OS1.5

Coastal Upwelling is a phenomenon in which cold and nutrient-enriched water from the Ekman layers reaches the surface enhancing the biological productivity of the upwelling region. In this work, an attempt is made to understand the influence of coastal upwelling on surface current variations during May 2018 to August 2018, when HF radar current observation (source: NIOT, India) is available. The wind-based Upwelling Index(UI_wind) showed coastal upwelling throughout the study period. But the SST based upwelling index (UI_sst) showed upwelling occurred only from May to the first week of June. Cross-shore components of HF radar-derived ocean surface current (CSSC)  showed strong similarity with UI_sst. The first phase of upwelling from UI_sst is observed to start on 5^th May and lasts till 14^th May with a maximum peak on around 10^th May and having a horizontal extension of ~40 km. Then, there is a break period for about three days and after that, the second phase of upwelling starts on 17^th May and lasts till 25^th May with a maximum peak on around 20^th May, but this time the horizontal extension is ~100 km which is much larger than during the first phase. A strong positive (from coast to offshore) CSSC is observed to start on around 5^th May and lasts till 13^th May with a maximum peak on around 10^th May and having a horizontal extension of ~40 km, as observed from UIsst. A reversal of CSSC (towards coast) is noted on 14^th May when the break of coastal upwelling is evident from UI_sst. The CSSC then again started intensifying 15^th May onwards and continued for ten days till 25^th May, similar to UI_sst.  The horizontal extension of the upwelling signature in the second phase of upwelling is ~70 km. Therefore, a 7-10 days of the coastal upwelling and its horizontal extension are identified in this study. This study suggests the use of high resolution (~6 km) HF radar current observation on the monitoring of coastal upwelling processes.

How to cite: Dey, S., Sil, S., and Mandal, S.: Influence of coastal upwelling on the surface current in the Bay of Bengal using HF radar and satellite observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15747, https://doi.org/10.5194/egusphere-egu21-15747, 2021.

In July 2016, a Seaglider equipped with a microstructure sensor system was deployed in the southern Bay of Bengal at 7° 54.0′ N, 89° 4.5′ E.  162 profiles (of which 146 were to 1000 m) of microstructure shear and temperature were collected as a time series at the same location.  Dissipation is calculated independently from both shear and temperature.  The time-average profile shows high dissipation (nearly 1×10^-5 W kg^-1) near the surface, dropping rapidly over the uppermost 50 m to ~1×10^-7 W kg^-1, followed by a more gradual decrease to ~5×10^-10 W kg^-1 at 300m.  A band of slightly higher dissipation around 500 m (~8×10^-10 W kg^-1) could facilitate an increased vertical flux of nutrients, heat, salinity, etc at these depths.  From 600 to 1000 m dissipation remains roughly constant at ~1×10^-10 W kg^-1.  Variability of the near surface dissipation in response to atmospheric forcing is also discussed.

How to cite: Damerell, G., Sheehan, P., Hall, R., Matthews, A., and Heywood, K.: Dissipation in the Bay of Bengal from a Seaglider, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1295, https://doi.org/10.5194/egusphere-egu21-1295, 2021.

Mesoscale eddies, coherent rotating structure with typical horizontal scale of ~100 km and temporal scales of a month, play a significant role in ocean energy and mass transports. Here both mesoscale cyclonic and anticyclonic eddies moving towards south in the northern Bay of Bengal during 20^th March 2017 to 20^th May 2017 are observed using a high resolution (~5 km) nitrogen-based nutrient, phytoplankton, zooplankton, and detritus (NPZD) ecological model embedded with Regional Ocean Modeling System (ROMS). Spatial maps of sea surface height anomaly (SSHA) from satellite-derived Archiving Validation, and Interpretation of Satellite Oceanographic (AVISO), and model are well matched. The centers and effective radii of both kind of eddies are identified using SSHA to proceed for their three-dimensional analysis. The extreme intensities of cyclonic and anticyclonic eddy centers are observed on 8^th April 2017 at 86.40°E, 18.19°N and 84.80°E, 16.52°N respectively. Both kind of eddies are vertically extended upto 800 m and have radius ~100 km at surface. At these two locations, time-depth variations of zonal and meridional currents, and other physical (temperature and salinity) and bio-physical (chlorophyll-a, phytoplankton, zooplankton, detritus nutrient, dissolved oxygen and NO₃ nutrient) parameters are studied particularly from 8^th March 2017 to 8^th May 2017. Further vertical distribution of zonal and meridional currents, and other parameters are studied along the eddy diameters at their extreme intensity. In the vertical structure of both current components, an opposite sense between cyclonic and anticyclonic eddies are clearly captured, while other variables show strong upwelling and downwelling nature around the cyclonic and anticyclonic eddy centers respectively. Abundances (scarcities) of chlorophyll-a, phytoplankton, zooplankton and detritus nutrient are observed at 50 – 150 m depth of the cyclonic (anticyclonic) eddy center. The concentration of chlorophyll-a, phytoplankton, zooplankton and detritus nutrient reach to maximum of 1 mg/m³, 0.35 mMol/m³, 0.22 mMol/m³ and 0.14 mMol/m³ at ~80 m depth for the cyclonic eddy, while these are absent for the anticyclonic eddy.

How to cite: Shee, A., Pramanik, S., Sil, S., and Das, S.: Vertical Analysis of Mesoscale Eddies in the Northern Bay of Bengal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14983, https://doi.org/10.5194/egusphere-egu21-14983, 2021.

The breakthrough in our knowledge of ocean eddies came with the results of the POLYGON-67 experiment in the central Indian Ocean carried out in January-April 1967 (see Koshlyakov et al, 2016). It was the first direct and unambiguous observation that proved an earlier hypothesis by V. B. Shtockman of the existence of mesoscale eddies in open ocean, not only next to strong jet-stream currents. Now it is well known that the currents in open ocean are almost everywhere dominated by meso-scale eddies also known as synoptic eddies (Robinson, 1983). POLYGON-67 experiment covered a rectangle bounded by 10-15°N and 63-66.5°E. The purpose of this work is to analyse the seasonal variability of meso-scale eddy activity in the area covered by POLYGON-67 using a modern and comprehensive data set produced by an operational data assimilation model over a period from 1998 to 2017.

