Displays

Nitrogen is a staple element for every living organism in addition to carbon, since all the major cellular components (e.g., DNA and RNA), proteins, and energy carrier molecules (e.g., ATP) are stemmed from these elements. Biological dinitrogen (N₂) fixation exerts an important control on oceanic primary production by providing bioavailable form of nitrogen (such as NH₄⁺) to photosynthetic microorganisms. We hypothesized that the oligotrophic nature of the Bay of Bengal might create a suitable niche for N₂ fixing microorganisms.

In the Bay of Bengal, fresh water influx driven stratification prevent the vertical influx of nutrients to the sunlit layers. Most of the riverine nutrients are used within estuarine and coastal regions, and thus these have negligible contribution on open ocean biological productivity. Atmospheric deposition contribution to the nutrients supply is equally low (< 3%) in the Bay. Thus, the recently observed high new production rates in the Bay of Bengal suggests the higher probability of N₂ fixation in this basin than the Arabian Sea. In addition, nitrogen isotopic composition of sedimentary organic matter (low δ¹⁵N values) in the Bay of Bengal can also be alluded to the presence of diazotrophy in the Bay. Hence, we further strengthened our hypothesis that N₂ fixers play a crucial role for the primary production in the Bay.

We commenced the first N₂ fixation study in the sunlit layer of the Bay of Bengal using ¹⁵N₂ gas tracer incubation experiments on a cruise expedition during summer monsoon 2018. N₂ fixation rates varied from 4 to 124 μmol N m^-2 d^-1 – these rates were very low compared to that observed in the Bay’s western counterpart in the Indian Ocean, i.e., the Arabian Sea. The contribution of N₂ fixation to primary production was small (< 1%). Noteworthily, the upper bound of observed N₂ fixation rates in our study was still higher than that measured in other oceanic regimes such as Eastern Tropical South Pacific, Tropical Northwest Atlantic, and Equatorial and Southern Indian Ocean. Strong monsoonal winds, turbidity due to copious riverine discharge and cloud cover over the Bay of Bengal might have inhibited N₂ fixation. Therefore, a more detailed study covering all the seasons is needed to understand the role of N₂ fixation rates on primary productivity in the Bay of Bengal.

How to cite: Singh, A., Saxena, H., Sahoo, D., Khan, M. A., Kumar, S., and Sudheer, A. K.: Low Dinitrogen Fixation Rates in the Bay of Bengal during Summer Monsoon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1264, https://doi.org/10.5194/egusphere-egu2020-1264, 2020.

Here we present high-resolution biogeochemical study using nitrogen/carbon isotope ratio measurement and molecular proxies from a sediment core (length = 2.9 m) collected from the center (588 mbsl; Lat: 16⁰49.88’N and Long: 71⁰ 58.55’ E) of the oxygen minimum zone off west coast of India. The core archives the depositional record covering 1.114 to 12.025 ky BP. The concentrations of total organic carbon (TOC) and total nitrogen (TN) range from 0.7 to 4.9 wt% and 0.068 to 0.5 wt % respectively. TOC and TN show parallel trends and the TOC/TN ratio varies within a narrow range of 8 to 11.5. The δ¹³C values range from -20.5‰ to -21.9‰ (V-PDB). The TOC/TN and δ¹³C suggest typical marine organic matter source. This observation is also further supported by the n-alkane distribution pattern where the dominance of nC₂₁ to nC₂₄ and the absence of odd alkane dominance over even suggest predominantly marine organic source. The δ¹⁵N profile shows a steady increase from 5.7‰ at 203 cmbsf (5.5 ky BP) to 7.5‰ at 2 cmbsf (~1ky BP) suggesting gradual increase in denitrification possibly liked to reduced ventilation in the Arabian Sea, whereas, between 5.5 to 12 ky BP, the δ¹⁵N values show marked fluctuations (5.2 to 7.1‰) possibly indicating variable level of oxygenation which in turn controlled the extent of denitrification. Possible influence of diagenesis (microbial degradation of organic matter) on the δ¹⁵N values also need to be investigated for a better understanding of the water column processes.

How to cite: Manaskanya, A., Mazumdar, A., Peketi, A., Fernandes, S., and Silva, R. D. A.: Last 12 ky record of various organic geochemical proxies in the Eastern Arabian Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8642, https://doi.org/10.5194/egusphere-egu2020-8642, 2020.

The land-locked northern boundary and seasonal high productivity in the Arabian sea (AS) leads to the formation and the maintenance of one of the most intense and thickest open ocean oxygen minimum zones (OMZ) there. Earlier studies based on both observation and model sensitivity experiments have reported that this perennial OMZ is highly sensitive to the strength of the monsoonal circulation and surface heating. Model simulations from the fifth phase of Coupled Model Intercomparison project (CMIP5) indicate significant changes in the Indian monsoonal circulation and the atmospheric heat fluxes under climate change. However, the future projection of AS OMZ under climate change remains largely uncertain and ill-understood. This is mainly due to a poor representation of the AS OMZ in the CMIP5 simulations and an important spread in their future oxygen projections for the region. Here we explore how downscaling CMIP5 global simulations with a high-resolution configuration of the Regional Ocean Modeling System (ROMS) model coupled to a nitrogen-based NPZD ecosystem model can help improving the representation of the AS OMZ and reduce the spread in CMIP5 projections. To this end, we performed a climatological “reference” simulation, i.e., the control simulation, where ROMS is forced with observed atmospheric and lateral boundary conditions, and a set of corresponding downscaled sensitivity experiment where ROMS is forced with atmospheric and lateral boundary conditions derived from global CMIP5 simulations. For the downscaling experiment, we chose two best performing models from the CMIP5 database based on their skill in simulating the present day (historical) climatology. The control simulation has been extensively validated against the observations for its skill in simulating the physical and biogeochemical variables. We explore the sensitivity of the downscaled oxygen distribution and OMZ to the regional model setup by varying the model resolution from 1/3deg to 1/12deg and expanding the model domain from a small AS-limited domain to one encompassing the full Indian Ocean. We show that the downscaled experiments improve the representation of different classes of oxygen (Oxic - O2 > 60mmol/l; Hypoxic - 60mmol/l >= O2 > 4mmol/l; and the Suboxic  - 4 mmol/l > O2 > 0 mmol/l) within the 0-1500m depth range. In particular, the downscaled experiments simulate a much smaller fraction of suboxic waters relative to hypoxic and oxic fractions, in agreement with observations.

How to cite: Vallivattathillam, P., Lachkar, Z., Levy, M., and Smith, S.: Projected response of Arabian sea Oxygen minimum zone to climate change: Insights from a set of downscaled experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13738, https://doi.org/10.5194/egusphere-egu2020-13738, 2020.

The South and East coast of South Africa is strongly influenced by the warm, fast-flowing Agulhas Current. The Agulhas Bank, a shallow shelf on the southern tip of Southern Africa, is a crucial area for productivity which support fisheries of high economic importance for South Africa.  In this context of climate change, perturbations of this diverse, complex and highly variable marine environment could affect the productivity and lead to dramatic social and economic consequences for the region. To predict potential changes over the eastern and central Bank, we employ a high-resolution global coupled ocean-biogeochemistry model, NEMO-MEDUSA, simulated to year 2099. We find that even though the Agulhas Bank is warming over the next century, primary production does not experience a significant decrease. Additionally, we show that the Agulhas Current might shift its position, with intensification surface current velocity on the Bank hence reducing water retention over the Bank. This change in local circulation over the Bank could have a serious impact on the ecosystem of the region.

