, near real-time monitoring is only possible with reanalyses, yet their quality needs to be evaluated. Here we present an assessment of oceanic transports in the ITF diagnosed from the Copernicus Marine Service (CMEMS) Global Reanalysis Ensemble Product (GREP) and the higher-resolution product GLORYS12V1. Validation data comes from several moorings in Makassar strait, Lombok strait, Ombai strait, and Timor passage, obtained as part of the well-known INSTANT (2004-2006) and MITF (2006-2011 and 2013-2017 in Makassar) campaigns. The campaigns provide a total of 11.5 years of in situ observations in Makassar, therefore allowing the assessment of the mean seasonal cycle of ITF transport and a thorough investigation of the shorter sampled outflow passages. The results showcase that reanalysis-based volume transports agree reasonably well with in situ observations, however, some aspects, such as asymmetries in the flow through each strait, are more accurately represented by GLORYS12V1. Also, in terms of mean integrated transports, the increased horizontal resolution of GLORYS12V1 leads to a better performance in the narrower straits of Lombok and Ombai. Furthermore, we draw attention to an apparent one-month lag between reanalyses and observations in Makassar strait transports, which we assess by studying the influence of the monsoon-driven (vertically varying) pressure gradient on the ITF.

How to cite: Fritz, M., Haimberger, L., and Mayer, M.: Suitability of ocean reanalyses for monitoring of oceanic exchanges through the Indonesian Throughflow, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6879, https://doi.org/10.5194/egusphere-egu23-6879, 2023.

09:05–09:15
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EGU23-15432
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OS1.7
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ECS
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Highlight
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On-site presentation
Lukas Fiedler, Vimal Koul, Eduardo Alastrué de Asenjo, Sebastian Brune, and Johanna Baehr

Ocean heat content observations in the Indian Ocean have revealed distinctive periods of significant multi-decadal trends — for example a cooling between 1990 and 1999 followed by an unprecedented warming between 2000 and 2009. However, a systematic assessment of the relative importance of anthropogenic forcings versus natural variability in driving such trends is still missing. Here, we utilise four state-of-the-art Single Model Initial- Condition Large Ensembles with MPI-ESM1.2-LR containing different factual and counterfactual forcing scenarios to address the problem. We are able to robustly attribute the unprecedented warming of the Indian Ocean between 2000 and 2009 to the increasing anthropogenic greenhouse gas emissions. Our results also reveal that the preceding cooling is likely to be intrinsic to Indian Ocean heat content variability, since none of the applied counterfactual scenarios exhibits such an observed decrease in Indian Ocean heat content. Furthermore, we trace the underlying reasons for the observed inherent cooling between 1990 and 1999 to a significant reduction in heat transported into the Indian Ocean from the Pacific Ocean by the Indonesian Throughflow. These results have implications for decadal predictions of Indian Ocean heat content.

How to cite: Fiedler, L., Koul, V., Alastrué de Asenjo, E., Brune, S., and Baehr, J.: Multi-decadal changes in the Indian Ocean heat content from a grand ensemble perspective, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15432, https://doi.org/10.5194/egusphere-egu23-15432, 2023.

09:15–09:25
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EGU23-15102
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OS1.7
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ECS
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On-site presentation
Marimel Gler, Andy Turner, Linda Hirons, Caroline Wainwright, and Charline Marzin

The Indian Ocean Dipole (IOD) is the main coupled mode of interannual variability in the equatorial Indian Ocean. The largest IOD event in 2019 is thought to have influenced the strong Indian monsoon precipitation, widespread Australian bushfires, and extreme rainfall and flooding in East Africa during that year. Despite its socio-economic importance, the region suffers large biases in weather and climate models used for seasonal forecasts and climate projections.

In this study, the performance of 42 models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) in reproducing the observed climate over the Indian Ocean is examined. Model simulations of precipitation and 850 hPa winds in the Atmospheric Model Intercomparison Project (AMIP) experiments for the period 1979-2014 are compared to observational and reanalysis data. Biases in the mean state during boreal summer (JJA) in the AMIP models are analysed to determine whether biases in the seasonal cycle established in JJA impact the IOD behaviour. Skill metrics are calculated to quantify the model performance in reproducing the observed JJA mean state and cluster analysis on the mean state biases is performed to characterise bias patterns in summer that may affect the Indian Ocean seasonal cycle and IOD. Results show that AMIP models simulate varying bias patterns in JJA and that the AMIP multi-model mean outperforms all individual models in reproducing the observed JJA mean state. For comparison, the Indian Ocean mean state biases are investigated in coupled models from the 20th-century all-forcings (CMIP) experiments to determine the impact of ocean-atmosphere coupling and coupled sea surface temperature biases on model performance. The IOD behaviour in the AMIP and CMIP models is assessed and the response of the atmospheric circulation to IOD forcing is examined by performing regression analysis. We investigate whether the ability of a model to capture characteristics of the IOD and simulate IOD teleconnection patterns is related to its representation of the mean state. We expand this work to investigate the variability in the Indian Ocean in the Met Office Global Seasonal Forecasting System version 6, GloSea6, with a focus on examining the systematic errors that develop in the region. The work will contribute to our understanding of Indian Ocean biases in weather and climate models, and their likely sources, and thus the wider implications for predictability of the IOD.  

How to cite: Gler, M., Turner, A., Hirons, L., Wainwright, C., and Marzin, C.: Indian Ocean mean state biases and IOD behaviour in CMIP6 multimodel ensemble, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15102, https://doi.org/10.5194/egusphere-egu23-15102, 2023.

09:25–09:35
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EGU23-3967
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OS1.7
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ECS
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Highlight
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On-site presentation
Edward Robinson, Philip Hosegood, Vasiliy Vlasenko, Nataliya Stashchuk, Clara Diaz, Nicola Foster, Joanna Harris, Clare Embling, and Kerry Howell

Tropical atoll habitats are often key conservation targets due to being inhabited by several vulnerable species such as reef manta rays and tropical coral species. These atolls are subject to both basin scale forcing through the Indian Ocean Dipole (IOD), monsoonal variation, and local processes. The steep slopes surrounding these atolls support highly dynamic, energetic nearshore ecosystems which vary over sub-kilometre spatial scales that are poorly resolved in general circulation models. Improving our understanding of how physical oceanographic processes control these local ecosystems, through both in-situ observations, and fine scale models, is critical for enabling informed policy decisions and efficient use of conservation resources. Here we summarise the impact of the local fine scale processes, which are heavily modulated by the monsoon and Indian Ocean Dipole (IOD), on a tropical atoll ecosystem in the central Indian Ocean (IO).