The 20-year long eddy resolving reanalysis of velocity fields in the Indian Ocean allows the study of seasonal variability, dynamics and generating mechanisms of eddy kinetic energy (EKE) in the tropical Indian Ocean, including the area covered by the original survey of POLYGON-67. In contrast to some other areas of the World Ocean, the EKE seasonality shows two maxima, the large one in April and the secondary one in October. The main mechanism of EKE generation is the barotropic instability which is evidenced by high correlation between EKE and enstrophy of large-scale currents, representing the strength of horizontal shear. It is found that the main contributor to the EKE variability within POLYGON-67 area is the advection of EKE across the boundaries during January-October, while the local generation has a comparable magnitude during August-December. The direction and strength of surface currents is consistent with the monsoon wind pattern in the area.

References

Koshlyakov, M.N., Morozov, E.G., and Neiman, V.G., 2016. Historical findings of the Russian physical oceanographers in the Indian Ocean. Geoscience Letters, 3:19; doi:10.1186/s40562-016-0051-6

Robinson, A.R. (Ed), 1983. Eddies in Marine Science. Springer, ISBN 978-3-642-69003-7, 612p.

How to cite: Shapiro, G. I. and Gonzalez-Ondina, J. M.: Generation and seasonal variability of eddy kinetic energy in the central Indian Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2157, https://doi.org/10.5194/egusphere-egu21-2157, 2021.

The Great Whirl (GW) is a quasi-permanent anticyclonic eddy that appears every summer monsoon in the western Arabian Sea off the horn of Africa. It generally forms in June, peaks in July-August, and dissipates afterward. While the annual cycle of the GW has been previously described, its year-to-year variability has been less explored. Satellite observations reveal that the leading mode of summer interannual sea-level variability in this region is associated with a typically ~100-km northward or southward shift of the GW. This shift is associated with coherent sea surface temperature and surface chlorophyll signals, with warmer SST and reduced marine primary productivity in regions with positive sea level anomalies and vice versa. Eddy-permitting (~25 km) and eddy-resolving (~10 km) ocean general circulation model simulations reproduce the observed pattern reasonably well, even in the absence of interannual variations in the surface forcing. This implies that the GW interannual variability partly arises from oceanic internal instabilities. Ensemble oceanic simulations further reveal that this stochastic oceanic intrinsic variability and the deterministic response to wind forcing each contribute to ~50% of the total GW interannual variability in July-August. The deterministic part appears to be related to the oceanic response  to Somalia alongshore wind stress and offshore wind-stress curl variations during the monsoon onset projecting onto the GW structure, and getting amplified by oceanic instabilities. After August, the stochastic component dominates the GW variability.

How to cite: Sadhvi, K., Suresh, I., Izumo, T., Vialard, J., Lengaigne, M., Penduff, T., and Molines, J.-M.: The role of oceanic internal instabilities on the Great Whirl interannual variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11170, https://doi.org/10.5194/egusphere-egu21-11170, 2021.

Atmospheric convectively coupled equatorial Kelvin waves (CCKWs) are a major tropical weather feature strongly influenced by ocean--atmosphere interactions. However, prediction of the development and propagation of CCKWs remains a challenge for models. The physical processes involved in these interactions are assessed by investigating the oceanic response to the passage of CCKWs across the eastern Indian Ocean and MC using the NEMO ocean model analysis with data assimilation. Three-dimensional life cycles are constructed for "solitary" CCKW events. As a CCKW propagates over the eastern Indian Ocean, the immediate thermodynamic ocean response includes cooling of the ocean surface and subsurface, deepening of the mixed layer depth, and an increase in the mixed layer heat content. Additionally, a dynamical downwelling signal is observed two days after the peak in the CCKW westerly wind burst, which propagates eastward along the Equator and then follows the Sumatra and Java coasts, consistent with a downwelling oceanic Kelvin wave with an average phase speed of 2.3 m s^-1. Meridional and vertical structures of zonal velocity anomalies are consistent with this framework. This dynamical feature is consistent across distinct CCKW populations, indicating the importance of CCKWs as a source of oceanic Kelvin waves in the eastern Indian Ocean. The subsurface dynamical response to the CCKWs is identifiable up to 11 days after the forcing. These ocean feedbacks on time scales longer than the CCKW life cycle help elucidate how locally driven processes can rectify onto longer time-scale processes in the coupled ocean--atmosphere system.

How to cite: Azaneu, M., Matthews, A., and Baranowski, D.: Subsurface oceanic structure associated with atmospheric convectively coupled equatorial Kelvin waves in the eastern Indian Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-984, https://doi.org/10.5194/egusphere-egu21-984, 2021.

Regional Ocean Model System (ROMS) has been simulated for the Sunda Strait, the Java Sea, and the Indian Ocean. The simulation was undertaken for thirteen months of data period (August 2013 – August 2014). However, we only used four months period for validation, namely September – December 2013. The input data involved the HYbrid Coordinate Ocean Model (HYCOM) ocean model output by considering atmospheric forcing from the European Centre for Medium-Range Weather Forecasts (ECMWF), without and with tides forcing from TPXO and rivers. The output included vertical profile temperature and salinity, sea surface temperature (SST), seas surface height (SSH), zonal (u), and meridional (v) velocity. We compared the model SST to satellite SST in time series, SSH to tides gauges data in time series, the model u and v component velocity to High Frequency (HF) radial velocity. The vertical profile temperature and salinity were compared to Argo float data and XBT. Besides, we validated the amplitude and phase of the ROMS seas surface height to amplitude and phase of the tides-gauges, including four constituents (M2, S2, K1, O1).