How to cite: Asdar, S., Roberts, M. J., Jacobs, Z., and Popova, E.: Impact of Climate Change on the Agulhas Bank, South Africa ― a 100 year projection with consequences for the squid fishery, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13104, https://doi.org/10.5194/egusphere-egu2020-13104, 2020.

This study estimates variability in meridional velocity and transport of the subtropical circulation in the south Indian Ocean using in-situ hydrographic observations, satellite altimetry and two reanalysis products for the period from 2006 to 2017. Previous studies used the zonal difference of satellite sea surface height (SSH) between the western and eastern parts of the basin as an index to variability in basinwide meridional geostrophic transport. This study estimates meridional geostrophic velocity in the upper 1800 m from in-situ observations and compares results with SSH variability. Results show that zonal SSH difference represents a surface trapped variability in meridional velocity, the amplitude of which is large in the upper 250 m and decreases to zero at about 1000 m depth. Zonal SSH difference is significantly correlated with zonally integrated meridional transport relative to 1000 m depth. It is likely that wind variability both in the south Indian Ocean and tropical Pacific Ocean is responsible for this surface trapped variability, as is suggested by past studies. Results of this study also show meridional velocity variability at subsurface, which peaks in magnitude at about 400 to 800 m depth and is not correlated with zonal SSH difference. Waves radiated from the eastern boundary are possibly responsible for the generation of this subsurface flow, but detailed forcing mechanisms are not known in this study. This subsurface flow can contribute to interannual variability in mode water transport and warrants a further study.

How to cite: Nagura, M.: Interannual to decadal variability in subtropical circulation transport in the south Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11992, https://doi.org/10.5194/egusphere-egu2020-11992, 2020.

Ocean-atmosphere interactions in the tropics have a profound influence on the climate system. El Niño–Southern Oscillation (ENSO), which is spawned in the tropical Pacific, is the most prominent and well-known year-to-year variation on Earth. Its reach is global, and its impacts on society and the environment are legion. Because ENSO is so strong, it can excite other modes of climate variability in the Indian Ocean by altering the general circulation of the atmosphere. However, ocean-atmosphere interactions internal to the Indian Ocean are capable of generating distinct modes of climate variability as well. Whether the Indian Ocean can feedback onto Atlantic and Pacific climate has been an on-going matter of debate. We are now beginning to realize that the tropics, as a whole, are a tightly inter-connected system, with strong feedbacks from the Indian and Atlantic Oceans onto the Pacific. These two-way interactions affect the character of ENSO and Pacific decadal variability and shed new light on the recent hiatus in global warming.

Here we review advances in our understanding of pantropical interbasins climate interactions with the Indian Ocean and their implications for both climate prediction and future climate projections. ENSO events force changes in the Indian Ocean than can feed back onto the Pacific. Along with reduced summer monsoon rainfall over the Indian subcontinent, a developing El Niño can trigger a positive Indian Ocean Dipole (IOD) in fall and an Indian Ocean Basinwide (IOB) warming in winter and spring. Both IOD and IOB can feed back onto ENSO. For example, a positive IOD can favor the onset of El Niño, and an El Niño–forced IOB can accelerate the demise of an El Niño and its transition to La Niña. These tropical interbasin linkages however vary on decadal time scales. Warming during a positive phase of Atlantic Multidecadal Variability over the past two decades has strengthened the Atlantic forcing of the Indo-Pacific, leading to an unprecedented intensification of the Pacific trade winds, cooling of the tropical Pacific, and warming of the Indian Ocean. These interactions forced from the tropical Atlantic were largely responsible for the recent hiatus in global surface warming.

Climate modeling studies to address these issues are unfortunately compromised by pronounced systematic errors in the tropics that severely suppress interactions with the Indian and Pacific Oceans. As a result, there could be considerable uncertainty in future projections of Indo-Pacific climate variability and the background conditions in which it is embedded. Projections based on the current generation of climate models suggest that Indo-Pacific mean-state changes will involve slower warming in the eastern than in the western Indian Ocean. Given the presumed strength of the Atlantic influence on the pantropics, projections of future climate change could be substantially different if systematic model errors in the Atlantic were corrected. There is hence tremendous potential for improving seasonal to decadal climate predictions and for improving projections of future climate change in the tropics though advances in our understanding of the dynamics that govern interbasin linkages.

How to cite: Lengaigne, M. and the Coauthors: Interactions of the Indian Ocean climate with other tropical oceans, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2707, https://doi.org/10.5194/egusphere-egu2020-2707, 2020.

Over the last decade the Indian Ocean has absorbed 60% of the global oceanic heat uptake and the fate of this heat and its impact on future change is unknown. Projections foresee accelerating sea level rise, more frequent extremes in monsoon rainfall, and decreasing oceanic productivity. Almost two-thirds of humanity live around the Indian Ocean, many in countries dependent on fisheries and rain-fed agriculture. Coastal population growth is conflating with climate change to further increase exposure and vulnerability of these populations. The Indian Ocean observing system (IndOOS), established in 2006, is a multi-national network of sustained oceanic measurements that underpin understanding and forecasting of weather and climate for the Indian Ocean region and beyond. However, gaps in the IndOOS have so far limited forecasting efforts, left large discrepancies in the basin energy budget, and kept us in the dark about ecosystem stressors. A three-year, international review of the IndOOS by more than 60 scientific experts provides a roadmap to an improved observing network — IndOOS-2 — that can better meet future scientific and societal challenges. Core findings include the need for 1) chemical, biological, and ecosystem measurements alongside physical parameters; 2) better resolved upper-ocean processes to yield improved sub-seasonal to seasonal forecasts; 3) expansion into the western Arabian Sea; and 4) expansion into key coastal regions and the deep ocean to better constrain heat and freshwater changes. IndOOS-2 will require new resources and partnerships, creating opportunities for Indian Ocean rim countries to enhance their monitoring and forecasting capacity as part of a growing Global Ocean Observing System community.

How to cite: McPhaden, M. J., Beal, L. M., Vialard, J., Roxy, M. K., and CLIVAR-GOOS IndOOS-2 Report, A. O. T.: IndOOS-2: A Roadmap to Better Observations of the Rapidly Warming Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2486, https://doi.org/10.5194/egusphere-egu2020-2486, 2020.

Modern numerical ocean models have matured over the last decades and are able to provide accurate fore- and hind-cast of the ocean state. The most accurate data could be obtained from the reanalysis where the model run in a hindcast mode with assimilation of available observational data. An obvious benefit of model simulation is that it provides the spatial density and temporal resolution which cannot be achieved by in-situ observations or satellite derived measurements. It is not unusual that even a relatively small area of the ocean model can have in access of 100,000 nodes in the horizontal, each containing vertical profiles of temperature, salinity, velocity and other ocean parameters with a temporal resolution theoretically as high as a few minutes. Remotely sensed (satellite) observations of sea surface temperature can compete with the models in terms of spatial resolution, however they only produce data at the sea surface not the vertical profiles. On the other hand, in-situ observations have a benefit of being much more precise than model simulations. For instance a widely used CTD profiler SBE 911plus has accuracy of about 0.001 °C, which is not achievable by models.

In the creation of a climatic atlas the higher accuracy of individual profiles provided by in-situ measurements may become less beneficial. Assuming the normal distribution of data at each location, the standard error of the mean (SEM) is calculated as SE=S/SQRT(N), where S is the standard deviation of individual data points around the mean, and N is the number of data points. The climatic data are obtained by averaging a large number of individual data points, and here the benefit of having more data points may become a greater advantage than the accuracy of a single observation.  