The IOD is experiencing increasingly extreme fluctuations with direct impacts on the depth of the thermocline throughout the western IO. In our observations from 2019, the IOD deepened the thermocline to an unprecedented depth of 100 m, subjecting mesophotic corals to temperatures typical of surface waters and causing significant bleaching. High resolution numerical modelling shows that internal waves, rather than alleviating bleaching, further exacerbate the heating effects preferentially advecting high temperature surface water to increased depths. The wave influence is, however, highly localised, necessitating designated studies at individual sites to understand the spatial heterogeneity in internal wave impacts.

At smaller sub-atoll scales, the IOD also influences the feeding behaviour of reef manta rays, which are more frequently detected in the presence of tidally forced surface-to-bottom temperature gradients. The site of most manta ray detections in the study area is a lipped gully, situated at 60-70 m depth, and colloquially named 'Manta Alley'. During deeper thermoclines, the cooling events within Manta Alley, with which increased reef manta presence is associated, are precluded from occurring due to the deep thermocline, impacting feeding behaviour.

Our results highlight the inherent dynamical complexity in these environments, with the impacts of basin scale processes cascading down to local scales. Improving our understanding of how these dynamics cross-interact with each other, as well as the local ecosystem, enhances the value of biological observations, presenting the opportunity for better informed and more effective conservation strategy.

How to cite: Robinson, E., Hosegood, P., Vlasenko, V., Stashchuk, N., Diaz, C., Foster, N., Harris, J., Embling, C., and Howell, K.: Ecosystem impacts due to thermocline depression by the 2019 extreme Indian Ocean Dipole event, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3967, https://doi.org/10.5194/egusphere-egu23-3967, 2023.

09:35–09:45
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EGU23-12048
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OS1.7
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ECS
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On-site presentation
Jenny Jardine, Sarah Wakelin, Jason Holt, Anna Katavouta, and Dale Partridge

The Bay of Bengal is a dynamic region that experiences intense freshwater runoff, extreme meteorological events, and seasonal reversing surface currents. The region is particularly susceptible to anthropogenic climate change, driven in part by large air-sea fluxes, persistent freshwater stratification, and low overturning rates. Predicting how this ecosystem is likely to change in the future is paramount for planning effective mitigation strategies. Using a relocatable, coupled physics-ecosystem model (NEMO-ERSEM), we investigate the future changes in surface circulation and coastal nitrate pathways in the Bay of Bengal from 1980 to 2060, using a “business-as-usual" (RCP 8.5) climate change scenario. We find that future surface currents during the Summer and Fall Inter-monsoon seasons are reduced in the north/north-eastern Bay and strengthened in the south-western Bay. Coastal nitrate transports around the Bay mirror this asymmetric change, with coastal nitrate transports at 17.5oN decreasing by 185.7 mol N s-1, despite increased riverine runoff from the Ganges and Irrawaddy River systems. This results in a positive feedback loop whereby the northern Bay becomes progressively fresher and more nutrient-rich, strengthening the barrier layer and increasing the risk of toxic algal blooms and eutrophication events. Conversely, in the south-western Bay (12oN), coastal nitrate transports increase by 1317.8 mol N s-1, driven primarily by an intensified Sri Lanka Dome, that promotes localised diatom blooms despite negligible changes in regional river runoff. This work highlights the need for more rigorous ecosystem modelling and future scenario testing. 

How to cite: Jardine, J., Wakelin, S., Holt, J., Katavouta, A., and Partridge, D.: An asymmetric change in circulation and nitrate transports around the Bay of Bengal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12048, https://doi.org/10.5194/egusphere-egu23-12048, 2023.

09:45–09:55
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EGU23-131
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OS1.7
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ECS
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On-site presentation
Medhavi Pandey and Haimanti Biswas

The Arabian Sea, a productive oceanic region in the North Indian Ocean, is under the direct influence of monsoon winds that impact the surface ocean processes. High biological productivity occurs due to natural nutrient enrichment events via coastal and open ocean upwelling (summer monsoon) and convective mixing (winter monsoon). Ample studies from this basin addressed the diatom community from the surface ocean, yet the key contributing diatom frustules to sedimentary phytodetritus has been overlooked. These microscopic biosilcifiers play an important role in the biological carbon pump by exporting significant organic carbon from the surface waters to the deep sea due to their ballasted silica shell (frustule). Hence, this is imperative to document the diatom genera that are transported efficiently to the sediment. The present study analyzed diatom frustule abundance (valves g-1) and identified the major diatom genera in core top sediments (0.5cm) of 10 locations from the Central (21, 19, 15, 13, and 11 °N along 64 °E) and Eastern Arabian Sea (21, 17, 15, 13, and 11 °N at 200 m isobath).  This is the first of this kind and found a comparable frustule distribution from the surface sediments of both Central (av. 5.16±1.23×104 valves g-1) and Eastern Arabian Sea (av. 5.80±7.14×104 valves g-1). Size-based classification revealed that the contributions of medium-sized (30-60 µm) frustules from both the central (49 %) and eastern (51%) Arabian Sea were quite high. And the contribution of large-sized frustules (>60 µm) was higher in the central Arabian Sea (39%) compared to the eastern part (19%). A total of 40 diatom genera with 18 in common from both locations were detected from the sedimentary phytodetritus with Coscinodiscus and Thalassiosira being the dominating ones. In the north-central (21, 19, 15 °N) Arabian Sea, the prevalence of large-sized diatoms (Coscinodiscus) was attributed to open ocean upwelling as well as convective mixing during summer and winter monsoons, respectively. Such large species can easily escape grazing and sink rapidly due to higher ballasting. Further, the presence of the oxygen minimum zone at the intermediate depth in this region might reduce the remineralization and grazing pressure within the mesopelagic during their transport to the abyss. Whereas relatively smaller diatoms (Thalassiosira, Pseudo-nitzschia, Fragilaria, Nitzschia) were in high abundance towards the south-central (13, 11 °N) that area remains nutrient-poor. In the Eastern Arabian Sea, Thalassiosira was noticed in high abundance towards the southeast (15, 13, 11 °N), whereas the northeast (17, 21 °N) was dominated by Coscinodiscus and mostly due to the prevalence of coastal upwelling and convective mixing, respectively. Likely, these diatoms (Coscinodiscus, Thalassiosira, Pseudo-nitzschia, Fragilaria, Nitzschia) play a key role in transferring the organic matter from the surface to sediments and thus actively contribute to carbon capture, elemental cycling, and supplying food source for the benthic biota. This study highlights for the first time the biogeochemical significance of these diatoms from this highly productive oceanic province.