How to cite: Mujiasih, S., Beckers, J.-M., and Barth, A.: Implementation and Validating of the Regional Ocean Model System (ROMS) for the Sunda Strait connecting the Java Sea to the Indian Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3457, https://doi.org/10.5194/egusphere-egu21-3457, 2021.

South East Asia seas, that include the South China Sea and the Indonesian Seas, transfer the warm and light waters of the surface branch of the global thermohaline circulation between the Pacific and Indian Oceans. To better understand the key contribution of South East Asia seas in the regional and global climate and ocean circulation, it is therefore essential to improve our knowledge of the functioning and variability at different scales of water, heat and salt budgets over this region. The complex topography of this region makes it difficult to study those budgets based on in-situ measurements only. Numerical studies are necessary and relevant to complement and interpret those measurements, however until now, most of numerical studies were performed at low resolution and/or on short periods.

To better quantify and understand the contributions of ocean, rivers and atmosphere to the variability at different scales of the water, heat and salt budgets over South East Asia seas, high resolution configurations (< 5 km) of the SYMPHONIE ocean model are developed over the area. State of the art datasets available from COPERNICUS and ECMWF are used to prescribe boundary conditions. Each term of the budgets is computed online in order to obtain rigorously closed budgets.

This methodology applied on the 2009-2018 period, that includes strong El Niño and La Niña years as well as neutral years, allows us to better characterize the seasonal to interannual variability of water, salt and heat budgets over the South East Asia seas, by quantifying and explaining the contribution of each factor (lateral fluxes, surface fluxes, rivers, internal variations, ENSO). We examine in particular the surface salinification of the South China Sea that was observed by previous authors between 2012 and 2016 (Zeng et al. 2018, doi:10.1002/2017GL076574) : our simulations suggest that it is mostly related to an increase of net lateral water influx at Luzon strait, itself induced by a deficit of precipitation over the region, rather than to an increase of the salinity of the inflowing water. We finally also explore the role of tides and mesoscale processes. This methodology, our key results and the future steps of this work, that include the on-going development of an ocean-atmosphere regional coupled model, will be synthetically summarized.

How to cite: Herrmann, M., Trinh Bich, N., Ulses, C., Marsaleix, P., Duhaut, T., To Duy, T., and Ouillon, S.: Water, heat and salt budgets over South East Asia seas : a high resolution modeling approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7991, https://doi.org/10.5194/egusphere-egu21-7991, 2021.

Using the Gauss–Markov decomposition method, this study investigates the mean structure and seasonal variation of the tropical gyre in the Indian Ocean based on the observations of surface drifters. In the climatological mean, the clockwise tropical gyre consists of the equatorial Wyrtki Jets (WJs), the South Equatorial Current (SEC), and the eastern and western boundary currents. This gyre system redistributes the water mass over the entire tropical Indian Ocean basin. Its variations are associated with the monsoon transitions, featuring a typical clockwise pattern in the boreal spring and fall seasons. The relative importance of the geostrophic and Ekman components of the surface currents as well as the role of eddy activity were further examined. It was found that the geostrophic component dominates the overall features of the tropical gyre, including the SEC meandering, the broad eastern boundary current, and the axes of the WJs in boreal spring and fall, whereas the Ekman component strengthens the intensity of the WJs and SEC. Eddies are active over the southeastern tropical Indian Ocean and transport a warm and fresh water mass westward, with direct impact on the southern branch of the tropical gyre. In particular, the trajectories of drifters reveal that during strong Indian Ocean Dipole or El Niño-Southern Oscillation events, long-lived eddies were able to reach the southwestern Indian Ocean with a moving speed close to that of the first baroclinic Rossby waves.

How to cite: Wu, W., Du, Y., Qian, Y.-K., Cheng, X., Wang, T., Zhang, L., and Peng, S.: Structure and Seasonal Variation of the Indian Ocean Tropical Gyre Based on Surface Drifters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6871, https://doi.org/10.5194/egusphere-egu21-6871, 2021.

The climatologically averaged sea surface height anomaly (SSHA) during the summer monsoon in the Bay of Bengal (BoB) shows two prominent negative anomalies, one in the southern BoB and another in the northern BoB. The occurrence of negative SSHA observed in the southern BoB has been extensively studied and is linked to Sri Lanka Dome (SLD), whereas negative SSHA observed in the north has received less attention. A pronounced thermal dome develops in the northern BoB with its mean position between 86-89^oE and 16-19^oN, as shown by the doming of isotherms. We refer to this oceanic thermal dome as the northern BoB Dome (NBD). The present study focuses on the evolution of the NBD using observation and a coupled OGCM-biogeochemical model. The formation of NBD occurs during the summer monsoon (May - September), at a time when the wind stress curl is positive. Interestingly, the cyclonic curl is positive in the entire northern BoB, yet the negative SSHA is confined to a small region. Our analysis shows that strong stratification in the northern BoB inhibits the entrainment of the cooler-nutrient-rich subsurface waters to the surface during the event of dome formation. Consequently, the mixed-layer temperature in the NBoB region stays above the temperature criteria for active convection (>28 ^oC). Further, the inhibition of entrainment of nutrients causes the NBD region to be lower in productivity than the SLD region, as seen in chlorophyll distribution. We compare the NBD's heat and nutrient budget with the SLD and show that the near-surface stratification differences make the two domes distinct from each other.

How to cite: Nambiathody, A., Vijayakumaran, V., and Chatterjee, A.: On the evolution of the northern Bay of Bengal Dome., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11200, https://doi.org/10.5194/egusphere-egu21-11200, 2021.

