The bottom boundary layer (BBL) is the portion of the water column which is directly affected by the drag of ocean currents on the seafloor. It is often assumed to coincide with the bottom mixed layer in which potential temperature is approximately uniform. Bottom mixed layers have an observed thickness of O(10 m), with maxima of O(100 m) in some basins. As a result, they are in general not represented in numerical models of large-scale circulation, which typically assume a vertical resolution of a few hundred meters near the bottom. The coarse vertical resolution near the bottom that is assumed in many numerical models implies that these models may not accurately represent the velocity shears near the bottom and the dissipation rates of kinetic energy by bottom drag, which depends on the near-bottom velocity. Here we present results from idealized numerical experiments of the circulation in a double (subtropical-subpolar) gyre, which are aimed at determining the effects of near-bottom vertical resolution on simulated ocean circulation and energetics. Results from experiments that resolve the BBL are compared to those from experiments that do not. In all experiments, the horizontal grid is fine enough to resolve the mesoscale eddy field. In our presentation, emphasis will be placed on energy dissipation by bottom drag in experiments with different near-bottom vertical resolutions. The implications of our results for the simulation of large-scale ocean circulation will then be clarified.

How to cite: Chen, S.-Y. (. and Marchal, O.: Resolution of the bottom boundary layer in an eddy-rich model of double-gyre circulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4023, https://doi.org/10.5194/egusphere-egu24-4023, 2024.

Comprehending how submesoscale dynamics and their potential interplay with tides affect climate models is challenging due to their small scales and high computational demands. To address this challenge, our approach integrates modelling and observational methods. In this study, we investigate the impact of internal tides, eddies and submesoscale currents on the frequency energy spectrum of the ocean. To this end, we apply a novel simulation with telescopic grid refinement to achieve a horizontal resolution finer than 600 m over large regions of the South Atlantic. This refined resolution allows us to accurately capture submesoscale turbulence and a relatively large part of the internal wave spectrum under realistic atmospheric conditions. By comparing simulations with and without tides, we find that without tidal forcing there is significantly less energy at the high frequency end of the spectrum. Validation with mooring and Pressure Inverted Echo Sounder data sets deployed over a two year period in the Walvis Ridge region indicates that the simulation with tides is more accurate in terms of high frequency energy levels. Using an eddy tracking algorithm allows us to differentiate energy spectra within the Agulhas rings from a ring-absent background state. Within these eddies, we observe a substantial shift towards higher power spectral densities of approximately one order of magnitude across both small and large scales.

How to cite: Epke, M. and Brüggemann, N.: Impact of tides and eddies on ocean energy spectra in submesoscale resolving simulations of the South Atlantic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2470, https://doi.org/10.5194/egusphere-egu24-2470, 2024.

This study provides an analysis of the ocean Lorentz Energy Cycle (LEC) simulated with an ICON-O configuration of 5km horizontal resolution. We focus on those aspects of the LEC related to the dissipation of mesoscale eddy energy. Since most processes relevant for eddy dissipation cannot be resolved even with 5km horizontal resolution, parameterizations are required to dissipate the energy. Typical parameterizations used in ocean models are bottom friction, vertical viscous dissipation and horizontal biharmonic dissipation. We analyse how these parameterizations are responsible for the simulated eddy energy dissipation. Dedicated sensitivity experiments estimate uncertainties arising from these parameterizations when typical friction parameters are modified. These experiments allow to assess the impact on the overall energy balance. Furthermore, we discuss how inertial and sub-inertial motions influence the overall energy dissipation. Overall, this study aims to provide the basis for future, more realistic diagnostics and parameterizations of the processes involved in eddy dissipation.

How to cite: Hillenkötter, D. and Brüggemann, N.: Sensitivity Analysis of the Lorentz Energy Cycle in the ICON-O Model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12074, https://doi.org/10.5194/egusphere-egu24-12074, 2024.

The character of internal tides energy flux in the northern South China Sea (SCS) is explored through an analysis of a fleet of underwater gliders. It is found that the lower mode diurnal internal tides with ~300 km wavelength in the middle basin originate predominantly at the Luzon Strait (LS) and propagate over 1000 km to the western SCS. The semidiurnal internal tides, however, originate from multiple regions, including the LS, the continental shelf, and the islands in the west part. The energy flux of the mode-1 diurnal internal tides attenuated rapidly within 450 km of the LS and was less pronounced after that. The estimated dissipation rate based on the mode-1 energy flux is about 10^-8 W/kg, underling the significant role of mode-1 diurnal internal tides in bolstering far-field mixing. This study provides a unique view of the spatial pattern, energy flux, and energy sink of the internal tides in the northern SCS, which could supplement the altimetry-based results and improve the parameterization in ocean models. 