How to cite: Pandey, M. and Biswas, H.: An account of the key diatom frustules from the surface sediments of the Central and Eastern Arabian Sea and their biogeochemical significance., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-131, https://doi.org/10.5194/egusphere-egu23-131, 2023.

09:55–10:05
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EGU23-358
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OS1.7
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ECS
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On-site presentation
Saumya Silori, Haimanti Biswas, and Damien Cardinal

The biogeochemistry of the Arabian Sea, the northwestern part of the Indian Ocean, is directly impacted by monsoonal wind reversal and is an area of strong ocean-atmospheric interaction. During the summer monsoon, coastal as well as open ocean upwelling occurs in the western, southeastern, and central parts of the Arabian Sea. The highest primary productivity rates are documented in these areas compared to the global oceans. Phytoplankton-derived particulate organic matter (POM) [Particulate organic carbon (POC) and nitrogen (PN)] play a central role in supporting the food chain as well as carbon export flux to the deep sea. Hence understanding the dynamics of POM concentrations and its stable carbon (δ13CPOC) and nitrogen (δ15NPN) isotopic ratios are important in delineating its sources and recycling. However, such studies are scarce from the Indian Ocean region. Here we present the first study describing the POM dynamics during the summer monsoon from the central Arabian Sea, addressing the interannual variability. We studied the monsoonal changes in POM and its isotopic signatures in the central Arabian Sea (21–11°N; 64°E) during August 2017 and 2018. A strong, low-lying atmospheric jet (Findlater Jet) blows across the basin during the southwest (SW) monsoon. Positive wind stress curl resulted in “open ocean upwelling” to the north of the jet’s axis, characterized by substantially shallower Mixed Layers Depths (MLDs) and higher POM contents relative to the jet’s axis and its south. The highest wind speeds were observed in the center of the transect due to the presence of the jet’s axis. And the negative curl to the south of the jet’s axis resulted in downwelling and, consequently, the deepest MLDs. The molar ratio between POC and PN (6.2 ± 1.9 in 2017; 6.4 ± 0.9 in 2018) was close to the canonical Redfield ratio (6.63). The δ13CPOC values (−26.3 ± 1.4‰ in 2017; 25.5 ± 1.4‰ in 2018) exhibited typical marine signature and a noticeable inter-annual difference. Relatively higher δ15NPN values in the north (7.68 ± 2.6‰ in 2017; 9.24 ± 3‰ in 2018) indicated the uptake of regenerated dissolved inorganic nitrogen from the oxygen minimum zone (OMZ). The lower δ15NPN values along the jet’s axis and to its south were attributed to the eastward advection of upwelled waters from the western Arabian Sea. Higher wind speeds and jet-induced wind stress curl in 2018 resulted in lower sea surface temperatures (SST) and higher nutrient concentrations. Despite the higher nutrient availability in 2018, POC contents did not exceed the values in 2017. However, considering the total nitrogen consumption (according to C:N: P = 106:16:1), the potential POC development in 2018 could be double the value in 2017. The interannual differences in SW monsoon onset and wind speeds seemed to directly control the nutrient supply, affecting plankton community structure and POM variability. Thus, any future changes in the physical forcing may directly influence the POC pool and consequent export flux to the mesopelagic.

How to cite: Silori, S., Biswas, H., and Cardinal, D.: Interannual variability in particulate organic matter associated with physical forcing in the central Arabian Sea assessed from (stable) carbon and nitrogen isotopes., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-358, https://doi.org/10.5194/egusphere-egu23-358, 2023.

10:05–10:15
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EGU23-7577
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OS1.7
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ECS
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On-site presentation
Tanya Marshall, Daniel Sigman, Lisa Beal, Alan Foreman, Alfredo Martínez-García, Stéphane Blain, Ethan Campbell, François Fripiat, Robyn Granger, Eesaa Harris, Gerald Haug, Dario Marconi, Sergey Oleynik, Patrick Rafter, Raymond Roman, Kolisa Sinyanya, Sandi Smart, and Sarah Fawcett

The Agulhas Current in the southwest Indian Ocean is the strongest western boundary current on Earth. The major role of the Agulhas Current in driving significant heat and salt fluxes is well known, yet its biogeochemical fluxes remain largely uncharacterised. Here, we use nitrate isotopes (δ15N, δ18O, and Δ(15-18) = δ15N-δ18O) to evaluate nutrient supply mechanisms that ultimately support new production in the southwest Indian Ocean. Across the greater Agulhas region, thermocline nitrate-δ15N is lower (4.9-5.8‰) than the underlying Subantarctic Mode Water source (δ15N of 6.9‰) and the upstream source regions (where nitrate-δ15N ranges from 6.4-7.0‰), which we attribute to local N2 fixation. Using a one-box model to simulate the newly-fixed nitrate flux, we estimate a local N2 fixation rate of 7-25 Tg N.a-1, amounting to ~30-95% of the whole Indian Ocean nitrogen gain estimated by models. Thermocline and mixed-layer nitrate Δ(15-18) is also low, due to both N2 fixation and coupled partial nitrate assimilation and nitrification. This local nitrogen cycling imprints an isotopic signal on Indian Ocean nitrate that persists in Agulhas rings that “leak” into the South Atlantic and are subsequently transported northwards. If this signal is retained in calcifying organisms (e.g., foraminifera) deposited on the seafloor, it could be used to trace past Agulhas leakage, yielding quantitative insights into the strength of the Atlantic Meridional Overturning Circulation over time. In addition to local N2 fixation, the nitrate isotopes reveal three physical mechanisms of subsurface nitrate supply: i) inshore upwelling driven by the current and winds, ii) entrainment at the edges of a mesoscale eddy, and iii) density-driven overturning at the current edge induced by strong horizontal velocity and density shears. All these nitrate supply mechanisms are evident as incidences of relatively high-Δ(15-18) nitrate in the thermocline and surface yet the intensity and subsurface expression of some of them is not apparent in the physical data, highlighting the utility of the nitrate isotopes for exploring physical ocean processes. The high mesoscale variability that likely drives subsurface nitrate supply to Agulhas Current surface waters is common to all western boundary currents, implying that vertical nitrate entrainment is quantitatively significant in all such systems. We posit that along with N2 fixation, physical mechanisms of upward nitrate supply enhance ocean fertility and possibly carbon export in the South Indian Ocean. Higher rates of warming, and thus thermal stratification, are expected to decrease Indian Ocean productivity more rapidly in the future than that of other ocean basins. However, a coincident increase in eddy kinetic energy across boundary currents may enhance the upward nutrient supply, partially offsetting the stratification-driven decline in productivity.