The first baroclinic mode Rossby wave is known to be of critical importance to the annual sea level variability in the southern tropical Indian Ocean (STIO; 0°–20°S, 50°–115°E). In this study, an analysis of continuously stratified linear ocean model reveals that the second baroclinic mode also has significant contribution to the annual sea level variability (as high as 81% of the first baroclinic mode). The contributions of residual high-order modes (3 # n # 25) are much less. The superposition of low-order (first and second) baroclinic Rossby waves (BRWs) primarily contribute to the high energy center of sea level variability at ;108S in the STIO and the vertical energy penetration below the seasonal thermocline. We have found that 1) the low-order BRWs, having longer zonal wavelengths and weaker damping, can couple more efficiently to the local large-scale wind forcing than the high-order modes and 2) the zonal coherency of the Ekman pumping results in the latitudinal energy maximum of low-order BRWs. Overall, this study extends the traditional analysis to suggest the characteristics of the second baroclinic mode need to be taken into account in interpreting the annual variability in the STIO.

How to cite: Huang, K.: Characteristics and mechanism of annual sea level variability in the southern tropical  Indian Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8619, https://doi.org/10.5194/egusphere-egu21-8619, 2021.

The Seychelles-Chagos Thermocline Ridge (SCTR) in the western tropical Indian Ocean is known as a region of off-equatorial upwelling contrasting to equatorial upwelling in the Pacific and Atlantic where the most wide open-ocean upwelling occurs corresponding to ascending branch of one of the meridional overturning cells in the Indian Ocean, yet detailed stratification, upwelling intensity, and dynamics of SCTR upwelling variability are still poorly understood. Here, we present observational results on the SCTR upwelling based on ship-based data collected during April-May 2019 as a part of the Korea-US inDian Ocean Scientific Research Program (KUDOS). The upwelling structure is confirmed from 20 ℃ and 10 ℃ isotherms (D20 and D10) shoaling up in the center of SCTR, from 200 m to 100 m (D20) and from 600 m to 400 m (D10), respectively. Horizonal divergence at the upper 250 m within an 1° by 1° area in the SCTR center (8 °S, 61 °E) estimated from currents measurements along the boundaries (1.0 x 10^-3 Sv) supports a mean upwelling intensity of 7.0 x 10^-3 m day^-1 (1.0 x 10^-3 Sv divided by the area). The upwelling intensity generally decreases with depth but shows multiple peaks within the upper water column, yielding the maximum peak (5.0 x 10^-2 m day^-1) at 60 m and the minimum peak (1.4 x 10^-2 m day^-1) at 230 m, with negative peaks (downwelling) at depths around 100 and 210 m. Our results on the observed structure and intensity of SCTR upwelling are discussed in comparison to time-varying local wind stress curl-driven Ekman pumping, D20-based Seychelles Upwelling Index (SUI), and Indian Dipole Mode Index (DMI). Detailed observations on the structure and intensity of SCTR upwelling presented here have important implications on time-varying SCTR upwelling (e.g., weakened upwelling peaked in fall 2019) and climate via meridional overturning circulation in the upper Indian Ocean.

How to cite: Noh, S. and Nam, S.: Observations on Seychelles-Chagos Thermocline Ridge (SCTR) upwelling during April-May 2019 in the western tropical Indian Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14203, https://doi.org/10.5194/egusphere-egu21-14203, 2021.

Regions of salinity maxima (S_max) in the world oceans experience spiciness changes which in-turn subduct and advect towards equator along the shallow meridional overturning cell. Advection results in stabilizing vertical salinity gradient (salinity increasing with depth) along westward and equatorward edges of S_max region whereas eastward and poleward edges holds destabilizing vertical salinity gradient with maximum salinity at surface (salinity decreasing with depth). Based on this contrast vertical salinity gradient, subtropical south Indian Ocean salinity maxima region is divided into two boxes along 30ºS. Seasonal evolultion of spiciness with respect to atmospheric and oceanic forcings, are investigated by using high frequency (3-day), high resolution (0.25º) ECCO (Estimating the Circulation and Climate of the Ocean) estimate. It is observed that in both these boxes, spice generation mechanisms are different. During austral winter, 25-sigma isopycnal outcrops to the north of 30S (northern box), along which the Temperature/Salinity changes subduct and hence spiciness anomalies are formed below the mixed layer. However, destabilization of vertical salinity gradient to the south of 30S (southern box), concomitant with weak stratification results in convective mixing at the mixed layer base and hence the spiciness changes penetrate the main thermocline. Seasonal mixed layer heat and salt budget analysis show that the surface heat and freshwater fluxes are the main forcings controlling monthly evolution of spiciness in the northern box, whereas in the southern box entrainment and meridional advection terms are mainly contributing for the spiciness changes.

How to cite: Kaundal, M., Nadimpalli, J., and Dash, M.: Seasonal contribution to the spice generation in subtropical south Indian salinity maxima, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1537, https://doi.org/10.5194/egusphere-egu21-1537, 2021.

An atmospheric channel with the monsoon circulation system and the Walker circulation and an ocean channel with Indonesian through-flow, connect the tropical Indian Ocean and the Pacific, which strongly modulate the Indo-Pacific climate change on different time scales. The atmospheric channel transports 0.35 Sv water vapor from the Indian Ocean to the Pacific on the mean state, while the Indonesia throughflow transports ~15 Sv of freshwater from the western Pacific to the Indian Ocean. These two aspects of freshwater transportation play an important role in maintaining the salinity balance in the tropical Indian Ocean (TIO). On the interannual-decadal time scale, a sea surface salinity dipole mode has been revealed in the tropical Indian Ocean (S-IOD) with salinity anomalies in the central equator and the southeastern TIO is opposite, corresponding to significant wind anomaly along the equator and precipitation and thermocline depth anomalies in the southeastern TIO. The ocean advection forced by wind anomalies along the equator and precipitation and thermocline depth anomalies in the southeastern TIO dominating the SSS variations of the S-IOD, respectively. The modulation of the Indo-Pacific Walker Circulation and its related ocean wave processes transported from the western Pacific through the waveguide in the Indonesian Seas are main factors for the development of S-IOD and its variability, which is forced by the Interdecadal Pacific Oscillation (IPO). Further analyses indicate that the long-term trend of SSS in the global ocean with the salty regions getting saltier and fresh regions getting fresher is modulated by the internal variability associated with the IPO, with the most significant regions in the western tropical Pacific and the southeastern Indian Ocean. Specifically, the IPO leads to a ~40% offset of SSS radiative-forced trend in the western tropical Pacific and ~170% enhancement of the trend in the southeastern Indian Ocean since the mid-20th century.      