In this study we have created an ocean climate atlas for the northern part of the Indian Ocean including the Red Sea and the Arabian Gulf using model generated data. The data were taken from Copernicus Marine Environment Monitoring Service (CMEMS) reanalysis product GLOBAL_REANALYSIS_PHY_001_030 with 1/12° horizontal resolution and 50 vertical levels for the period 1998 to 2017. The model component is the NEMO platform driven at the surface by ECMWF ERA-Interim reanalysis. The model assimilates along track altimeter data, satellite Sea Surface Temperature, as well as in-situ temperature and salinity vertical profiles where available. The monthly data from CMEMS were then averaged over 20 years to produce an atlas at the surface, 10, 20, 30, 75, 100, 125, 150, 200, 250, 300, 400, and 500 m depths.  The standard error of the mean has been calculated for each point and each depth level on the native grid (1/12 degree).

The atlas based on model simulations was compared with the latest version of the World Ocean Atlas (WOA)  2018 published by the NCEI.  WOA has objectively analysed climatological mean fields on a ¼  degree grid. The differences between the mean values and SEMs from observational and simulated atlases are analysed, and the potential causes of mismatch are discussed.

How to cite: Shapiro, G. I., Gonzalez-Ondina, J. M., Francis, X., Lim, H. S., and Almehrezi, A.: Could an ocean climate atlas generated by a model compete with an observational one?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1582, https://doi.org/10.5194/egusphere-egu2020-1582, 2020.

The ocean exchanges heat and mass with the atmosphere in form of shortwave and longwave radiations, precipitation, and evaporation. The regional scale ocean processes governed by this exchange play a vital role in modulating the local dynamics of the Indian Ocean. For instance, the meso-scale eddies and waves control the ocean vertical temperature structure, mixed layer depth, and the thermocline. The Indian Ocean Observing System (IndOOS) recommends the need of proper understanding of heat budget in the Indian Ocean to resolve the mesoscale and submesoscale processes, which trigger large scale ocean circulation, cyclonic eddies, plumes etc. In a regional domain, the stability of ocean also depends on the local parameters, namely, wind pattern, precipitation, runoff and exchange of heat and mass fluxes near the domain boundary. The main objective of this study is to understand the effect of atmospheric wind and solar radiation on the ocean surface and sub-surface characteristics using Modular Ocean Model (MOM5).

A regional domain in the Bay of Bengal (BoB) is selected, which has unique features, such as, large amount of freshwater flux, seasonal wind reversal and high amount of solar radiation due the geographic location. The dynamics in BoB is important for understanding the Indian summer and winter monsoon seasons and associated weather patterns. A regional ocean modelling approach is adopted using MOM5 with horizontal grid resolution (0.25⁰) while maintaining the vertical grid-size as 1m near the surface region which increases with depth. For the regional domain, radiation open boundary condition (OBC) is implemented on three lateral boundaries of domain, based on the technique proposed by Orlanski (1976). The OBC at the lateral boundaries help in smooth exchange of current and tracers. K-profile parameterization (KPP) vertical mixing scheme is used that accounts for effects of shear, wave breaking, and double diffusion. The model is started from a state of rest and simulated for a period of 10 years using 6-hourly solar radiation (Japanese 25-year reanalysis (JRA-25)) and daily averaged wind stress (SODA reanalysis) dataset. After five years of model spin-up, the last five years of simulated output is considered to ensure consistency of model results. Heat budget calculation shows good agreement with WHOI OA Air-Sea Fluxes (OAFlux). Smooth exchange of mass and fluxes is observed near boundary, which confirms successful implementation of OBC. Implementation of KPP scheme enhances mixing in the upper ocean layers with more realistic thermocline formation and turbulent kinetic energy (TKE). The model is able to mimic the seasonal variability in the ocean currents enforced due to winds. The Sea Surface Temperature (SST) is in good agreement with SODA reanalysis data.

A plume like mesoscale feature in the SST plot is captured in the present study (that is also observed in microwave SST), but found to be missing in earlier BoB study with sponge boundary conditions. Finer scale resolution (0.125⁰) study is in progress, which is expected to show secondary mesoscale structures and their evolution. The results from this study would help in better understanding of the influence regional-scale processes on local ocean dynamics.

How to cite: Tirodkar, S., Behera, M. R., and Balasubramanian, S.: A regional study of Bay of Bengal processes using radiation boundary condition in Modular Ocean Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-513, https://doi.org/10.5194/egusphere-egu2020-513, 2020.

Within the Coordinated Regional Climate Downscaling Experiment (CORDEX) framework, a high-resolution Regional Earth System Model (RESM) was used to understand the effects of various parameterizations of the attenuation of short-wave radiation (SWR) penetrating into the ocean. The RESM comprises of the Max Planck Institute Ocean Model and the Hamburg Ocean Carbon Cycle model (MPIOM/HAMOCC) coupled via the OASIS coupler to the Regional atmosphere Model (REMO), and the Hydrological Discharge model (HD). Two runs of the RESM for the historical period 1950-2017 were performed. In the first run, the model utilized a simple light attenuation parameterization based on the Jerlov water types when the attenuation coefficient varies spatially depending on the water type but does not vary in time. In the second run, the feedback between the ocean and atmosphere through the marine ecosystem was implemented by using the parameterization of light attenuation coefficient as the function of not only water attenuation itself but also phytoplankton concentration, which was implemented in both the physical and biogeochemical model blocks. The obtained model results correspond well to the observed climatic characteristics. In the calculation with phytoplankton-dependent light attenuation parameterization, the average SST was lower than in the case of Jerlov-based parameterization. The greatest difference in SST (more than 1 °C) occurs in the spring and summer periods during the phytoplankton bloom. The SST differences in autumn and winter are less pronounced and do not exceed 0.2 °C and 0.6 °C, respectively. Also, during the period of intensive heating (spring and summer) the SWR in the ocean upper layers calculated by the feedback-based model run is more strongly absorbed in these layers and, as a result, a significant cooling of subsurface layers (25-200 m) occur (up to 1-1.5 ° С). The phytoplankton primary production and its dispersion in the feedback-based model run turned out to be higher, especially during the periods of winter and summer blooms, and the surface concentration of dissolved nitrates was lower than in the reference run (Jerlov-based parameterization) almost the whole year.

The work was supported by the Russian Science Foundation (Project 19-47-02015) and by the grant DST/INT/RUS/RSF/P-33/G of the Department of Science and Technology, Govt. of India.

How to cite: Martyanov, S. D., Sein, D. V., Ryabchenko, V. A., Dvornikov, A. Y., and Kumar, P.: The influence of water temperature-phytoplankton feedback in a Regional Earth System Model upon the hydrography and biogeochemistry of the northern Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7886, https://doi.org/10.5194/egusphere-egu2020-7886, 2020.

Remote wind forcing plays a strong role in the Northern Indian Ocean, where oceanic anomalies can travel long distances within the coastal waveguide. Previous studies for instance emphasized that remote equatorial forcing is the main driver of the sea level and currents intraseasonal variability along the west coast of India (WCI). Until now, the main pathway for this connection between the equatorial and coastal waveguides was thought to occur in the eastern equatorial Indian Ocean, through coastal Kelvin waves that propagate around the Bay of Bengal rim and then around Sri Lanka to the WCI. Using a linear, continuously stratified ocean model, the present study demonstrates that two other mechanisms in fact dominate. First, the equatorial waveguide also intersects the coastal waveguide at the southern tip of India and Sri Lanka, creating a direct connection between the equator and WCI. Rossby waves reflected from the eastern equatorial Indian Ocean boundary indeed have a sufficiently wide meridional scale to induce a pressure signal at the Sri Lankan coast, which eventually propagates to the WCI as a coastal Kelvin wave. Second, local wind variations in the vicinity of Sri Lanka generate strong intraseasonal signals, which also propagate to the WCI along the same path. Sensitivity experiments indicate that these two new mechanisms (direct equatorial connection and local wind variations near Sri Lanka) dominate the WCI intraseasonal sea level variability, with the “classical” pathway around the Bay of Bengal only coming next. Other contributions (Bay of Bengal forcing, local WCI forcing) are much weaker.