How to cite: Marshall, T., Sigman, D., Beal, L., Foreman, A., Martínez-García, A., Blain, S., Campbell, E., Fripiat, F., Granger, R., Harris, E., Haug, G., Marconi, D., Oleynik, S., Rafter, P., Roman, R., Sinyanya, K., Smart, S., and Fawcett, S.: Nutrient fluxes in the greater Agulhas Current region: signals of local and remote Indian Ocean nitrogen cycling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7577, https://doi.org/10.5194/egusphere-egu23-7577, 2023.

Posters on site: Fri, 28 Apr, 14:00–15:45 | Hall X5

Chairpersons: Muhammad Adnan Abid, Peter Sheehan, Caroline Ummenhofer
X5.299
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EGU23-2528
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OS1.7
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Highlight
Yuhong Zhang and Yan Du

This study analyzed the downwelling Rossby waves in the south Indian Ocean induced spring asymmetric mode and the relationship with the Indian Ocean Dipole (IOD) event based on observations and reanalysis data sets. The westward downwelling Rossby waves favor significant sea surface temperature (SST) warming in the Seychelles thermocline dome that triggers atmosphere response and the asymmetric mode in spring. The zonal sea level pressure gradient causes anomalous easterly winds in the central and eastern equatorial IO, cooling the SST off Sumatra-Java. Meanwhile, the remainder of the downwelling Rossby waves reach the west coast, transform to northward coastal-trapped waves, and then reflect as eastward downwelling Kelvin waves along the equator. The downwelling Kelvin waves reach the Sumatra-Java coast during late spring to early summer, favoring SST warming in the southeastern tropical Indian Ocean. Thus, there are two types of ocean-atmosphere response almost at the same time along the equator. The final SST status depends on which process is stronger, and as a consequence, triggers a negative or a positive phase of the IOD event in the fall season. The results show four positive and three negative IOD events related to the above processes from 1960 to 2019. The strong downwelling Rossby waves are easier to induce intense asymmetric mode and negative IOD event, usually associated with preceding strong El Niño in the Pacific. In contrast, the weak downwelling Rossby waves tend to induce weak asymmetric mode and positive IOD event, usually associated with preceding weak El Niño or anomalous anti-cyclonic atmospheric circulation in the southeastern IO.

How to cite: Zhang, Y. and Du, Y.: Oceanic Rossby waves induced two types of ocean-atmosphere response and opposite Indian Ocean Dipole phases, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2528, https://doi.org/10.5194/egusphere-egu23-2528, 2023.

X5.300
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EGU23-2532
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OS1.7
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Jiechao Zhu, Yuhong Zhang, Xuhua Cheng, Xiangpeng Wang, Qiwei Sun, and Yan Du

Abstract: The distribution of sea surface salinity (SSS) in the Arabian Sea (AS) and Bay of Bengal (BoB) is in contrast due to differences in air-sea freshwater fluxes and river runoff inputs.The monsoon-induced inter-basin water exchange plays an important role in regional salinity balance and atmosphere-ocean feedback in the North Indian Ocean. The satellite SSS dataset reveals that significant intraseasonal variability of SSS occurs in the region south of the Indian Peninsula with the strongest amplitude in winter. A case study in autumn-winter of 2016 showed that the Northeast Monsoon Current (NMC) and mesoscale eddies play a dominant role in the intraseasonal variability of the SSS in the region south of the Indian peninsula. In November, the East India Coastal Current (EICC) transports the low-salinity water southward to the region east of Sri Lanka. Meanwhile, a cyclonic eddy develops and propagates westward south of the NMC. Both NMC and the cyclonic eddy advects the low-salinity water westward to the region south of the Indian Peninsula. Then, an anticyclonic eddy generates in the north of the NMC. Thus, an eddy pair forms for more than one and a half months, which develops and propagates westward, transporting low-salinity water westward. The perturbation of mesoscale eddies and SSS gradient leads to the significant intraseasonal variability of SSS there.

Key words: Sea Surface Salinity; intraseasonal variability; mesoscale eddies; North Indian Ocean;

How to cite: Zhu, J., Zhang, Y., Cheng, X., Wang, X., Sun, Q., and Du, Y.: Effect of mesoscale eddies on the transport of low-salinity water from the Bay of Bengal into the Arabian Sea during winter, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2532, https://doi.org/10.5194/egusphere-egu23-2532, 2023.

