Effects of an along-shelf current on the generation of internal tides near the critical latitude

The effects of along-shelf barotropic geostrophic currents on internal wave generation by the $K_1$ tide interacting with a shelf at near-critical latitudes are investigated. The horizontal shear of the background current results in a spatially varying effective Coriolis frequency which modifies the slope criticality and potentially creates blocking regions where freely propagating internal tides cannot exist. This paper is focused on the barotropic to baroclinic energy conversion rate, which is affected by a combination of three factors: slope criticality, size and location of the blocking region where the conversion rate is extremely small and the internal tide (IT) beam patterns. All of these are sensitive to the current parameters. In our parameter space, the current can increase the conversion rate up to 10 times.

Analysis of Characteristics and Turbulent Mixing of Seawater Mass in Lombok Strait

The Lombok Strait, as one of the outlet straits, is part of the ITF route, which is directly adjacent to the Indian Ocean. There is a sill in the Lombok Strait, which is a place for internal wave generation. Leg-1 data from the Japan Agency for Marine-Earth Science and Technology in collaboration with the Agency for the Assessment and Application of Technology which is part of the Tropical Ocean Climate Study Expedition including CTD Yoyo and ADCP taken using ship vehicles R/V Kaiyo. CTD Snapshot from PUSHIDROSAL using the KRI Spica 934 vehicle part of the Opssurta Baruna Jaya 2 Expedition. Determination of seawater mass stratification with the criteria for the thermocline layer is ≥ 0.05 °C.m-1. Four types of water masses were identified, Java Sea, mixed seawater mass (Java Sea - ITF) which occurred diapycnal mixing, North Pacific Subtropical Water (NPSW) and North Pacific Intermediate Water (NPIW). The seawater mass stratification in the Lombok Strait based on temperature, salinity and density which are seen to follow the internal tidal pattern. The average values for energy dissipation and vertical diffusivity for each layer and replication were 5.73 x 10-7 W.Kg-1 and 3.67 x 10-2 m2.s-1 for CTD Yoyo and 2.25 x 10-6 W.Kg-1 and 7.38 x 10-2 m2.s-1 for CTD Snapshot. The value obtained is greater than the open ocean and straits in other studies. The high shear value confirms this in the thermocline layer. The Richardson gradient value> 0.25 is relatively constant in the thermocline layer.

Internal Wave Generation by a Combination of Tidal and Steady Currents

<p>In the presence of topography, two main contributors for internal wave energy are tide-topography interaction transferring energy from the barotropic tide to internal tides, and lee wave generation when geostrophic currents or eddying abyssal flows interact with topography. In the past few decades, many studies considered the respective contribution of the oscillating flows or steady background flows, but few investigations have considered both.  </p><p>In this talk, we consider the joint effects of tidal and steady currents to investigate internal wave generation and propagation on the Amazon shelf, a hotspot for internal solitary wave (ISW) generation. The Amazon Shelf is off the mouth of the Amazon River in the southwest tropical Atlantic Ocean, affected by strong tidal constituents over complex bottom bathymetry and a strong western boundary current, the North Brazilian Current (NBC). Both satellite observations and numerical modelling are used in this study. Satellite observations provide a clear visualization of the wave characteristics, such as temporal and spatial distributions, propagating direction and its relation to background currents. Based on parameters from satellite observations and reanalysis dataset, we set up a model to numerically investigate the dynamics of the ISW generation. We demonstrate that the small-scale topography contributes to a rich generation of along-shelf propagating ISW, which significantly contribute to the ocean mixing and potentially cause sediment resuspension. Moreover, the ISW-induced currents also contribute to the sea surface wave breaking as observed by satellite measurements. In addition, statistics based on a decade of satellite images and numerical investigations on seasonal variations of the ISWs and the NBC improve our understanding of the generation and evolution of these nonlinear internal waves in the presence of background currents.</p>

Nonlinear internal wave generation over the Yermak Plateau

<p>North of the critical latitude (78.4), internal waves of the M<sub>2</sub> tidal frequency can no longer freely propagate, and the energy conversion from the barotropic to the internal tides vanishes. Near the continental slopes around the Arctic Ocean, internal wave energy is enhanced and comparable to values at mid-latitudes (Rippeth et al. 2015, Levine et al. 1985). Observations on the northern flank of the Yermak Plateau (YP) has characterized the region as one of enhanced internal wave activity and nonlinear internal waves have been observed (Czipott et al. 1991, Padman and Dillon 1991).</p><p>The YP is a bathymetry feature stretching out into the Fram Strait north-west of Svalbard. The YP plays a prominent role in the Arctic’s heat balance due to its interaction with the West-Spitsbergen current which is a main contributor to the heat transport into the Arctic Ocean. Nonlinear waves generated over the YP are a significant energy source for mixing and can therefore modulate and force exchange processes.</p><p>To study the nonlinear internal wave generation mechanisms over the YP, we used a high resolution, nonlinear, non-hydrostatic model. We found that nonlinear internal waves are forced not by the M<sub>2</sub> but the K<sub>1</sub> tide which has been observed to have significant variability over the YP (Padman et al. 1992). Barotropic, diurnal shelf waves generated on the eastern side of the YP propagates counter-clockwise, amplifying the cross-slope currents. This amplification is the necessary condition for nonlinear internal wave generation over the YP.</p>

AN INTERNAL WAVE GENERATION METHOD FOR THE NON-HYDROSTATIC MODEL SWASH

Numerical wave propagation models are commonly used as engineering tools for the study of wave transformation in coastal areas. In order to simulate waves in the nearshore zone correctly, the generation and absorption of waves at the boundaries of the models need to be modelled accurately. In numerical models, incident waves are usually generated by prescribing their horizontal velocity component at the boundary of the computational domain over the vertical direction. Additionally, in order to absorb and to prevent re-reflections in front of the numerical wave generator, a weakly reflective wave generation boundary condition is applied in which the total velocity signal is a superposition of the incident velocity signal and a velocity signal of the reflected waves. However, this method is based on the assumption that the reflected waves are small amplitude shallow water waves propagating perpendicular to the boundary of the computational domain and hence this method is weakly reflective for directional and dispersive waves. Within the present study, an internal wave generation method combined with sponge layers is applied in the non-hydrostatic model SWASH, in order to more accurately generate waves and avoid re-reflections at the boundaries.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/5M3aU03XJvI

Internal Wave Generation in Tropical Upwelling Regions: the Angolan and Peruvian Shelves

<p>The Angolan and Peruvian shelves are located in upwelling regions along the eastern boundaries of the tropical Atlantic and Pacific Oceans. They are sites of important fisheries supported by high productivity which is driven by fluxes of nutrients from deep to near surface water along the coast. Mixing associated with internal waves is believed to play a role in this process. Recent field observations have shown the presence of an active internal wave field that includes internal solitary waves. In this talk results of high-resolution two-dimensional simulations of internal wave generation by tide-topography interactions on the Angolan and Peruvian shelves are presented. The simulations show the generation of internal wave beams at near-critical slopes and the generation of high-frequency internal solitary waves. The high-frequency IW spectrum is enhanced when small scale bathymetric ripples are included. Wave generation during winter and summer stratifications will be compared.</p>