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.

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 32^oS 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.

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.

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.

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.

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.

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 17.5^oN 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 (12^oN), 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.

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.

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 (δ¹³C_POC) and nitrogen (δ¹⁵N_PN) 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 δ¹³C_POC 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 δ¹⁵N_PN 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 δ¹⁵N_PN 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.

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.