X5.301
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EGU23-6781
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OS1.7
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ECS
Peter Sheehan, Adrian Matthews, Benjamin Webber, Alejandra Sanchez-Franks, Nicholas Klingaman, and Pn Vinayachandran

The southwest monsoon delivers over 70% of India’s annual rainfall and is crucial to the success of agriculture across much of South Asia. Monsoon precipitation is known to be sensitive to sea surface temperature (SST) in the Bay of Bengal (BoB). Here, we use a configuration of the Unified Model of the UK Met Office coupled to an ocean mixed layer model to investigate the role of upper-ocean features in the BoB on southwest monsoon precipitation. We focus on the pronounced zonal and meridional SST gradients characteristic of the BoB; the zonal gradient in particular has an as-yet unknown effect on monsoon rainfall. We find that the zonal SST gradient is responsible for a local decrease in rainfall over the southern BoB of approximately 5 mm day−1, and an increase in rainfall over Bangladesh and northern India of approximately 1 mm day−1. This increase is remotely forced by a strengthening of the monsoon Hadley circulation. The meridional SST gradient acts to decrease precipitation over the BoB itself, similarly to the zonal SST gradient, but does not have comparable effects over land. The impacts of barrier layers and high-salinity sub-surface water are also investigated, but neither has significant effects on monsoon precipitation in this model; the influence of barrier layers on precipitation is felt in the months after the southwest monsoon. Models should accurately represent oceanic processes that directly influence BoB SST, such as the BoB cold pool, in order to faithfully represent monsoon rainfall.

How to cite: Sheehan, P., Matthews, A., Webber, B., Sanchez-Franks, A., Klingaman, N., and Vinayachandran, P.: On the influence of the Bay of Bengal’s sea surface temperature gradients on rainfall of the South Asian monsoon, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6781, https://doi.org/10.5194/egusphere-egu23-6781, 2023.

X5.302
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EGU23-3426
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OS1.7
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ECS
Marina Azaneu, Adrian Matthews, Karen Heywood, and Rob Hall

Ocean stratification can modulate the upper ocean response and its feedback to atmospheric forcing. Surface freshwater input by advection and precipitation, for example, can change the upper ocean stratification and produce barrier layers. The existence of a barrier layers can affect SST in several ways, for example by reducing entrainment of cooler water at the base of the mixed layer, and consequently may impact air--sea interactions. In the southeastern Indian Ocean, eddies are abundant and can act on transporting warm and fresh waters westward, thus possibly contributing to the formation of barrier layers. Here we initially evaluate the importance of eddy activity in contributing to barrier layer formation and intraseasonal variability in the southern Indian Ocean. Using 15 years (2005-2019) of ocean reanalysis daily data, we estimate how much of the spatial and time variability of barrier layer thickness is related to eddy activity, which is determined by calculating eddy kinectic energy. With the establishment of a relationship between eddy activity and barrier layer thickness, we then proceed to estimate the relationship between barrier layer thickness and local SST anomalies. This way, we seek to infer the significance of eddy activity in affecting SST through barrier layer formation, and thus its potential impact in air--sea interactions and coupled weather systems such as the MJO.

How to cite: Azaneu, M., Matthews, A., Heywood, K., and Hall, R.: Eddy activity and its role in barrier layer thickness variability in the southeast Indian Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3426, https://doi.org/10.5194/egusphere-egu23-3426, 2023.

X5.303
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EGU23-5143
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OS1.7
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Highlight
Ke Huang

Using mooring observations and reanalysis, we show that anomalously strong westward Equatorial Undercurrent (wEUC) developed in June–July in 2016 and 1998 in the Indian Ocean, which coincided with extreme Indian Ocean Dipole (IOD) and El Niño events. Simulations show that equatorial Kelvin and Rossby waves were excited by winds associated with El Niño and positive IOD events during 2015 and 1997, and their negative phases during 2016 and 1998. The constructive relationship between the delayed-time contributions of eastern-boundary-reflected-waves that excited by the easterlies in 2015 and 1997 and the direct contributions of wind-forced-waves that excited by the westerlies in 2016 and 1998 resulted in the intensified wEUC. Slow intermediate-order baroclinic-modes, rather than fast low-order baroclinic-modes, dominated the strong wEUC. The eastern-boundary-reflected-waves dominated in 1997–1998 and directly wind-forced-waves dominated in 2015–2016. Our results emphasize the importance of constructive interactions of the directly-wind-forced and boundary-reflected waves in driving the interannual variability of Indian Ocean wEUC.

How to cite: Huang, K.: Successive Co-occurring IOD and ENSO Unprecedentedly Intensify Indian Ocean Westward Equatorial Undercurrent During the Summers of 1998 and 2016, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5143, https://doi.org/10.5194/egusphere-egu23-5143, 2023.

X5.304
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EGU23-6736
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OS1.7
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ECS
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Shanshan Pang, Xidong Wang, and Jérôme Vialard

Previous studies have hypothesized that climatologically thick salinity-stratified Barrier Layers (BL) in the North Indian Ocean (NIO) could influence the upper ocean heat budget, sea surface temperature (SST) and monsoon. Here, we investigate the performance of state-of-the-art climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6) in simulating the barrier layer thickness (BLT) in the NIO. CMIP6 models generally reproduce the main features of the BLT seasonal cycle and spatial distribution, but with a shallow November-February (NDJF) BLT bias in regions with thick observed BLT (eastern equatorial Indian Ocean [EEIO], Bay of Bengal [BoB] and southeastern Arabian Sea [SEAS]). CMIP6 models display an easterly equatorial zonal surface wind bias linked to dry rainfall and cold SST biases in the southern BoB, through the Bjerknes feedback loop. The easterly equatorial bias is also responsible for the shallow isothermal layer depth (ILD) and BLT bias in the EEIO. The underestimated rainfall over the BoB leads to higher sea surface salinity (SSS) and too deep mixed layer depth (MLD), resulting in the BoB BLT bias. The intensity of the easterly equatorial bias also contributes to the inter-model spread in BoB BLT bias, through the propagation of EEIO ILD signals into the coastal waveguide. Finally, the SEAS BLT bias is due to a too deep MLD, which is predominantly controlled by the high SSS related to attenuated monsoonal currents around India and a reduced inflow of BoB low-salinity water. The BL effect on the mixed layer entrainment cooling does not seem to operate in CMIP6 simulations. Rather, deep salinity-related MLD biases in the BoB result in a diminished cooling rate in response to winter negative surface heat fluxes, and hence alleviate cold BoB SST biases. This suggests that salinity effects alleviate the biases that develop through the positive Bejrknes feedback loop between BoB SST, BoB rainfall and equatorial wind stresses in CMIP6.

How to cite: Pang, S., Wang, X., and Vialard, J.: How well do CMIP6 models simulate salinity barrier layers in the North Indian Ocean?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6736, https://doi.org/10.5194/egusphere-egu23-6736, 2023.