How to cite: Zhang, Y., Du, Y., and Sun, Q.: Sea surface salinity dipole mode in the tropical Indian Ocean and its relationship with Indo-Pacific climate , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15293, https://doi.org/10.5194/egusphere-egu21-15293, 2021.

This study identifies a new triggering mechanism of the Indian Ocean Dipole (IOD) from the Southern Hemisphere. This mechanism is independent from the El Niño/Southern Oscillation (ENSO) and tends to induce the IOD before its canonical peak season. The joint effects of this mechanism and ENSO may explain different lifetimes and strengths of the IOD. During its positive phase, development of sea surface temperature cold anomalies commences in the southern Indian Ocean, accompanied by an anomalous subtropical high system and anomalous southeasterly winds. The eastward movement of these anomalies enhances the monsoon off Sumatra-Java during May-August, leading to an early positive IOD onset. The pressure variability in the subtropical area is related with the Southern Annular Mode, suggesting a teleconnection between high-latitude and mid-latitude climate that can further affect the tropics. To include the subtropical signals may help model prediction of the IOD event.

How to cite: Zhang, L.-Y., Du, Y., Cai, W., Chen, Z., Tozuka, T., and Yu, J.-Y.: Triggering the Indian Ocean Dipole from the Southern Hemisphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3655, https://doi.org/10.5194/egusphere-egu21-3655, 2021.

The 2019 positive Indian Ocean Dipole (IOD) was the strongest event since the 1960s which developed independently without coinciding El Niño. The dynamics is not fully understood. Here we show that in March-May, westward propagating oceanic Rossby waves, a remnant consequence of the weak 2018 Pacific warm condition, led to anomalous sea surface temperature warming in the southwest tropical Indian Ocean (TIO), inducing deep convection and anomalous easterly winds along the equator, which triggered the initial cooling in the east. In June-August, the easterly wind anomalies continued to evolve through ocean-atmosphere coupling involving Bjerknes feedback and equatorial nonlinear ocean advection, until its maturity in September-November. This study clarifies the contribution of oceanic Rossby waves in the south TIO in different dynamic settings and reveals a new triggering mechanism for extreme IOD events that will help to understand IOD diversity.

How to cite: Du, Y., Zhang, Y., Zhang, L.-Y., Tozuka, T., and Cai, W.: Thermocline warming induced extreme Indian Ocean Dipole in 2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3639, https://doi.org/10.5194/egusphere-egu21-3639, 2021.

The Indian Ocean Dipole (IOD) is one of the dominant modes of variability of the tropical Indian Ocean and it has been suggested to have a crucial role in the teleconnection between the Indian summer monsoon and El Nino Southern Oscillation (ENSO). The main ideas at the base of the influence of the IOD on the ENSO-monsoon teleconnection include the possibility that it may strengthen summer rainfall over India, as well as the opposite, and also that it may produce a remote forcing on ENSO itself. The Indian Ocean has been experiencing a warming, larger than any other basins, since the 1950s. During these decades, the summer monsoon rainfall over India decreased and the frequency of Indian Ocean Dipole (IOD) events increased. In the future the IOD is projected to further increase in frequency and amplitude with mean conditions mimicking the characteristics of its positive phase. Still, state of the art global climate models have large biases in representing IOD and monsoon mean state and variability, with potential consequences for properties and related teleconnections projected in the future. This works collects a review study of the influence of the IOD on the ISM and its relationship with ENSO, as well as new results on IOD projections comparing CMIP5 and CMIP6 models.

How to cite: Cherchi, A., Terray, P., Ratna, S. B., Meccia, V., and K.P., S.: On the relationship between Indian Ocean Dipole, Indian summer monsoon and ENSO, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14562, https://doi.org/10.5194/egusphere-egu21-14562, 2021.

Today, precipitation and wind patterns over the equatorial Indian Ocean and surrounding lands are paced by monsoon and Walker circulations that are controlled by the seasonal land-sea temperature contrast and the inter-annual convection over the Indo-Pacific Warm Pool, respectively. The annual mean surface westerly winds are particularly tied to the Walker circulation, showing interannual variability coupled with the gradient of Sea Surface Temperature (SST) anomaly between the tropical western and southeastern Indian Ocean, namely, the Indian Ocean Dipole (IOD). While the Indian monsoon pattern has been widely studied in the past, few works deal with the evolution of Walker circulation despite its crucial impacts on modern and future tropical climate systems. Here, we reconstruct the long-term westerly (summer) and easterly (winter) wind dynamics of the equatorial Indian Ocean (10°S−10°N), since the Last Glacial Maximum (LGM) based on i) primary productivity (PP) records derived from coccolith analyses of sedimentary cores MD77-191 and BAR94-24, retrieved off the southern tip of India and off the northwestern tip of Sumatra, respectively and ii) the calculation of a sea surface temperature (SST) anomaly gradient off (south) western Sumatra based on published SST data. We compare these reconstructions with atmospheric circulation simulations obtained with the general coupled model AWI-ESM-1-1-LR (Alfred Wegener Institute Earth System Model).

Our results show that the Indian Ocean Walker circulation was weaker during the LGM and the early/middle Holocene than present. Model simulations suggest that this is due to anomalous easterlies over the eastern Indian Ocean. The LGM mean circulation state may have been comparable to the year 1997 with a positive IOD, when anomalously strong equatorial easterlies prevailed in winter. The early/mid Holocene mean circulation state may have been equivalent to the year 2006 with a positive IOD, when anomalously strong southeasterlies prevailed over Java-Sumatra in summer. The deglaciation can be seen as a transient period between these two positive IOD-like mean states.