We further show that the direct connection between the equatorial waveguide and WCI is negligible at seasonal timescale, but not at interannual timescales where it contributes to the occurrence of anoxic events. By providing an improved understanding of the mechanisms that control the WCI thermocline and oxycline variability, our results could have socio-economic implications for regional fisheries and ecosystems.

How to cite: Suresh, I., Vialard, J., Lengaigne, M., Izumo, T., and Pillathu Moolayil, M.: Importance of wind variations and intersecting waveguides near Sri Lanka for the intraseasonal sea level variability along the west coast of India , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19919, https://doi.org/10.5194/egusphere-egu2020-19919, 2020.

Argo trajectory data is used to estimate the velocities of mid-depth (1000db) currents in the North Indian Ocean (NIO). Based on these estimated velocities rather than an assumed level of “no motion”, the structure of upper ocean absolute geostrophic currents can be derived more accurately from the Argo temperature and salinity profiles. The derived flow field reveals that eastward zonal velocities have a striation-like structure in the Arabian Sea, while barely observed in the Bay of Bengal. The striation-like structure is most prominent in the layer from 500db, with a meridional scale of about 300km. Both the meridional scale and the distribution of these mid-depth striations are unique as compared to the other ocean basins. The nonlinear 1 1/2 -layer reduced gravity model and the baroclinic Rossby wave triad interaction theory capture the essential factors controlling the characteristics of the quasi-zonal striation structure. Compared to the Pacific Ocean, the narrower meridional scale in the NIO is because of the smaller basin scale in the equatorial zone rather than semiannual wind stress forcing period or slope of the eastern boundary. Coastal trapped Kelvin waves contribute significantly to the generation of the zonal striation in the Arabian Sea.

How to cite: Du, Y., Xia, Y., Qiu, B., Cheng, X., Wang, T., and Xie, Q.: The characteristics of the mid-depth striations in the North Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6513, https://doi.org/10.5194/egusphere-egu2020-6513, 2020.

With sea surface temperatures (SST) exceeding 30˚C in May, the southeastern Arabian Sea (SEAS) hosts one of the warmest open ocean region globally, which appears to play an important role in the summer monsoon onset. Freshwater input from the Bay of Bengal precede the SEAS warm pool build-up by a few months, and are believed to influence its temperature through its impact on oceanic stability and vertical mixing of heat. SSS interannual variations in the SEAS region have not been extensively described before, and their potential feedback on the warm pool build-up and the monsoon are still debated. In the present study, we describe the SEAS SSS seasonal and interannual variability, its driving mechanisms and potential impact on the monsoon. To that end, we analyse experiments performed with a regional 25-km ocean model, both forced and coupled to a regional atmospheric model. The forced and coupled simulations both reproduce the main oceanic features in the SEAS region, including the salinity seasonal cycle and interannual variability. Winter salinity stratification inhibits the vertical mixing of heat, thereby warming the mixed layer by ~0.5°C.month^-1. This salinity-induced warming is however compensated by a salinity-induced cooling by air-sea fluxes. Salinity stratification indeed yields a thinner mixed layer which is more efficiently cooled by negative surface heat fluxes at this season. Overall, salinity has thus a negligible impact on the SST seasonal cycle. SEAS SSS interannual variations are largely remotely driven by the Indian Ocean Dipole (IOD), an indigenous interannual climate mode in the equatorial Indian Ocean. The IOD remotely impacts coastal currents along the Indian coastline, and hence modulates freshwater transport from the Bay of Bengal into the SEAS. This yields positive SSS anomalies in the SEAS during the boreal winter that follows positive IOD events. Those SSS anomalies however do not appear to significantly alter the interannual surface layer heat budget. Coupled model sensitivity experiments, in which the influence of haline stratification on vertical mixing is neglected, further confirm that the SEAS winter freshening does not significantly influence the SEAS warm-pool build-up nor the monsoon onset

How to cite: Valiya Parambil, A., Lengaigne, M., Vialard, J., Krishnapillai Sukumarapillai, K., and Madhavan Girijakumari, K.: Variability of Sea Surface Salinity in the Southeastern Arabian Sea: Driving mechanisms and influence on the Arabian Sea mini Warm Pool, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21178, https://doi.org/10.5194/egusphere-egu2020-21178, 2020.

The Indonesian Throughflow (ITF) serves as an important oceanic teleconnection for Indo-Pacific climate, altering heat and buoyancy transport from the Pacific to the Indian Ocean. Equatorial Pacific wind forcing transmitted through the ITF impacts interannual to interdecadal Indian Ocean thermocline depth and heat content, with implications for preconditioning Indian Ocean Dipole events. Yet the modulation of Indian Ocean thermal properties at seasonal timescales is still poorly understood. Here we synthesize coral δ¹⁸O records, instrumental indices (El Niño Southern Oscillation (ENSO), Asian Monsoon), and simulated ocean variability (sea surface salinity (SSS) and temperature (SST), heat content, mixed layer depth) from state-of-the-art NEMO ocean model hindcasts to explore drivers of seasonal to multi-decadal variability. All coral sites are located within main ITF pathways and are influenced by monsoon-driven, buoyant South China Sea (SCS) surface waters during boreal winter that obstruct surface ITF flow and reduce heat transport to the Indian Ocean. Makassar and Lombok Strait coral δ¹⁸O co-varies with simulated SSS, subsurface heat content anomalies (50-350m) and mixed layer depth at the coral sites and in the eastern Indian Ocean. At decadal timescales, simulated boreal winter ocean variability at the coral sites additionally indicates a potential intensification of the SCS buoyancy plug from the mid- to late-20^th century. Notably, the variability in these coral and model responses reveals sensitivity to phase changes in the Interdecadal Pacific Oscillation and the East Asian Winter Monsoon. These results collectively suggest that the paleoproxy records are capturing important features of regional hydrography and Indo-Pacific exchange, including responses to regional monsoon variability. Such proxy-model comparison is critical for understanding the drivers of variability related to changes in ITF oceanic teleconnections over the 19^th and 20^th centuries.

How to cite: Murty, S., Ummenhofer, C., Scheinert, M., Behrens, E., Biastoch, A., and Böning, C.: Drivers of oceanic exchange through the Indonesian Throughflow since the late 1800s – a synthesis of coral δ18O and high-resolution ocean models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11181, https://doi.org/10.5194/egusphere-egu2020-11181, 2020.