How to cite: Gao, Z., Chen, Z., Huang, X., Yang, H., Wang, Y., Ma, W., and Luo, C.: Estimating the Energy Flux of Internal Tides in the Northern South China Sea Using Underwater Gliders, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4384, https://doi.org/10.5194/egusphere-egu24-4384, 2024.

Despite the importance of the Arctic Ocean for the large-scale circulation and climate, there is still a knowledge gap in our understanding of the spatial characteristics of the Arctic Ocean circulation, especially for the mesoscale. This research diagoses the kinetic energy (KE) and its transfer features of a rich-eddy Arctic Ocean from eddy-resolved sea-ice model FESOM (Finite-Element/volumE Sea ice-Ocean Model)  at about 1km in horizontal resolution, revealing the KE spectra at spatial scales and the seasonality. There are two peaks in the kinetic energy spectral density, one at the gyre scale of the Arctic boundary currents (centered at 1700-2000km), and the other associated with the mesoscale (centered at 60km), both of which exhibit a power-law scaling typical for large circulation and ocean eddies, respectively. Energy transfer diagnosis shows that there is inverse-cascade KE pathway at a hundred of kilometers to several killometers, of which the KE transfer value from larger scales to smaller scales is negative. The seasonality of the eddy KE spectrum and energy transfer in upper ocean is found to be highly consistent to sea ice loss trend, while eddy KE in deep ocean shows not significant seasonal variations. 

How to cite: Liu, C., Wang, Q., Danilov, S., Koldunov, N., Mueller, V., Li, X., Sidorenko, D., and Zhang, S.: Spatial scales of kinetic energy and its transfer across large-meso scales in the Arctic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-431, https://doi.org/10.5194/egusphere-egu24-431, 2024.

North-western Indian Ocean (NIO), especially Arabian Sea (AS), experiences intense seasonally reversing winds and undergoes a seasonal change in the turbulence energetics. Given the importance of Arabian Sea dynamics on the Indian monsoon, a numerical investigation was carried out to study the energy budget using MOM 5. Model domain is between 32 to 118⁰ E and 28⁰ S to 30⁰ N and is forced with daily varying 10 year averaged atmospheric fluxes. Lateral open boundaries are prescribed with sea surface height anomaly (SSHA) in combination with radiation condition. Vertical profiles of temperature and salinity are also prescribed at lateral boundaries. Using the averaged forcing, model is run for 10 years as spin up and brought into equilibrium and then forced with 5 years inter-annually varying dataset, which is used for analysis. Though the model domain covers quite an extensive part of Indian Ocean, but our analysis is limited in the NIO between 8⁰S to 30⁰N and 32 to 82⁰E (hereafter called as Analysis domain). Model produced data is seasonally averaged into 4 seasons, DJF (December to February), MAM (March to May), JJAS (June to September), and ON (October-November). Currents are well represented by model and are validated with OSCAR currents. Ocean properties such as Sea Surface Temperature, Sea Surface Salinity and Mixed Layer Depth produced by model are within reasonable bias. The investigation of TKE indicates that it has seasonal and spatial preference. Depth averaged over 30m TKE, indicates that it is strongest in JJAS and DJF along the Somalia coast and equatorial region of NIO respectively. In MAM, TKE is strong in south-western AS, close to equator and in ON, a reminiscence of TKE is seen along the Western AS (WAS). Spatial average vertical profile of TKE for 3 regions, [1: 8⁰S to 30⁰N and 32 to 82⁰E (Analysis domain), 2: 4⁰S to 4⁰N and 56 to 82⁰E (Western Equatorial Indian Ocean, WEIO), and 3: 4 to 12⁰N and 45 to 55⁰E (WAS)] is shown. Analysis reveals that TKE is highest in upper 200m and decreases with depth, and in DJF, it is maximum in WEIO and least in WAS, but quite interestingly, after 150m till about 250m, TKE increases in WAS. This could indicate strong subsurface turbulent activity in WAS in DJF. In JJAS, TKE is almost 4 times that of DJF and is highest in WAS and least in WEIO. In WAS, turbulence produced due to buoyancy is suppressed by production flux being negative which indicates inverse energy cascade in DJF. In JJAS, production flux is strongly positive in WAS with negative production in the Great Whirl eddy and in the equatorial region. Turbulence is produced by buoyancy as well near horn of Africa, yet dissipation is weak in the said region, which could be due to strong positive transport. Production shows weak positive features in WAS and is strongly negative in equatorial region in MAM and ON. Buoyancy flux is negative in WAS in MAM and ON, indicative of stable stratification.