How to cite: Zhou, X., Duchamp-Alphonse, S., Kageyama, M., Bassinot, F., Shi, X., Beaufort, L., and Lohmann, G.: Variations of the Indian Ocean Walker circulation since the Last Glacial Maximum revealed by reconstructed and simulated zonal wind intensity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12762, https://doi.org/10.5194/egusphere-egu21-12762, 2021.

The Bay of Bengal (BoB) has the long-stood enigma of an oxygen minimum zone, which maintains traces of oxygen without becoming fully anoxic. This may, besides biological feedback loos stabilizing the low oxygen concentrations in those waters, have to do with low primary production in the BoB’s surface waters and the lack of subsequent respiration of organic material in intermediate oxygen poor waters. Recently, a small but significant decrease of global marine primary production has been reported based on ocean color data, which was mostly ascribed to decreases in primary production in the northern Indian Ocean, particularly in the Bay of Bengal.

Available reports on primary production from the BoB are limited, and due their spatial and temporal variability difficult to interpret. Primary production in the BoB has historically been described to be driven by diatom and chlorophyte clades, however, this is not consistent with newer data, which instead show an abundance of smaller, visually difficult to detect cyanobacterial primary producers. By combining the available metagenomic and biogeochemical datasets with satellite-based ocean color observations, a pattern can be derived showing a shift in community composition of primary producers in the BoB over the last two decades. This shift is driven by a decrease in chlorophyte abundance, and a coinciding increase in cyanobacterial abundance, despite stable concentrations of total chlorophyll. Statistical analysis indicated a correlation of this community change in the BoB to decreasing nitrate concentrations, which may provide an explanation for both, the decrease of eukaryotic nitrate-dependent primary producers and the increase of small unicellular cyanobacteria related to Prochlorococcus, which have a comparably higher affinity to nitrate. A potential change in primary producer community composition and especially a decrease in primary production in the BoB may thus have a significant impact on the distribution of low oxygen waters in this basin and may possibly mitigate their further expansion, therefore arguing against the BoB being at a tipping point to develop full anoxia.

How to cite: Löscher, C.: Is the Bay of Bengal at a tipping point?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7497, https://doi.org/10.5194/egusphere-egu21-7497, 2021.

The variability of the South Asian Monsoon (SoAM) in warmer climatic conditions is not established yet. The Mid-Pliocene Warm Period (MPWP, 3.264 to 3.025 ma) is the most recent such event when the boundary conditions were similar to present with similar CO₂ concentration (more than 400 ppmv) and temperature (2-3°C higher than present). It presents the best analogue for understanding the impacts of future global warming on SoAM. The high-resolution study of denitrification from the eastern Arabian Sea can provide an insight into the SoAM variability during MPWP. Denitrification is the process by which nitrate is reduced to nitrogen gas (N₂ or N₂O) during organic matter decay in oxygen minima zones in the water column. The denitrification process enriches the nitrate pool with ¹⁵N, which is incorporated in the particulate organic matter. Denitrification is governed by the surface water productivity related to SoAM strength and the water column ventilation. We analyzed the nitrogen isotopic ratio of sedimentary organic matter (SOM, δ¹⁵N_SOM) to examine the denitrification in the eastern Arabian Sea. Total nitrogen (TN %) and total organic carbon (TOC%) are used to estimate the surface water productivity from the sediment collected during expedition IODP 355, Hole U1456A. We find that the δ¹⁵N_SOM values vary between 7-9 ‰ during 3.22-3.15 Ma and 2.9-2.75 Ma indicating high denitrification. High δ¹⁵N_SOM values coincide with high productivity as shown by both TN and TOC. It shows two major periods in the late Pliocene (3.22-3.15 Ma and 2.92-2.75 Ma) associated with stronger denitrification and high productivity. These results indicate the intensification of SoAM during warmer periods of Late Pliocene and at the start of intensification of Northern hemisphere glaciation. The enhanced denitrification during this period could possibly be due to a reduction in deep water ventilation and monsoon driven upsurge in productivity.

How to cite: Behera, P. and Tiwari, M.: Monsoon variability during Mid Pliocene Warm Period: Evidence from oceanic denitrification at eastern Arabian Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12089, https://doi.org/10.5194/egusphere-egu21-12089, 2021.

Under the impact of natural and anthropogenic climate variability, upwelling systems are known to change their properties leading to associated regime shifts in marine ecosystems. These often impact commercial fisheries and societies dependent on them. In a region where in situ hydrographic and biological marine data are scarce, this study uses a combination of remote sensing and ocean modelling to show how a stable seasonal upwelling off the Kenyan coast shifted into the territorial waters of neighboring Tanzania under the influence of the unique 1997/ 98 El Niño and positive Indian Ocean Dipole event. The formation of an anticyclonic gyre adjacent to the Kenyan/ Tanzanian coast led to a reorganization of the surface currents and caused the southward migration of the Somali–Zanzibar confluence zone and is attributed to anomalous wind stress curl over the central Indian Ocean. This caused the lowest observed chlorophyll-a over the North Kenya banks (Kenya), while it reached its historical maximum off Dar Es Salaam (Tanzanian waters). We demonstrate that this situation is specific to the 1997/ 98 El Niño when compared with other the super El-Niño events of 1972,73, 1982–83 and 2015–16. Despite the lack of available fishery data in the region, the local ecosystem changes that the shift of this upwelling may have caused are discussed based on the literature. The likely negative impacts on local fish stocks in Kenya, affecting fishers’ livelihoods and food security, and the temporary increase in pelagic fishery species’ productivity in Tanzania are highlighted. Finally, we discuss how satellite observations may assist fisheries management bodies to anticipate low productivity periods, and mitigate their potentially negative economic impacts.

How to cite: Jacobs, Z., Jebri, F., Srokosz, M., Raitsos, D., Painter, S., Nencioli, F., Osuka, K., Samoilys, M., Sauer, W., Roberts, M., Taylor, S., Scott, L., Kizenga, H., and Popova, E.: A Major Ecosystem Shift in Coastal East African Waters during the 1997/98 Super El Niño as Detected Using Remote Sensing Data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11951, https://doi.org/10.5194/egusphere-egu21-11951, 2021.