The dominant mode of sea surface temperature (SST) variability in the southeast Indian Ocean off the coast of Western Australia is called Ningaloo Niño/Niña. An unprecedented Ningaloo Niño, or marine heatwave, occurred during the austral summer of 2010/2011 with mean SSTs at 3°C above the long-term mean and had drastic impacts on the ecosystem. This event was attributed to a combination of an anomalous strong Leeuwin Current and high local air-sea heat fluxes. A number of local and remote forcing mechanisms have been investigated in recent years, however, little is known about the depth-structure of these ocean extremes and their general connections to large-scale ocean interannual to decadal variability. Using a suite of simulations with a high-resolution global Ocean General Circulation Model from 1958-2016, we investigate eastern Indian Ocean variability with focus on Ningaloo Niño and corresponding cold Ningaloo Niña events. In particular, we are interested in the impacts of large-scale ocean and climate variability, such as the Indonesian Throughflow, El Niño - Southern Oscillation and the Indian Ocean Dipole (IOD), on the study region. Spatial composites reveal large-scale surface and subsurface anomalies that extend from the western Pacific across the Indonesian Archipelago into the tropical eastern Indian Ocean. In particular, strong anomalies in temperature, salinity and mixed layer depth are found to the west of Sumatra and Java, a region that is generally strongly impacted by the IOD. We therefore investigate the connection with Ningaloo Niño/Niña events, at surface and subsurface, with a focus on 2010/2011 where a strong negative IOD event occurred prior to the unprecedented Ningaloo Niño. Furthermore, we find that major heatwaves in 2000 and 2011 are associated with pronounced fresh anomalies. Sensitivity experiments allow us to assess the relative role of buoyancy and wind-forcing as drivers of the observed patterns. Our work can provide valuable contributions for advancing the understanding of Ningaloo Niño/Niña drivers from surface to depth and regional to large scales.

How to cite: Ryan, S., Ummenhofer, C., Gawarkiewicz, G., Wagner, P., Scheinert, M., Biastoch, A., and Böning, C.: Interannual variability of the Eastern Indian Ocean with focus on the Ningaloo Niño and negative Indian Ocean Dipole event in 2010/2011, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12313, https://doi.org/10.5194/egusphere-egu2020-12313, 2020.

Small pelagic fisheries play a critical role in food security and economic stability for East African coastal communities ― a region of least developed countries. Using satellite and field observations together with modelling, we show the links between the small pelagic fisheries along the East African coast and the changes in Western Indian Ocean currents due to the interannual variability of the monsoonal wind field. The annual variations in phytoplankton biomass and fisheries yield are strongly associated. During the Northeast monsoon, the enhanced phytoplankton biomass is triggered by local wind-driven upwelling. During the Southeast monsoon, however, the enhanced phytoplankton biomass is due to two current induced mechanisms: coastal “dynamic uplift” upwelling; and westward advection of waters with higher nutrient concentrations. This biological response to the Southeast monsoon is greater than that to the Northeast monsoon. Interannually, an extreme increase (decrease) in chlorophyll concentrations is induced by strengthened (weakened) surface currents, which occur during anomalously “strong” (“weak”) Southeast monsoon years. For years where the effects of El Niño / La Niña are weak, the Southeast monsoon wind strength over the south tropical Indian Ocean is the main driver of year-to-year variability. Such changes have important implications for the predictability of fisheries yield, its response to climate change, policy and resource management.  

How to cite: Jebri, F., Jacobs, Z., Raitsos, D., Srokosz, M., Painter, S., Kelly, S., Roberts, M., Scott, L., Taylor, S., Palmer, M., Kizenga, H., Shaghude, Y., Wihsgott, J., and Popova, E.: Interannual monsoon wind variability over the South tropical Indian ocean drives East African small pelagic fisheries, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2381, https://doi.org/10.5194/egusphere-egu2020-2381, 2020.

The Dansgaard-Oeschger oscillations and Heinrich events described in Greenland ice core records are also expressed in the climate of the tropical realm as for instance documented in Arabian Sea sediments. However, little is known about these fluctuations beyond the reach of the Greenland ice cores. Here, we present high-resolution organic- and inorganic geochemical, sedimentological as well as micropaleontological data from two cores retrieved off the coast of Pakistan, extending the monsoon record to the past 200,000 years in millennial scale resolution.

The stable oxygen isotope (δ¹⁸O) record of the planktic foraminifera G. ruber shows a strong correspondence to Greenland ice core δ¹⁸O, whereas the deepwater δ¹⁸O signal of benthic foraminifera (U. peregrina and G. affinis) reflects patterns similar to those observed in Antarctic ice core records. Strong shifts in benthic δ¹⁸O during stadials are interpreted to show frequent injections of oxygen-rich intermediate water masses of Southern Ocean origin into the Arabian Sea. Alkenone-derived SSTs vary between 23 and 28°C. Highest temperatures were encountered during interglacial MIS 5. Millennial scale SST changes of 2°C magnitude are modulated by long-term SST fluctuations. Interstadials (of glacial phases) and the cold phases of interglacials are characterized by sediments enriched in organic carbon (TOC) whereas sediments with low TOC contents appear during stadials. Abrupt shifts (50-60 year duration) at climate transitions, such as interstadial inceptions, correlate with changes in productivity-related and anoxia-indicating proxies. Interstadial inorganic data consistently show that enhanced fluxes of terrestrial-derived sediments are paralleled by productivity maxima, and are characterized by an increased fluvial contribution from the Indus River. The hydrogen isotopic composition of terrigenous plant waxes indicates that stadials are dry phases whereas humid conditions seem to have prevailed during interstadials. In contrast, stadials are characterized by an increased contribution of aeolian dust probably from the Arabian Peninsula. Heinrich events are especially dry and dusty, indicating a dramatically weakened Indian summer monsoon and increased continental aridity.

These results strengthen the evidence that North Atlantic temperature changes and shifts in the hydrological cycle of the Indian monsoon system are closely coupled, and had a massive impact 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., Groeneveld, J., Hollstein, M., Mohtadi, M., Schefuß, E., and Steinke, S.: Millennial scale monsoon variability in the northeastern Arabian Sea: A multiproxy approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2678, https://doi.org/10.5194/egusphere-egu2020-2678, 2020.

Sixteen years satellite observations are used to investigate the frontogenesis, frontal variability and its impact on chlorophyll in the Arabian Sea. Large frontal probability (FP) and high chlorophyll mainly happens near the coast, e.g., near Somalia and Oman, and its value generally decreases with offshore distance. An Empirical Orthogonal Function (EOF) is used to disentangle the spatial and temporal variability of front and chlorophyll. Prominent seasonal cycle of frontal activities is identified, peaking in summer when southwest wind prevails. The seasonality for chlorophyll is same with wind and front near Somalia, which largely impacted by monsoon. During summer, the southwest monsoon drives offshore Ekman transport and induces coastal upwelling. It transports subsurface cold water and nutrients to the surface layer, which generates fronts and enhances chlorophyll, respectively. The frontal activities can be used as an indicator to determine the chlorophyll level that high chlorophyll happens when frontal probability gets higher than 2%. At anomalous field, stronger wind can induce higher frontal activities and chlorophyll. The impact of wind on frontogenesis can extend 1,000km offshore and a simplified linear regression is applied to quantify their relationship. The variability of wind leads chlorophyll by lags increasing with distance, indicating a horizontal offshore transport of coastal water. In winter, the northeast wind is not upwelling favorable, thus the frontal activities and chlorophyll are greatly reduced off Somalia. During this period, large chlorophyll is found in the north off Oman because of mixing, thus its relationship with front is less pronounced. In the upwelling regions, fronts act as an intermedia process that connecting the wind forcing and responses of ecosystem. The frontal activities in Arabian Sea is fundamentally important to improve our understanding of monsoon related ocean dynamics.

How to cite: Wang, Y., Ma, W., Zhou, F., and Fei, C.: Frontal variability and its impact on chlorophyll in the Arabian Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9188, https://doi.org/10.5194/egusphere-egu2020-9188, 2020.