X5.305
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EGU23-7728
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OS1.7
Fine scale frontal structure and biological response in an ring-eddy dipole in the Mozambique Channel
(withdrawn)
Pierrick Penven, Jean-Francois Ternon, Margaux Noyon, and Steven Herbette
X5.306
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EGU23-2165
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OS1.7
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ECS
Hyo-Jin Park, Soon-Il An, Soong-Ki Kim, Wenju Cai, Agus Santoso, Daehyun Kim, and Jong-Seong Kug

Indian Ocean Dipole phenomenon (IOD) refers to a dominant zonal contrast pattern of sea surface temperature anomaly (SSTA) over tropical Indian Ocean (TIO) on interannual time scales. Its positive phase, characterized by anomalously warm western TIO and anomalously cold southeastern TIO, is usually stronger than its negative phase, namely a positively skewed IOD. Here, we investigate causes for the IOD asymmetry using a prototype IOD model, of which physical processes include both linear and nonlinear feedback processes, El Nino’s asymmetric impact, and a state-dependent noise. Parameters for the model were empirically obtained using various reanalysis SST data sets. The results reveal that the leading cause of IOD asymmetry without accounting seasonality is a local nonlinear process, and secondly the state-dependent noise, the direct effect by the positively skewed ENSO and its nonlinear teleconnection; the latter two have almost equal contribution. However, the contributions by each process are season dependent. For boreal summer, both local nonlinear feedback process and the state-dependent noise are major drivers of IOD asymmetry with negligible contribution from ENSO. The ENSO impacts become important in boreal fall, along with the other two processes.

 

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2018R1A5A1024958)

How to cite: Park, H.-J., An, S.-I., Kim, S.-K., Cai, W., Santoso, A., Kim, D., and Kug, J.-S.: Main drivers of Indian Ocean dipole asymmetry revealed by a simple IOD model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2165, https://doi.org/10.5194/egusphere-egu23-2165, 2023.

X5.307
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EGU23-12052
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OS1.7
Eun-Ran Baek, Minju Kim, Dong-Jin Kang, and Jung-Hoon Kang

This study investigated the occurrence and distribution of microplastics utilizing zooplankton samples collected in the Western Indian Ocean because there is no information concerning epipelagic zone in the open ocean. We collected microplastics from three water layers [surface mixed layer(SML), middle layer(ML), lower layer(LL)] within 200 m using a Multiple Opening/Closing Net and Environmental Sensing (opening: 1 ㎡) at 22 stations of 1 degree interval between 5°N and 16°S along the 67°E of Western Indian Ocean in 2017. The microplastics were consistently found in almost all samples and the microplastic abundance ranged between 0.00-2.01 particles/㎥ from the 3 layers. And the average microplastic abundance was highest in the lower layer (0.30±0.09 particles/㎥) and lowest in middle layer (0.26±0.08 particles/㎥). The percentage of fiber was highest in the SML (55.7%) and the LL (45.9%), and the percentage of film was highest in ML (46.8%). The microplastic abundance in the size of 1.0-5.0 ㎜ was highest in SML (42.0%), while the abundance in the size of 0.2-0.5 ㎜ was highest in ML(56.8%) and LL(54.5%). The stations can be divided into four sections including upwelling characterized by Seychelles-Chagos Thermal Ridge (SCTR) based on the 20℃-isotherm depth (D20). The average microplastic abundance was the highest in SML (0.23±0.06 particles/㎥) in 1°S~5°S, and in LL (0.50±0.25 particles/㎥) at latitudes of 10°S~16°S and in LL (0.32±0.16 particles/㎥) at latitudes between 5°N~EQ. However, the average microplastic abundance at latitudes of 6°S ~9°S corresponding to the upwelling zone was highest in the ML (0.65±0.38 particles/㎥) with the high percentage of film (68.7%). Cluster analysis by microplastics occurred in each water layers showed that the stations were divided into 3 groups in each layer. Groups in SML and LL were mainly clustered by fiber, whereas groups in ML was mainly clustered by film, which was associated with the upwelled region of Seychelles-Chagos Thermal Ridge (SCTR). Fourier transform infrared spectroscopy analysis showed that the main types of microplastics were dominated by fiber (40.6%) and film (73.2%) characterized by polycarbonate. Present results showed that meridional and vertical distribution of microplastics in the epipelagic zone varied with the physical characteristics of upwelling zone characterized by Seychelles-Chagos Thermal Ridge (SCTR) in the Western Indian Ocean.

How to cite: Baek, E.-R., Kim, M., Kang, D.-J., and Kang, J.-H.: The occurrence and distribution of microplastics in epipelagic zone of the western Indian Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12052, https://doi.org/10.5194/egusphere-egu23-12052, 2023.

X5.308
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EGU23-8672
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OS1.7
Anna Katavouta, Jeff Polton, Jennifer Jardine, Dale Partridge, Svetlana Jevrejeva, and Jason Holt

The Indonesian Seas act as a main pathway of water transport from the Pacific to the Indian Ocean, known as the Indonesian Throughflow (ITF). Climate-induced changes in the regional water properties within the Indonesian Seas could have extensive impacts on the large-scale ocean budgets, as the ITF will carry these signals from the Indonesian Seas across the Indian Ocean’s upper thermocline. Here, we investigate the impacts of climate change on the Indonesian Seas’ dissolved inorganic carbon (DIC) budget using a regional ocean physics/biogeochemistry model for South East Asia that downscales climate projections from an Earth System Model under the RCP 8.5 scenario. The regional model has a horizontal resolution of about 9 km, uses a hybrid depth-terrain following vertical coordinate system and explicitly includes tides so as to better resolve the shelf-seas processes. A transport-based framework is used to explore the role of climate-induced changes of the ITF on the carbon storage within the Indonesian Seas. Specifically, the DIC trends are separated into: (i) an “added contribution” associated with the uptake of additional carbon from the atmosphere due to carbon emissions, and (ii) a “dynamic redistribution” of the pre-existing ocean DIC associated with changes in the circulation due to climate change. Our analysis reveals that in the next decades, although carbon emissions will lead to an ocean carbon uptake and an increase in the DIC within the Indonesian Seas, a plausible climate-induced weakening in the ITF can lead to either an increase or a decrease in the DIC at different depths associated with different water masses. Hence, the effects of global carbon emissions on the carbon budget within the Indonesian Seas, and particularly whether local waters will experience a lower or higher increase in DIC than the rest of the ocean, are controlled by the dynamical redistribution associated with the response of the ITF to climate change.   