Bioavailable nitrogen (N) and phosphorus (P) determine the strength of the ocean’s carbon (C) uptake and variation in their ratio (N:P) is key to phytoplankton growth. A similarity between C:N:P ratio (106:16:1) in plankton and deep-water inorganic nutrients was observed by Alfred C. Redfield, who suggested that biological processes in the surface ocean controlled deep ocean chemistry. Recent studies suggest that the ratio varies geographically. The veracity in C:N:P ratio could be attributed to the characteristic physical and biogeochemical processes, which play an important role in regulating the elemental dynamics in ocean. Basins like the northern Indian Ocean due to its geographic setting and monsoonal wind forcing provide a natural laboratory to explore the role of environmental factors, physical and biogeochemical processes on C:N:P stoichiometry.

We sampled the Bay of Bengal for its C, N, and P contents in the organic and inorganic pool from surface to 2000 m at 8 stations (5 coastal, 3 open ocean) during spring 2019. Mesoscale anticyclonic eddies were identified in our sampling area, which were associated with low nutrient concentrations in the photic depth. Mean (NO₃^- + NO₂^-):PO₄^3- ratio was 0.6 at eddy and 4.7 at non eddy stations. On the other hand, C:N:P in the organic matter was same at eddy and non-eddy locations. Mean C:N:P ratio in particulate organic matter was 254:39:1 and 244:37:1 in the photic depth of the coastal and open ocean stations, respectively. Biological N₂ fixation contributed ~0.1-0.4% to the N:P ratio of export flux, which ultimately contributes to the (NO₃^- + NO₂^-):PO₄^3- ratio in subsurface waters. Our results highlight the importance of physical and biological processes in changing elemental stoichiometry.

How to cite: Sahoo, D., Saxena, H., Nazirahmed, S., Kumar, S., Sudheer, A., Bhushan, R., Sahay, A., and Singh, A.: Role of eddies and N2 fixation in shaping C:N:P proportions in the Bay of Bengal during spring, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11603, https://doi.org/10.5194/egusphere-egu21-11603, 2021.

The Arabian sea (AS) hosts one of the most intense oxygen minimum zones (OMZ) in the open ocean. This OMZ is formed and maintained by the peculiar geography and the associated monsoonal productivity in the AS and  is highly sensitive to the strength of monsoonal circulation and surface heating. Model projection from the fifth phase of Coupled Model Intercomparison project (CMIP5) indicate significant changes in both the Indian monsoonal circulation, atmospheric heat fluxes and primary productivity under climate change, but the response of the AS OMZ to these changes remain largely ill-understood. The poor representation of the AS OMZ and lack of oxygen diagnostics in the CMIP5 simulations pose major limitations in exploring the response of AS OMZ to future climate change. In this study, we use a set of regional downscaled experiments with a high-resolution configuration of the Regional Ocean Modeling System (ROMS) model coupled to a nitrogen-based NPZD ecosystem model to examine the sensitivity of the AS OMZ response to a range of CMIP5 forcing anomalies and to model resolution. Our downscaled set of experiments are based on a climatological control simulation forced with observed climatological atmospheric and lateral boundary conditions, to which climate change anomalies derived from CMIP5 simulations are added to construct climate change forcing fields. The control simulation has been extensively validated against observations. We explore the sensitivity of the downscaled oxygen distribution and OMZ to the regional model setup by varying the model horizontal resolution from 1/3 - 1/12 degree. In agreement with the set of available coarse resolution CMIP5 projections, our downscaled experiments show a future increase in the oxygen levels within the core of AS OMZ. The downscaled experiments improve the realistic representation of different classes of water (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. We find that the projected oxygen changes in the AS OMZ are largely driven by the Apparent Oxygen Utilisation (AOU), which vary with forcing and model resolution, leading to a wide spread in the AS OMZ response to climate change.

How to cite: Vallivattathillam, P., Lachkar, Z., Lévy, M., and Smith, S.: Arabian sea Oxygen Minimum Zone projections under climate change: sensitivity to forcing and model resolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11051, https://doi.org/10.5194/egusphere-egu21-11051, 2021.

The central Arabian Sea (CAS) is productive during both the summer and winter monsoons owing to different physical processes. We analysed four years (2013-2016) record of chlorophyll and dissolved oxygen (DO) concentration from a Bio-Argo float deployed in this region. Though the surface blooms were observed during both the monsoons and sub-surface chlorophyll was also persistently observed, the intensity and duration of the bloom have been decreasing over the past few years. Also, the winter blooms were more prominent compared to the summer bloom in the study region. Our analysis shows that the observed inter-annual variability in the summer bloom can be attributed to the variability in wind speed, oceanic stratification and advection of nutrient rich water from the western Arabian Sea. During both the monsoons, stratification played an important role in reducing the productivity in recent years. We also found that during the winter monsoon, the upwelling Rossby wave propagating from the west coast of India influenced productivity as north as 15ºN. The chlorophyll data from Bio-Argo float shows that the total surface chlorophyll concentration has been decreasing during the study period. Consequently the DO concentration has also been decreased. An increase in the deeper water is speculated to be due to the decrease in surface productivity. This is in contradiction to the previous studies on intensification of Arabian Sea OMZ. Also, in the event of recent reports on decreasing trend in productivity in the Arabian Sea, the present study provides new insights on the possible effect of declining productivity on the DO concentration under the climate change regime.

How to cite: Mathew, T.: Observed variability of monsoon blooms in the north-central Arabian Sea and its implication on oxygen concentration: A Bio-Argo study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16286, https://doi.org/10.5194/egusphere-egu21-16286, 2021.