Mesoscale eddies that known as a dominant reservoir of kinetic energy has been studied extensively for its dynamics and variation.In order to maintain energy budget equilibrium,the energy stored in mesoscale eddies is dissipated by small scale processes around centimeters.Submesoscale processes that lie between mesoscale and microscale motions effectively extract energy from mesoscale motions and transfer to smaller scales.The Bay of Bengal(the BOB) receives large fresh water from precipitation and river runoff resulting in strong salinity fronts that conducive to the generation of submesoscale processes.Using the Regional Ocean Modeling System(ROMS) data with two horizontal resolutions:a high-resolution(~1.6km) that is partially resolve submesoscale,and a low-resolution(~7km) that not resolves submesoscale,we focus on the seasonality of submesoscale processes in the Bay of Bengal.To ensure that only the submesoscale motions is considered,we choose 40km as the length to separate submesoscale from the flow field.Results show that submesocale processes is ubiquitous in the BOB,mainly trapped in the mixed layer.As resolution increasing,submesoscale motions become much stronger.Seasonality of submesoscale in the BOB is apparent and is different from the Gulf stream region  which is strongest in winter and weakest in summer.Submesoscale features in this region mostly present in fall,which the most important mechanisms is frontogenesis due to strong horizontal buoyancy flux associated with large strain.Submesoscale motions is also vigorous in winter.The proposed mechanism is that the depth of mixed layer is deep enough which contributes to the occurrence of mixed layer instability.During the whole year,mesoscale strain field is weakest in summer,which makes submesoscale weakest.

How to cite: Li, L. and Cheng, X.: Seasonality of Submesoscale processes in the Bay of Bengal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12390, https://doi.org/10.5194/egusphere-egu2020-12390, 2020.

Air-Sea interaction in the Bay of Bengal has a strong coupling with the Monsoon rains over the South Asian region. The wet and dry spells, or active-break cycles of the Asian summer monsoon are governed by different modes of intra-seasonal variability with implied northward and westward propagation. Multiple hypotheses exist as to how air-sea interaction and the ocean mixed layer influence the propagation of Monsoon Intra-seasonal Oscillations (MISO), but the multi-scale nature of atmosphere-ocean coupling is not well understood. Multi-country collaborative initiatives MISOBOB (Oceanic Control of Monsoon Intra-seasonal Oscillations in the Tropical Indian Ocean and the Bay of Bengal-USA), RIO-MISO (Role of the Indian Ocean on Monsoon Intra-Seasonal Oscillations-USA), and OMM (Ocean Mixing and Monsoons-India) have led to a combination of ocean observations, atmospheric observations, and associated modeling to study this phenomenon.

We present observations analyzed using the OMNI (Ocean Moored Buoy Network for Northern Indian Ocean) buoy network of India and RAMA 15N mooring along with MISOBOB field program in June 2018, which captured the onset of the 2018 Monsoon from a heavily instrumented ship that simultaneously made measurements in the atmospheric and oceanic boundary layers. The shortwave and net heat fluxes show dramatic changes during the active phase with the in-situ net heat flux reversing sign. The Monsoon onset cooled all of the Central and North Bay of Bengal by 1.5 K, leading to large heat losses in the Bay, as the oceanic surface mixed layer deepened from 20m to about 40m. This talk will also explore the role of sub-surface salinity stratification in modulating cooling of the upper ocean at multiple locations across the Bay, providing a basin-wide view. Observations suggest that the air-sea interaction and ocean stratification in the Bay likely has strong feedback on the organized convection in the atmosphere.

How to cite: Tandon, A., Shroyer, E., Venkatesan, R., Lucas, A., Farrar, J. T., and McPhaden, M.: Capturing the 2018 Monsoon Onset in the Bay of Bengal from in-situ ship and mooring network observations , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3793, https://doi.org/10.5194/egusphere-egu2020-3793, 2020.

A strong Low-Level Jet (LLJ), also known as the Findlater jet, develops over the Arabian Sea during the Indian summer monsoon. This jet is an essential source of moisture for monsoonal rainfall over the densely-populated Indian subcontinent and is a key contributor to the Indian Ocean oceanic productivity by sustaining the western Arabian Sea upwelling systems. The LLJ intensity fluctuates intraseasonally within the ~20- to 90-day band, in relation with the northward-propagating active and break phases of the Indian summer monsoon. Our observational analyses reveal that these large-scale regional convective perturbations  only explain about half of the intraseasonal LLJ variance, the other half being unrelated to large-scale convective perturbations over the Indian Ocean. We show that convective fluctuations in two regions outside the Indian Ocean can remotely force a LLJ intensification, four days later. Enhanced atmosphericdeep convection over the northwestern tropical Pacific yields westerly wind anomalies that propagate westward to the Arabian Sea as baroclinic atmospheric Rossby Waves. Suppressed convection over the eastern Pacific / North American monsoon region yields westerly wind anomalies that propagate eastward to the Indian Ocean as dry baroclinic equatorial Kelvin waves. Those largely independent remote influences jointly explain ~40% of the intraseasonal LLJ variance that is not related to convective perturbations over the Indian Ocean (i.e. ~20% of the total), with the northwestern Pacific contributing twice as much as the eastern Pacific. Taking into account these two remote influences should thus enhance the ability to predict the LLJ.

Related reference: Swathi M.S, Takeshi Izumo, Matthieu Lengaigne, Jérôme Vialard and M.R. Ramesh Kumar:Remote influences on the Indian monsoon Low-Level Jet intraseasonal variations, accepted in Climate Dynamics.

How to cite: Izumo, T., Swathi, M. S., Lengaigne, M., Vialard, J., and Ramesh Kumar, D.: Remote influences on the Indian monsoon Low-Level Jet intraseasonal variations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19411, https://doi.org/10.5194/egusphere-egu2020-19411, 2020.

The interannual-to-decadal variability of heat content and sea level in the South Indian Ocean (SIO) is strongly influenced by its connection with the Pacific and large-scale climatic forcing in the Indo-Pacific region primarily associated with El Niño-Southern Oscillation (ENSO). Besides the advection by the Indonesian Throughflow, signals generated in the Pacific can enter the SIO as coastally trapped Kelvin waves and propagate along the coast of Western Australia. In the southeast tropical and subtropical Indian Ocean, these signals along the eastern boundary can radiate westward as Rossby waves and eventually impact sea level and heat content in the SIO interior and near the western boundary. Local wind forcing, through Ekman pumping over the open ocean and coastal upwelling, is also able to generate Rossby waves and/or modify those emanated from the eastern boundary.

As measured by Argo floats and satellite altimetry, a decade-long increase of the upper-ocean heat content and sea level in the SIO in 2004-2013 ended with a remarkable drop returning to the initial values in 2004. This basin-wide heat release was associated with one of the strongest on record El Niño events in 2014-2016. Surprisingly, the basin-averaged heat content and sea level quickly recovered during the weak La Niña event in 2017-2019. Here we present an analysis of the evolution and mechanisms of 2014-2016 cooling and subsequent warming in the SIO subtropical gyre. We show that the 2014-2016 El Niño did contribute to the reduced heat content in the eastern SIO, while the local wind forcing (via increased Ekman upwelling) largely contributed to the heat reduction in the western SIO. We find no evidence to support that the 2017-2018 warming was forced by the weak La Niña, because the upper-ocean heat content in eastern SIO was still below normal during 2016-2018. The recovery largely occurred in the western SIO due to local wind forcing (via increased Ekman downwelling) primarily associated with changes in the strength of the southeasterly trade winds.