How to cite: Katavouta, A., Polton, J., Jardine, J., Partridge, D., Jevrejeva, S., and Holt, J.: Exploring the Climate-change induced dissolved inorganic carbon trends in the Indonesian Seas and their link to a changing Indonesian Throughflow using a regional downscaling of future climates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8672, https://doi.org/10.5194/egusphere-egu23-8672, 2023.

X5.309
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EGU23-11289
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OS1.7
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ECS
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Highlight
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Ruisi Qi, Ying Zhang, Yan Du, and Ming Feng

The spatio-temporal characteristics of the interannual variability and long-term trend of the marine heatwaves (MHWs) and related dynamic mechanisms in the western equatorial Indian Ocean (WEIO) are investigated using satellite observations. A prominent MHW hot spot is found in a region of the WEIO (48°E-54°E, 2°S-2°N), with a mean MHWs' intensity, duration, and frequency of 1.54°C, 13.33 days, and 1.97 times, respectively. MHWs in the hot spot region have significant interannual variability after removing the long-term trend, associated with Indo-Pacific major climate modes. In 1982/1983, 1983/1984, 1987/1988, 1997/1998, 2006/2007, 2009/2010, 2011/2012, 2012/2013, 2014/2015, 2015/2016, and 2019/2020, the MHWs occurred with longer duration, higher frequency, and more total days. These years correspond to a positive Indian Ocean Dipole, or an El Niño event, or both. The occurrence of MHWs accompanied by anomalous positive sea surface height suggests that oceanic planetary wave processes modulate MHWs in the WEIO. Westward-propagating downwelling equatorial Rossby waves triggered by anomalous equatorial easterly winds drive the convergence of warm upper-ocean water and weaken the upwelling of cool subsurface water, which favor anomalously warm sea surface temperature (SST) and the occurrence of MHWs. In addition, the westward-propagating off-equatorial downwelling Rossby waves in the southern tropical Indian Ocean also affect MHWs in the WEIO through the propagation and reflection of waves. The annual MHW frequency, duration, and total days in the hot spot region increase up to 1.56 times, 4.95 days, and 31.72 days per decade, respectively, related to the significant increase in mean SST under global warming.

How to cite: Qi, R., Zhang, Y., Du, Y., and Feng, M.: Characteristics and Drivers of Marine Heatwaves in the Western Equatorial Indian Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11289, https://doi.org/10.5194/egusphere-egu23-11289, 2023.

X5.310
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EGU23-9682
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OS1.7
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ECS
Marie Montero, Clément de Boyer Montégut, Jérôme Vialard, William Llovel, Thierry Penduff, Jean-Marc Molines, Stephanie Leroux, Nicolas Reul, and Jean Tournadre

The Bay of Bengal (BoB) Sea Surface Salinity (SSS) is highly contrasted and variable, in response to the large monsoonal wind and freshwater forcing. In addition to this strong seasonal cycle, previous studies have underlined strong SSS non-seasonal variations associated with the Indian Ocean Dipole (IOD) and mesoscale eddies. In this study, we quantify the relative contributions of externally forced (wind, freshwater) and internally generated (mesoscale eddies) SSS non-seasonal variability in the BoB. To that end, we use Ocean General Circulation Model 10-member ensemble experiments from the IMHOTEP (IMpacts of freshwater discHarge interannual variability on Ocean heaT-salt contents and rEgional sea-level change over the altimetry Period) project.
The model reproduces the large forced interannual SSS signals in the Northernmost part of the BoB and along the east coast of India, associated with the East Indian Coastal Current (EICC) modulation by the IOD. The internal SSS variability is largest in boreal fall in the North-Western BoB and more tightly controlled by the climatological SSS gradient distribution than by that of eddy kinetic energy. The external atmospheric forcing dominates the total variability in the regions of strongest variability, near the Ganges mouth and along the east coast of India in boreal fall and winter. Internal variability, however, contributes to 50-70% of the variability further offshore in boreal fall and winter. This confirms the strong role of eddies in controlling the freshwater extension up to ~700 km away from the coast, through stirring of the intense gradient between the coastal fresh and offshore saltier water. We finally discuss the consequences of these findings for comparing model and observations, in view of the chaotic nature of internal eddy variability.

How to cite: Montero, M., de Boyer Montégut, C., Vialard, J., Llovel, W., Penduff, T., Molines, J.-M., Leroux, S., Reul, N., and Tournadre, J.: Relative contribution of eddies ant atmospheric forcing to the Bay of Bengal non-seasonal Sea Surface Salinity Variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9682, https://doi.org/10.5194/egusphere-egu23-9682, 2023.

X5.311
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EGU23-11965
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OS1.7
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ECS
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Jessica A. Hargreaves, Nerilie Abram, and Jennie Mallela

Future climate trends indicate that changes in temperature and precipitation are likely to influence global supply chains, agricultural productivity, water security, health and well-being; particularly in densely populated nations across the southeast Indian Ocean region. The Indonesian Throughflow is an ocean current that transports low-latitude, warm and relatively fresh water from the western Pacific into the eastern Indian Ocean. It is thought that variability and changes in the Indonesian Throughflow have significant impacts on the climate and oceanography of the Indo-Pacific region. The short coverage of observational records makes assessments of hydrological changes across the region challenging on longer timescales, with changes before the 1970s being particularly unreliable. An extended record of Indonesian Throughflow variability needs to be established to contextualise changes and improve model projections of future variability.