A winter monsoon cruise was undertaken in the northern Arabian Sea to understand the bio-physical interaction responsible for the occurrence of phytoplankton bloom in the region. The observation shows strong convective mixing with a dense and deeper mixed layer (MLD: 100-140 m) and well-oxygenated upper water column (>95% saturation). The chlorophyll concentration was low (0.1 -0.3 µg/l) despite having ample nitrate (~2.5 µM) in the surface layer. The region, however, was deprived of micro phytoplankton, especially diatomic species and Noctiluca Scintillans, and was dominated by the picophytoplankton (77%-85%). The mean Si/N ratio in the upper 100 m was 0.72 indicating “Silicate stressed” condition for the proliferation of diatoms. Even a deeper mixed layer could not penetrate into the silicicline (~150m) which was deeper than the nitracline (~110m). In addition, the euphotic depth (~49m) was much shallower than the mixed layer depth suggesting the Sverdrup critical depth limitation in the northern Arabian Sea. We further show that the bloom initiated only when the mixed layer shoals towards the euphotic zone. Our observations suggest that two primary factors, the stoichiometric ratio of nutrients, especially Si/N ratio, in the mixed layer and re-stratification of the upper water column, govern the phytoplankton blooming in the northern Arabian Sea during the later winter monsoon.

How to cite: Shenoy, L.: What controls the initiation of algal bloom during the winter monsoon in the Arabian Sea??, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16285, https://doi.org/10.5194/egusphere-egu21-16285, 2021.

Seasonal variability is a powerful component of the spatio-temporal dynamics of plankton communities, especially in the regions with oxygen-depleted waters. The Arabian Sea and the Black Sea are typical representatives of these regions. In both, the dinoflagellate Noctiluca scintillans (Macartney) Kofoid & Swezy, 1921, is one of the abundant plankton species which forms algal blooms. Sampling on coastal stations in the upper mixed layer by the plankton nets with the 120-140 µm mesh size was carried out in 2004-2010. Monthly data were averaged over years. A comparison of seasonal patterns of Noctiluca abundance pointed to the persistence of a bimodal seasonal cycle in both regions. The major peak was observed during spring in the Black Sea and during the winter (Northeast) monsoon in the Arabian Sea. The timing of the second (minor) peak was different over regions as well. This peak was modulated by advection of seasonally fluctuating velocity of coastal currents which transport waters enriched by nutrients by coastal upwelling. The abundance of Noctiluca of the major peak (with the concentration around 1.5*10⁶ cells m^-3) was from one to two orders as much high in the western Arabian Sea compared to the northern Black Sea. The remotely sensed chlorophyll-a concentration during the time of the major seasonal peak exhibited a fivefold difference over these regions. In terms of nutrientconcentration in the upper mixed layer (in particular, nitrates and silicates), a difference of about one order of magnitude was observed.

How to cite: Piontkovski, S., Al Hashmi, K., Zagorodnaya, Y., Serikova, I., Evstigneev, V., Prusova, I., and Alabri, N.: Seasonal variations of the dinoflagellate algae Noctiluca scintillans abundance in the western Arabian Sea and the northern Black Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1479, https://doi.org/10.5194/egusphere-egu21-1479, 2021.

The Indian Ocean is a dynamic region that is heavily influenced by immense freshwater runoff, extreme meteorological events and the seasonal reversal of monsoonal currents. Providing essential resources for over one-third of the global population, the Northern Indian Ocean is a key area of research: increased freshwater run-off, low overturning velocities and high air-sea fluxes result in the region being highly susceptible to climate fluctuations, and execess nutrients, particularly nitrates accumulated through agricultural run-off, directly influence marine biogeochemical cycles. The South Asia Nitrogen Hub (SANH) is a GCRF project designed to assess, monitor and predict the physical and biogeochemical response of the Northern Indian Ocean to such anthropogenic changes. To address key questions in SANH, a relocatable physical-biogeochemical (NEMO-ERSEM) was configured across the region, which includes the Eastern Arabian Sea and the Bay of Bengal. A 22-year hindcast run (1993-2015) at ~11km resolution allows the physical-biogeochemical processes (including from mesoscale eddies, extreme meteorological events and varying runoff) to be viewed at scale that is otherwise impossible with observational campaigns. In conjunction with the large-scale model domain, 6 smaller high-resolution (~1-2km) coastal models were configurated around the Indian subcontinent, allowing a more focussed view at processes that directly impact coastal populations. Here, we will present initial results from the large-scale hindcast run, the coastal regions, and explore the advantages and caveats of relocatable modelling.

How to cite: Jardine, J., Katavouta, A., Partridge, D., Polton, J., Holt, J., and Wakelin, S.: Assessing the long-term physical-biogeochemical interactions in the North Indian Ocean using a coupled relocatable model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9706, https://doi.org/10.5194/egusphere-egu21-9706, 2021.

Coral larvae can be transported over great distances by ocean currents, establishing ecological and genetic connectivity between distal coral reefs. Understanding these patterns of connectivity and how they vary through time is essential for effective marine spatial planning, particularly in the SW Indian Ocean which is an under-studied region. However, tracking coral larval dispersal directly is generally unfeasible due to their size, necessitating indirect observations or numerical models. We have developed a regional configuration of the Coastal and Regional Ocean Community Model (CROCO) in the SW Indian Ocean at 1/50^o, spanning from the East African coast to the Chagos Archipelago, to simulate surface currents and gain insight into likely coral larval dispersal pathways and connectivity. The configuration is forced by the ERA-5 atmospheric reanalysis at the surface, and the 1/12^o CMEMS GLORYS12V1 reanalysis and barotropic tides at the lateral ocean boundaries. We will be carrying out a 25-year interannual simulation and a climatological control simulation. Using lagrangian particle tracking, we will estimate patterns of connectivity between reef sites across the region (with a particular focus on connectivity across Seychelles), and how significant and predictable the temporal variability in connectivity is. Early progress towards this goal will be presented. The longer-term ambition of this project is to assess our predicted connectivity against independent connectivity estimates from genetic studies and previous regional simulations at a lower resolution.

How to cite: Vogt-Vincent, N., Johnson, H., and Burt, A.: Modelling coral reef connectivity in the SW Indian Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-959, https://doi.org/10.5194/egusphere-egu21-959, 2021.