Because sea level is a good proxy for the oceanic heat content in the SIO, we extend our analysis back to 1993 using satellite altimetry records. Using a simple model of wind-forced Rossby waves, we estimate the relative contributions of sea level signals propagating from the eastern boundary, the origin of which is strongly linked to ENSO, and the local wind forcing in the SIO interior to the observed sea level variability. The local wind forcing appears to dominate the sea level (and, hence, the upper-ocean heat content) variability in the western SIO, especially in 2013-2019, while the ENSO-related signals are dominant in the eastern SIO. The local wind forcing over the SIO interior effectively suppressed the cooling associated with the most recent 2014-2016 El Niño event. In contrast, the cooling associated with the strongest on record 1997-1998 El Niño was amplified by the local wind forcing in the basin’s interior.

How to cite: Volkov, D., Rudko, M., and Lee, S.-K.: Dramatic reduction and quick recovery of the South Indian Ocean heat content and sea level in 2014-2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12760, https://doi.org/10.5194/egusphere-egu2020-12760, 2020.

We conducted an onboard measurement of dissolved- and particulate ²³⁴Th in seawater of upper Indian Ocean. The study region covers the meridional section of upper (<500 m depth) Indian Ocean (3⁰N to 15⁰S at 67⁰E in July 2017, and 5⁰S to 13⁰S at 60⁰E and 5⁰S to 24⁰S 67⁰E in April 2018). Dissolved and particulate (>1.2 μm) ²³⁴Th ranged 0.8 – 2.7 dpm L^-1 and 0.05 – 0.7 dpm L^-1, respectively. In July 2017, the large deficiency of dissolved ²³⁴Th were consistently observed at ~50m depth where the subsurface chlorophyll maximum (SCM) present, along the entire section (5⁰S to 13⁰S). After then, the ²³⁴Th/²³⁸U were almost ~1 in ≥100 m depths. In contrast, in April 2018, the significant deficits of dissolved ²³⁴Th were observed in entire upper water columns, 0 – 200m depths. This difference in distribution patterns between two years appears to be related to the annual-/seasonal- variations of SCM patterns. In 2018, SCM were shown in 70 – 80 m depths near equator (5⁰S degree), and gradually deepens in lower latitude (SCM presents in 130 m depths in 24⁰S). Interestingly, the unusually lowest dissolved ²³⁴Th (and very low particulate ²³⁴Th also,) were observed in 5⁰S 60⁰E, near the Seychelles–Chagos thermocline ridge (SCTR) region. There are two hypotheses to explain this extremely lower concentrations of ²³⁴Th. The one is that the large input of lithogenic particles from SCTR, seems to be due to largest ²³⁴Th removal in the water column of extremely shallow area (<300 m of bottom depth). The other is that unusually strong eastward currents (>1 m/s of zonal velocity, based on ADCP observations) can laterally transport the ²³⁴Th. In this presentation, we will also present the preliminary results of vertical export fluxes of some particulate trace elements (Al, Fe, Mn, Cu, Zn, Ni, Pb, and etc.) in the upper Indian Ocean estimated by using this ²³⁴Th tracer.

How to cite: Kim, S. H., Kim, I., and Lee, H.: Latitudinal distributions of 234Th in the upper western Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21824, https://doi.org/10.5194/egusphere-egu2020-21824, 2020.

Surface seawater carbon dioxide was observed from 3 °S to 27 °S along 67 °E of the Indian Ocean in April 2018 and 2019. Partial pressure of CO₂(pCO₂) in the surface seawater and the atmosphere were observed every two minutes using an underway CO2 measurement system (General Oceanics Model 8050) installed on R/V Isabu. Surface water temperature and salinity were measured as well. The pCO₂ was measured using Li-7000 NDIR. Standard gases were measured every 8 hours in five classes with concentrations of 0 µatm, 202 µatm, 350 µatm, 447 µatm, and 359.87 µatm. The fCO₂ of atmosphere remained nearly constant at 387 ± 2 µatm, but the surface seawater fCO₂ peaked at about 3 °S and tended to decrease toward the north and south. The distribution of fCO₂ in surface seawater according to latitude tends to be very similar to that of sea surface temperature. In order to investigate the factors that control the distribution of fCO₂ in surface seawater, we analyzed the sea surface temperature, sea surface salinity, and other factors. The effects of salinity are insignificant, and the surface fCO₂ distribution is mainly controlled by sea surface temperature and other factors that can be represented mainly by biological activity and mixing.

How to cite: Kang, D.-J., Choi, S.-H., Kim, D., and Lee, G.-M.: Meridional Distribution of Surface CO2 along 67°E of the Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20849, https://doi.org/10.5194/egusphere-egu2020-20849, 2020.

Spatial distribution of the chlorophyll-a and phytoplankton community composition related to different water masses were studied during regional cruise in February-March 2006 across the Arabian/Persian Gulf and the Sea of Oman, the marginal seas of the Western Indian Ocean.

Chlorophyll-a concentrations were measured using in vitro method with fluorescence detection and also were assessed as in vivo fluorescence measured by submersible fluorometer. Nearly four hundred species of phytoplankton were enumerated and identified using microscopy in the samples collected at the same stations.

High phytoplankton abundance was associated with diatom-dominated phytoplankton blooms in the central and northwestern part of the Gulf, in the Strait of Hormuz and in the Sea of Oman. The average concentration of in vitro measured surface chlorophyll-a in the studied area was 2.5 mg/m³, with the maximum over 9 mg/m³. The relationships between the concentrations of satellite remotely sensed chlorophyll and in vitro measured chlorophyll-a were found to be mostly in good agreement. The highest concentrations of the surface chlorophyll (> 4 mg/m³) were observed in the areas where diatom-dominated blooms were identified. It was revealed a significant relationship between the phytoplankton composition and water masses indexed by salinity.

The main significance of this study is in the first data set of in vitro measured precise chlorophyll-a concentrations that were obtained along with phytoplankton abundance and taxonomic diversity from the entire region of the Arabian/Persian Gulf and the Sea of Oman. This data set can be used for remote sensing measurements validation and as a baseline for future studies of the biological productivity changes in the Western Indian Ocean.

How to cite: Polikarpov, I., Saburova, M., and Al-Yamani, F.: Chlorophyll and phytoplankton spatial distribution in the Arabian/Persian Gulf and the Sea of Oman, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21350, https://doi.org/10.5194/egusphere-egu2020-21350, 2020.

The Bay of Bengal (BoB) has long stood as a biogeochemical enigma with subsurface waters containing extremely low, but persistent, concentrations of oxygen (O₂) in the nanomolar range which - for some, yet unconstrained reason- are prevented from becoming anoxic. One reason for this may be
the low productivity of the BoB waters due to nutrient limitation, and the resulting lack of respiration of organic material at intermediate waters. Thus, the parameters determining primary production are key to understanding what prevents the BoB from developing anoxia. Primary productivity in the sunlit surface layers of tropical oceans is mostly limited by the supply of reactive nitrogen through upwelling, riverine flux, atmospheric deposition, and biological dinitrogen (N₂) fixation. In the BoB, a stable stratification limits nutrient supply via upwelling in the open waters, and riverine or atmospheric fluxes have been shown to support only less than one quarter of the nitrogen for primary production. This leaves a large uncertainty for most of the BoB’s nitrogen input, suggesting a potential role of N₂ fixation in those waters.
Here, we present a survey of N₂ fixation and carbon fixation in the BoB during the winter monsoon season. We detected a community of N₂ fixers comparable to other OMZ regions, with only a few cyanobacterial clades and a broad diversity of non-phototrophic N₂ fixers present throughout the water column (samples collected between 10 m and 560 m water depth). While similar communities of N₂ fixers were shown to actively fix N₂ in other OMZs, N₂ fixation rates were below the detection limit in our samples covering the water column between the deep chlorophyll maximum and the OMZ.
Consistent with this, no N₂ fixation signal was visible in δ¹⁵N signatures. We suggest that the absence of N₂ fixation may be a consequence of a micronutrient limitation or of an O₂ sensitivity of the OMZ diazotrophs in the BoB. To explore how the onset of N₂ fixation by cyanobacteria compared to nonphototrophic N₂ fixers would impact on OMZ O₂ concentrations, a simple model exercise was carried out. We observed that both, photic zone-based and OMZ-based N₂ fixation are very sensitive to even minimal changes in water column stratification, with stronger mixing increasing organic matter production and export, which would exhaust remaining O₂ traces in the BoB.       