Christmas Island, located in the southeast Indian Ocean (not to be confused with the Pacific Ocean Kiritimati Island), is located along an outflow of the Indonesian Throughflow. This Island is an ideal location to develop new palaeo-reconstructions of sea surface temperature and hydroclimate, extending our understanding of Indonesian Throughflow variability. Here we present a newly developed coral palaeoclimate reconstruction for Christmas Island, covering the last 118 years at approximately monthly-fortnightly resolution. Corals are sensitive recorders of critical environmental variables, including sea surface temperature and hydroclimate through the analysis of paired stable oxygen isotopes (δ18O) and trace element (Sr/Ca) ratios. This reconstruction consists of a composite of four newly developed coral records and one previously published record and provides a newly developed δ18Osw variability record for the region. The newly developed δ18Osw coral reconstruction correlates strongly with salinity variability, however, presents a weak relationship to in-situ precipitation, indicating that coral hydroclimate reconstructions from Christmas Island likely isolate salinity variability associated with changes in the strength of the Indonesian Throughflow. This relationship highlights the importance that ocean advection plays on δ18Osw variability at this site. Comparisons to both observational records of the Indonesian throughflow, and previously published coral δ18Osw records from the Ombai Strait (Timor), a major outflow passage, reveal strong relationships to variability at Christmas Island. The Christmas Island reconstruction provides a unique opportunity to extend current knowledge of the Indonesian Throughflow beyond the observational record. This Christmas Island record also provides an opportunity to evaluate the impact that interannual to multidecadal variability has on the climate across the southeast tropical Indian Ocean.

How to cite: Hargreaves, J. A., Abram, N., and Mallela, J.: 118-year hydroclimate reconstruction from Christmas Island (Indian Ocean); an extended record of variability in the Indonesian Throughflow, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11965, https://doi.org/10.5194/egusphere-egu23-11965, 2023.

Posters virtual: Fri, 28 Apr, 14:00–15:45 | vHall CR/OS

Chairpersons: Peter Sheehan, Caroline Ummenhofer, Muhammad Adnan Abid
vCO.5
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EGU23-2472
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OS1.7
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ECS
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Yifan Xia and Yan Du

The middepth zonal velocity resembles a system of eastward/westward jets with a considerably smaller width than the larger-scale ocean surface circulation. Such a phenomenon always occurs in a turbulent ocean that presents eddy or eddy–mean flow interactions. In this study, the upper-ocean absolute geostrophic currents in the southern Indian Ocean are constructed using Argo temperature and salinity data from the middepth (1000 m) zonal velocity derived from the Argo float trajectory. The results reveal alternating quasi-zonal striation-like structures of middepth zonal velocity in the equatorial and southern tropical Indian Ocean, with a meridional scale of 300 km. The triad of baroclinic Rossby wave instability plays a significant role in near-equatorial striations. In the south, the  unstable vertical structure leads to strong baroclinic instability, which increases the eddy kinetic energy in the middepth layer, thus contributing to a turbulent PV gradient. The convergence/divergence of the eddy PV flux generates the quasi-zonal striations. The meridional scale of the striations is controlled by the most unstable wavelength of baroclinic instability, which explains the observations.

How to cite: Xia, Y. and Du, Y.: Middepth Zonal Velocity in the Southern Tropical Indian Ocean: Striation-Like Structures and Their Dynamics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2472, https://doi.org/10.5194/egusphere-egu23-2472, 2023.

vCO.6
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EGU23-2164
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OS1.7
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Highlight
Hema Achyuthan and Nagasundaram Mohan

A sediment core retrieved from a water depth of 250 m near the Andamans Forearc Basin (AFB), the Landfall Island, North Andaman, reflects a record of sediment provenance and monsoonal shift since the mid to late Holocene. The core represents radiocarbon ages ranging from 6,078 to 1,658 yrs BP (from~ 6,500 yrs BP to the present). The core is dominantly clayey silt with incursions of coarser components that occur around 6,000, 5,400, and 3,400 yrs BP. Grain size variation indicates a cyclic variation of wetter and drier conditions matching changes in the intensity of the Indian Summer Monsoon (ISM), which was at its greatest intensity around 6,400, 5,300, and 3,300-3,000 yrs BP. Geochemical parameters including higher CaCO3 content, εNd, and 18O in Globigerinoides ruber are consistent with the long-term trend from cooler, wetter conditions to warmer, drier conditions at present. Chemical weathering intensity, which lags behind climate changes on land, shows a pulse of highly weathered sediment deposited at about 4,000 BP. Clay minerals represented by smectite, illite, kaolinite, and chlorite in varying amounts indicate high kaolinite content and K/C ratio specify intense Southwest Monsoon (SWM) and stronger bedrock weathering in the hinterland (~6,500–5,400 years BP). Incidence of smectite (48.82 to 25.09 %) and chlorite/illite (C/I) ratio (0.56 to 0.28) indicate an overall weakened southwest monsoon since 6,000 to 2,000 years BP with a brief incursion of extremely reduced SWM around 4,400 to 4,200 years BP. This is corroborated by the oxygen isotope on G. ruber that indicates a significant shift in the isotopic values ~4,300 years BP (−3.39‰), indicating a weak SWM. Fluctuations in the intensity of SWM are also observed for 2,000 years to the present. Sandy sediment was supplied from the Andaman Islands, Irrawaddy, and the Salween sea. Since the Mid Holocene period, longer periods of aridification and shorter periods of wetter conditions increased in the region after approximately 4,300 yrs BP. A correlation of monsoonal events using the Godhavari marine sediment core (Ponton et al.,2012)  and our data is noted that Bronze Age Harappan urbanism flourished since 4,500 yrs BP along the river banks in the western region of the present semi-arid Desert and the Deccan owing to intensified rain-fed agriculture. Since approximately 3,900 yrs ago, the total settled area and many settlement sizes declined, abandoned, and a significant shift in site numbers and density towards the southeast and west is recorded. During the Iron Age, after ca. 3,200 yrs BP, adaptation to semi-arid conditions in western Rajasthan, central and south India appears to have been well established with a significant number of sites in areas receiving <500 mm of rainfall. Weak monsoon precipitation led to conditions adverse to both inundation and rain-based farming and encouraged pastoralism. Monsoonal-fed rivers were active during the short-wet periods and gradually dried or became seasonal, affecting habitability along their courses. 

How to cite: Achyuthan, H. and Mohan, N.: Mid-Holocene Monsoon Weakening: A major cause for societal change in the Indian subcontinent, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2164, https://doi.org/10.5194/egusphere-egu23-2164, 2023.