How to cite: Chauhan, R., Behera, M., and Balasubramanian, S.: Modelling the seasonal and spatial variation of Turbulent Kinetic Energy budget in the north-western Indian Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-578, https://doi.org/10.5194/egusphere-egu24-578, 2024.

Understanding the potential impacts of internal waves on phytoplankton communities in oligotrophic oceans remains an important research challenge. In this study, we elucidated the impact of internal waves on phytoplankton communities through a comprehensive 154-hr time-series of observations in the South China Sea (SCS). We identified distinctive variations in phytoplankton pigment biomass and composition across the upper, middle, and lower layers of the euphotic zone, which we attributed to the perturbations triggered by internal waves. Phytoplankton other than Prochlorococcus in the lower, nutrient-replete layer likely benefitted from allochthonous nutrients introduced by internal waves, but their growth rates were constrained by light limitation, and their pigment biomass was held in check by microzooplankton grazing. In contrast, in the upper, nutrient-depleted layer, the relative abundance of Prochlorococcus increased, likely because of the ammonium regenerated by zooplankton. The middle layer, characterized as the deep chlorophyll maximum layer, exhibited a dynamic equilibrium characterized by nutrient and light co-limitation. This equilibrium resulted in high nitrate assimilation and growth by phytoplankton. The balancing of those rates by significant grazing losses maintained total chlorophyll a concentrations at a high level. Based on these findings, we proposed a three-layer euphotic zone structure characterized by distinct physiological conditions, nutrient-light dynamics, grazing pressure, and phytoplankton responses to internal waves. This three-layer paradigm elucidated the intricate interplay between internal waves and phytoplankton communities and provided insights into the mechanisms that govern primary production and carbon cycling in oligotrophic oceanic ecosystems.

How to cite: xiao, W., ma, L., bai, X., Laws, E., guo, C., liu, X., Chiang, K.-P., gao, K., and huang, B.: Responses of Phytoplankton Communities to Internal Waves inOligotrophic Oceans, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4161, https://doi.org/10.5194/egusphere-egu24-4161, 2024.

Hourly satellite-tracked surface drifter data are utilized to study energy transfer from eddies to Near-Inertial Waves (NIWs). Spatial velocity gradients are computed from two consecutive velocity estimates derived from the same drifter, providing variable spatial resolutions of (1 km). The eddy-to-NIW energy transfer can be positive or negative, with the positive transfer (forward energy cascade) dominant. The global integrated energy transfer rate (ε) is 0.025 TW, with the anticyclonic eddy contribution dominant over the cyclonic eddy contribution. Given that the global near-inertial wind work (W) is 0.2 TW, the eddy-to-NIW energy transfer efficiency (ε/W) is about 13%, which is one order of magnitude larger than that in low resolution simulations. This result may still underestimate the Eulerian energy transfer by a factor of 2. To our knowledge, this is the first time that this energy transfer is calculated from global drifter observations, providing a baseline for comparison in future studies.

How to cite: Chen, Z.: Energy Transfer between Mesoscale Eddies and Near-inertial Waves from Surface Drifter Observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5641, https://doi.org/10.5194/egusphere-egu24-5641, 2024.

Internal solitary waves (ISWs) were observed by mooring data in the South China Sea in August and September 2014. Normally the arrival pattern of ISWs is relatively regular and the ISW amplitudes have a positive correlation with the magnitude of semidiurnal tidal currents near the Luzon Strait. However, the ISW amplitudes observed in the second spring tide of September are significantly large when an anti-cyclonic eddy is passing the mooring. When an ISW passes the eddy center, its amplitude reduces to nearly zero, but when an ISW passes the eddy edge, its amplitude increases by 50%. A deepened thermocline always damps the ISW amplitudes, whilst the eddy-induced background currents at different locations may have varied effects on the ISW amplitudes, e.g., the background current at the eddy center tends to damp the ISW amplitude but that at the eddy edge amplifies the ISW amplitude. The different effects of a deepened thermocline and eddy-induced background currents near the eddy edge may damp or amplify the ISW amplitude.