How to cite: Löscher, C., Mohr, W., Bange, H., and Canfield, D.: No N2 fixation in the Bay of Bengal?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19117, https://doi.org/10.5194/egusphere-egu2020-19117, 2020.

Subantarctic Mode Water (SAMW) is formed by deep mixing in winter in the Subantarctic Zone and transported into the adjacent subtropical gyres after subduction, which plays a vital role in heat, freshwater, carbon and nutrient budgets in the global oceans. The changes in SAMW properties and its impact on spiciness variation in the southern Indian Ocean have been investigated using the gridded Argo dataset in 2004-2018. Annual mean potential temperature and salinity of the SAMW have undergone significant variations during 2004-2018, with an increase (a decrease) trend for potential temperature (salinity). An analysis of decomposition shows that the heaving process contributes to warming and salinification while spiciness causes cooling and freshening, both of which modulate the SAMW properties. A strong deepening of the isopycnal surfaces caused by positive wind stress curl anomalies over the subtropical southern Indian Ocean leads to warming/salinification heaving contribution to the changes in SAMW. The cooling/freshening contribution from spiciness process is due to a southward shift of sea surface potential density favoring colder and fresher water into the interior ocean, which is driven by an increase in wintertime sea surface temperature and salinity in the SAMW formation region. The colder and fresher water carried with the SAMW spreads along isopycnal surfaces via the Indian Ocean subtropical gyre, which results in cooling and freshening spiciness trends over the all basin of the subtropical southern Indian Ocean.

How to cite: Zhang, Y., Du, Y., and Feng, M.: Changes in Subantartic Mode Water Properties and its Impact on Spiciness Variation in the Southern Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21228, https://doi.org/10.5194/egusphere-egu2020-21228, 2020.

An ocean downscaling model product, forced under the RCP8.5 future climate change scenario, has been used to understand the ocean heat balance of the Indian Ocean in a warming climate. Towards the end of the 21th century, the model simulates a significant reduction of Indonesian Throughflow (ITF) transport, which reduces the Pacific to Indian Ocean heat transport by 0.20 PW; whereas across S in the southern Indian Ocean (SIO), the southward heat transport is reduced by 0.28 PW, mainly contributed from the weakening western boundary current, the Agulhas Current (0.21 PW). The projected weakening of the Agulhas Current is to compensate for the reduction of the ITF transport, with additional contribution from the spin-down of the SIO subtropical gyre. Thus, being amplified by the ocean circulation changes in the SIO, the projected Indian Ocean warming trend is much faster than the direct air-sea heat flux input.

How to cite: Ma, J., Feng, M., Lan, J., and Hu, D.: Projected future changes of meridional heat transport and heat balance of the Indian Ocean , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11975, https://doi.org/10.5194/egusphere-egu2020-11975, 2020.

The southern tip of Africa is the gateway between the Indian and Atlantic oceans, one of the most widely recognized chokepoints of the meridional overturning circulation (MOC). The oceanic circulation in this region connects a western boundary current, the Agulhas Current, to an eastern boundary current, the Benguela Current, a connection not replicated elsewhere and quite important, not only because of its peculiarity, but also because of its role in the MOC. During the last few decades numerous international research programs have collected large amounts of oceanographic data of this region. All these efforts, however, have been largely focused on the deep-ocean, leaving the coastal region practically unattended. In this presentation we will use the results of a suite of process-oriented numerical experiments to discuss the circulation along the Agulhas Bank (AB)—the shelf region sandwiched between the eastern and western margins of the African continent; in particular to illustrate its connections and interactions with the deep-ocean region. As we shall show these shelf/deep-ocean interactions, are not only important to the shelf but also to the Indian/Atlantic interoceanic exchange.

How to cite: Matano, R.: The Agulhas Bank Circulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3153, https://doi.org/10.5194/egusphere-egu2020-3153, 2020.

The Agulhas Leakage is considered to influence the Atlantic Meridional Overturning Circulation and the climate system via transport of saltier and warmer water masses from the Indian Ocean into the Atlantic Ocean (Laxenaire et al. 2018). Therefore, reconstructing the time of the possible onset of the Agulhas Leakage will allow an improvement of global palaeo-climate models. Since the Agulhas Leakage is known to disperse the Indian Ocean biota into the Atlantic Ocean, Caley et al. (2012) proposed that the Agulhas Leakage exists since about 1.3Ma.

Here, we provide new evidence for an early Pleistocene existence of the Agulhas Leakage by comparing the test size evolution of G. menardii between the tropical eastern Atlantic Ocean ODP Site 667 and the Indian Ocean IODP Site 1476, which is located in the Mozambique Channel.

At Site 667 and parallel to a climate cooling trend, we observe a test size decrease from a maximal axial length (max δY) of 875µm during the Mid-Pliocene Warmth Period (3.2Ma) to a maximal axial length of 520µm by the end of Pliocene (ca. 2.6Ma). This trend is followed by a relatively rapid test size increase until ca. 2.1Ma, during which the size more than doubles (max δY = ca. 1200µm). This pattern in the test size evolution of G. menardii was also observed in the western tropical Atlantic Ocean (Knappertsbusch 2016).

In the Mozambique Channel, we do not observe a decrease of the test size at the end of the Pliocene. The values stay almost stable throughout the Pliocene (max δY = ca. 900µm) until 2.3Ma. Between 2.3 and 2Ma, the maximal test size increases to a value very similar to that observed in the eastern tropical Atlantic (max δY = ca. 1250µm).

It has been observed that relatively large G. menardii specimens occurred in the Mozambique Channel, while the Atlantic only harboured relatively small specimens during the late Pliocene and earliest Pleistocene, and that both localities show a similar test size at ca. 2Ma. This suggests the possibility of a dispersal of the Indian Ocean giant G. menardii into the Atlantic between 2.3 and 2Ma, probably via a strengthening Agulhas Leakage.

Caley, T., Jiraudeau, J., Malaizé, B., Rossignol, L. & Pierre, C. (2012), ‘Agulhas leakage as a key process in the modes of Quaternary climate changes’, PNAS 109(18), 6835–6839.

Knappertsbusch, M. W. (2016), ‘Evolutionary prospection in the Neogene planktic foraminifer Globorotalia menardii and related forms from ODP Hole 925B (Ceara Rise, western tropical Atlantic): evidence for gradual evolution superimposed by long distance dispersal?’, Swiss Journal of Palaeontology.

Laxenaire, R., Speich, S., Blanke, B., Chaigneau, A., Pegliasco, C. & Stegner, A. (2018), ‘Anticyclonic Eddies Connecting the Western Boundaries of Indian and Atlantic Oceans’, Journal of Geophysical Research: Oceans 123, 7651–7677.

How to cite: Friesenhagen, T. and Knappertsbusch, M.: New Micropalaeontological Evidence for an Early Pleistocene Existence of the Agulhas Leakage, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3889, https://doi.org/10.5194/egusphere-egu2020-3889, 2020.