How to cite: Xu, J.: Observations of different effects of an anti-cyclonic eddy on internal solitary waves in the South China Sea , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6907, https://doi.org/10.5194/egusphere-egu24-6907, 2024.

Tides, and more specifically the internal gravity wave field generated by the tidal current flowing over bathymetric features, play a significant role in the energy cascade and in setting the global state of the ocean by intensifying mixing. Accounting for these dynamics in numerical models is important for accurately depicting energy transfer across scales, and an increasing number of models use explicit tidal forcing. Yet, in fixed coordinates models, the strong vertical motions generated by internal waves trigger the tendency of advection to produce numerical errors, leading on the vertical to spurious numerical mixing that can easily be as high as the physical one. This can severely bias model output and impact the overall simulated dynamics by spuriously mixing water masses and modifying the stratification, which in turn impacts the propagation of internal waves. We propose a fresh way to look at advection schemes, along with a method to make their diffusivity more selective, and eventually protect the relevant vertical scales from being spuriously eroded. A case study on the South East Asian Seas using the Symphonie model is presented, since these seas are known for being the generation site of exceptionally strong internal tides.

How to cite: Garinet, A., Marsaleix, P., and Herrmann, M.: Reducing spurious numerical mixing in simulations under strong tidal forcing : a case study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11635, https://doi.org/10.5194/egusphere-egu24-11635, 2024.

We find internal gravity waves everywhere in the oceans. The interaction of these waves with other waves or eddies are key to the energy transfers in the ocean and can also lead to wave breaking and density mixing, which in turn drives the large-scale mean flow. The importance of wave-wave interactions compared to wave-mean flow interactions, however, remains largely unknown. We present two methods to better understand both processes: First, the scattering of waves by other waves is evaluated numerically using the kinetic equation derived from the assumption of weak interactions. Second, the interaction of waves with a local coherent meso-scale eddy and a random eddy field is simulated using a novel numerical model called the Internal Wave Energy Model based on the six-dimensional radiative transfer equation. We find mainly a forward energy cascade due to wave-wave interactions. Waves loose energy to the local eddy at its rim in a critical layer like behaviour, while the eddy field scatters the waves along a constant frequency cone.

How to cite: Sebastia Saez, P., Eden, C., and Chouksey, M.: Interactions of internal gravity waves with other waves and mesoscale eddies, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12521, https://doi.org/10.5194/egusphere-egu24-12521, 2024.

In this study, the effects of cross-front winds on a submesoscale dense filament are investigated using high-resolution turbulence and velocity observations, and idealized numerical simulations. Our study area is the Baltic Sea, which is characterized by strong frontal gradients and pronounced submesoscale dynamics. Embedded in a large-scale frontal region, our observations reveal the existence of a 3-4 km wide, dense filament with an asymmetric structure resulting from the interactions between cross-front winds and the two submesoscale fronts laterally bounding the filament. These two fronts are driven by either downgradient winds, directed from the lighter surrounding waters toward the dense center of the filament, or upgradient winds, directed in the opposite direction. While the effect of a wind stress that is aligned with the frontal jet has been investigated in numerous previous studies, especially field data focusing on the role of cross-front winds are largely lacking at the moment. We find that for downgradient winds, when the surface Ekman transport and the frontal jet are aligned, both the frontal jet and the cross-front secondary circulation are enhanced. The latter supports a tendency for frontal re-stratification, suppression of turbulence, and mixed-layer shoaling. For the front with upgradient wind forcing, the Ekman transport and the frontal jet nearly cancel, and also the cross-front secondary circulation is strongly suppressed. Restratification by the secondary circulation is weak in this case, and the well-mixed turbulent surface layer is approximately twice as deep compared to the other side of the filament with downgradient forcing. We show that these mechanisms are consistent with results from idealized numerical simulations of frontal regions with downgradient and upgradient wind forcing.

How to cite: Umlauf, L., Schmitt, M., and Peng, J.-P.: Cross-front wind forcing of a dense submesoscale filament, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18021, https://doi.org/10.5194/egusphere-egu24-18021, 2024.